X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2Ftypecheck%2FTcSimplify.lhs;h=f187cdcf088b6bb9f47e3b51b8e5b1721624497c;hb=fa2879a0a740ee3c3860822ff5bb93c97d4d6af6;hp=ad166c1776707d5f828670f67072626a6d0b154f;hpb=3160f854580e6d8df412c8cd34d93bae27175d67;p=ghc-hetmet.git diff --git a/ghc/compiler/typecheck/TcSimplify.lhs b/ghc/compiler/typecheck/TcSimplify.lhs index ad166c1..f187cdc 100644 --- a/ghc/compiler/typecheck/TcSimplify.lhs +++ b/ghc/compiler/typecheck/TcSimplify.lhs @@ -3,291 +3,1302 @@ % \section[TcSimplify]{TcSimplify} -Notes: -Inference (local definitions) -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -If the inst constrains a local type variable, then - [ReduceMe] if it's a literal or method inst, reduce it - [DontReduce] otherwise see whether the inst is just a constant - if succeed, use it - if not, add original to context - This check gets rid of constant dictionaries without - losing sharing. +\begin{code} +module TcSimplify ( + tcSimplifyInfer, tcSimplifyInferCheck, + tcSimplifyCheck, tcSimplifyRestricted, + tcSimplifyToDicts, tcSimplifyIPs, + tcSimplifySuperClasses, + tcSimplifyTop, tcSimplifyInteractive, + tcSimplifyBracket, + + tcSimplifyDeriv, tcSimplifyDefault, + bindInstsOfLocalFuns + ) where + +#include "HsVersions.h" + +import {-# SOURCE #-} TcUnify( unifyType ) +import HsSyn ( HsBind(..), HsExpr(..), LHsExpr, emptyLHsBinds ) +import TcHsSyn ( mkHsApp, mkHsTyApp, mkHsDictApp ) + +import TcRnMonad +import Inst ( lookupInst, LookupInstResult(..), + tyVarsOfInst, fdPredsOfInsts, newDicts, + isDict, isClassDict, isLinearInst, linearInstType, + isMethodFor, isMethod, + instToId, tyVarsOfInsts, cloneDict, + ipNamesOfInsts, ipNamesOfInst, dictPred, + fdPredsOfInst, + newDictsAtLoc, tcInstClassOp, + getDictClassTys, isTyVarDict, instLoc, + zonkInst, tidyInsts, tidyMoreInsts, + pprInsts, pprDictsInFull, pprInstInFull, tcGetInstEnvs, + isInheritableInst, pprDictsTheta + ) +import TcEnv ( tcGetGlobalTyVars, tcLookupId, findGlobals, pprBinders, + lclEnvElts, tcMetaTy ) +import InstEnv ( lookupInstEnv, classInstances, pprInstances ) +import TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, checkAmbiguity ) +import TcType ( TcTyVar, TcTyVarSet, ThetaType, TcPredType, + mkClassPred, isOverloadedTy, mkTyConApp, isSkolemTyVar, + mkTyVarTy, tcGetTyVar, isTyVarClassPred, mkTyVarTys, + tyVarsOfPred, tcEqType, pprPred, mkPredTy, tcIsTyVarTy ) +import TcIface ( checkWiredInTyCon ) +import Id ( idType, mkUserLocal ) +import Var ( TyVar ) +import Name ( Name, getOccName, getSrcLoc ) +import NameSet ( NameSet, mkNameSet, elemNameSet ) +import Class ( classBigSig, classKey ) +import FunDeps ( oclose, grow, improve, pprEquationDoc ) +import PrelInfo ( isNumericClass, isStandardClass ) +import PrelNames ( splitName, fstName, sndName, integerTyConName, + showClassKey, eqClassKey, ordClassKey ) +import Type ( zipTopTvSubst, substTheta, substTy ) +import TysWiredIn ( pairTyCon, doubleTy, doubleTyCon ) +import ErrUtils ( Message ) +import BasicTypes ( TopLevelFlag, isNotTopLevel ) +import VarSet +import VarEnv ( TidyEnv ) +import FiniteMap +import Bag +import Outputable +import ListSetOps ( equivClasses ) +import Util ( zipEqual, isSingleton ) +import List ( partition ) +import SrcLoc ( Located(..) ) +import DynFlags ( DynFlag(..) ) +import StaticFlags +\end{code} + + +%************************************************************************ +%* * +\subsection{NOTES} +%* * +%************************************************************************ + + -------------------------------------- + Notes on functional dependencies (a bug) + -------------------------------------- + +| > class Foo a b | a->b +| > +| > class Bar a b | a->b +| > +| > data Obj = Obj +| > +| > instance Bar Obj Obj +| > +| > instance (Bar a b) => Foo a b +| > +| > foo:: (Foo a b) => a -> String +| > foo _ = "works" +| > +| > runFoo:: (forall a b. (Foo a b) => a -> w) -> w +| > runFoo f = f Obj +| +| *Test> runFoo foo +| +| :1: +| Could not deduce (Bar a b) from the context (Foo a b) +| arising from use of `foo' at :1 +| Probable fix: +| Add (Bar a b) to the expected type of an expression +| In the first argument of `runFoo', namely `foo' +| In the definition of `it': it = runFoo foo +| +| Why all of the sudden does GHC need the constraint Bar a b? The +| function foo didn't ask for that... + +The trouble is that to type (runFoo foo), GHC has to solve the problem: + + Given constraint Foo a b + Solve constraint Foo a b' + +Notice that b and b' aren't the same. To solve this, just do +improvement and then they are the same. But GHC currently does + simplify constraints + apply improvement + and loop + +That is usually fine, but it isn't here, because it sees that Foo a b is +not the same as Foo a b', and so instead applies the instance decl for +instance Bar a b => Foo a b. And that's where the Bar constraint comes +from. + +The Right Thing is to improve whenever the constraint set changes at +all. Not hard in principle, but it'll take a bit of fiddling to do. + + + + -------------------------------------- + Notes on quantification + -------------------------------------- + +Suppose we are about to do a generalisation step. +We have in our hand + + G the environment + T the type of the RHS + C the constraints from that RHS + +The game is to figure out + + Q the set of type variables over which to quantify + Ct the constraints we will *not* quantify over + Cq the constraints we will quantify over + +So we're going to infer the type + + forall Q. Cq => T + +and float the constraints Ct further outwards. + +Here are the things that *must* be true: + + (A) Q intersect fv(G) = EMPTY limits how big Q can be + (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be + +(A) says we can't quantify over a variable that's free in the +environment. (B) says we must quantify over all the truly free +variables in T, else we won't get a sufficiently general type. We do +not *need* to quantify over any variable that is fixed by the free +vars of the environment G. + + BETWEEN THESE TWO BOUNDS, ANY Q WILL DO! + +Example: class H x y | x->y where ... + + fv(G) = {a} C = {H a b, H c d} + T = c -> b + + (A) Q intersect {a} is empty + (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d} + + So Q can be {c,d}, {b,c,d} + +Other things being equal, however, we'd like to quantify over as few +variables as possible: smaller types, fewer type applications, more +constraints can get into Ct instead of Cq. + + +----------------------------------------- +We will make use of + + fv(T) the free type vars of T + + oclose(vs,C) The result of extending the set of tyvars vs + using the functional dependencies from C + + grow(vs,C) The result of extend the set of tyvars vs + using all conceivable links from C. + + E.g. vs = {a}, C = {H [a] b, K (b,Int) c, Eq e} + Then grow(vs,C) = {a,b,c} + + Note that grow(vs,C) `superset` grow(vs,simplify(C)) + That is, simplfication can only shrink the result of grow. + +Notice that + oclose is conservative one way: v `elem` oclose(vs,C) => v is definitely fixed by vs + grow is conservative the other way: if v might be fixed by vs => v `elem` grow(vs,C) + + +----------------------------------------- + +Choosing Q +~~~~~~~~~~ +Here's a good way to choose Q: + + Q = grow( fv(T), C ) \ oclose( fv(G), C ) + +That is, quantify over all variable that that MIGHT be fixed by the +call site (which influences T), but which aren't DEFINITELY fixed by +G. This choice definitely quantifies over enough type variables, +albeit perhaps too many. + +Why grow( fv(T), C ) rather than fv(T)? Consider + + class H x y | x->y where ... + + T = c->c + C = (H c d) + + If we used fv(T) = {c} we'd get the type + + forall c. H c d => c -> b + + And then if the fn was called at several different c's, each of + which fixed d differently, we'd get a unification error, because + d isn't quantified. Solution: quantify d. So we must quantify + everything that might be influenced by c. + +Why not oclose( fv(T), C )? Because we might not be able to see +all the functional dependencies yet: + + class H x y | x->y where ... + instance H x y => Eq (T x y) where ... + + T = c->c + C = (Eq (T c d)) + + Now oclose(fv(T),C) = {c}, because the functional dependency isn't + apparent yet, and that's wrong. We must really quantify over d too. + + +There really isn't any point in quantifying over any more than +grow( fv(T), C ), because the call sites can't possibly influence +any other type variables. + + + + -------------------------------------- + Notes on ambiguity + -------------------------------------- + +It's very hard to be certain when a type is ambiguous. Consider + + class K x + class H x y | x -> y + instance H x y => K (x,y) + +Is this type ambiguous? + forall a b. (K (a,b), Eq b) => a -> a + +Looks like it! But if we simplify (K (a,b)) we get (H a b) and +now we see that a fixes b. So we can't tell about ambiguity for sure +without doing a full simplification. And even that isn't possible if +the context has some free vars that may get unified. Urgle! + +Here's another example: is this ambiguous? + forall a b. Eq (T b) => a -> a +Not if there's an insance decl (with no context) + instance Eq (T b) where ... + +You may say of this example that we should use the instance decl right +away, but you can't always do that: + + class J a b where ... + instance J Int b where ... + + f :: forall a b. J a b => a -> a -If the inst does not constrain a local type variable then - [Free] then throw it out as free. +(Notice: no functional dependency in J's class decl.) +Here f's type is perfectly fine, provided f is only called at Int. +It's premature to complain when meeting f's signature, or even +when inferring a type for f. -Inference (top level definitions) -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -If the inst does not constrain a local type variable, then - [FreeIfTautological] try for tautology; - if so, throw it out as free - (discarding result of tautology check) - if not, make original inst part of the context - (eliminating superclasses as usual) -If the inst constrains a local type variable, then - as for inference (local defns) +However, we don't *need* to report ambiguity right away. It'll always +show up at the call site.... and eventually at main, which needs special +treatment. Nevertheless, reporting ambiguity promptly is an excellent thing. -Checking (local defns) -~~~~~~~~ -If the inst constrains a local type variable then - [ReduceMe] reduce (signal error on failure) +So here's the plan. We WARN about probable ambiguity if -If the inst does not constrain a local type variable then - [Free] throw it out as free. + fv(Cq) is not a subset of oclose(fv(T) union fv(G), C) -Checking (top level) -~~~~~~~~~~~~~~~~~~~~ -If the inst constrains a local type variable then - as for checking (local defns) +(all tested before quantification). +That is, all the type variables in Cq must be fixed by the the variables +in the environment, or by the variables in the type. -If the inst does not constrain a local type variable then - as for checking (local defns) +Notice that we union before calling oclose. Here's an example: + class J a b c | a b -> c + fv(G) = {a} +Is this ambiguous? + forall b c. (J a b c) => b -> b -Checking once per module -~~~~~~~~~~~~~~~~~~~~~~~~~ -For dicts of the form (C a), where C is a std class - and "a" is a type variable, - [DontReduce] add to context +Only if we union {a} from G with {b} from T before using oclose, +do we see that c is fixed. -otherwise [ReduceMe] always reduce +It's a bit vague exactly which C we should use for this oclose call. If we +don't fix enough variables we might complain when we shouldn't (see +the above nasty example). Nothing will be perfect. That's why we can +only issue a warning. -[NB: we may generate one Tree [Int] dict per module, so - sharing is not complete.] -Sort out ambiguity at the end. +Can we ever be *certain* about ambiguity? Yes: if there's a constraint -Principal types -~~~~~~~~~~~~~~~ -class C a where - op :: a -> a + c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY -f x = let g y = op (y::Int) in True +then c is a "bubble"; there's no way it can ever improve, and it's +certainly ambiguous. UNLESS it is a constant (sigh). And what about +the nasty example? + + class K x + class H x y | x -> y + instance H x y => K (x,y) + +Is this type ambiguous? + forall a b. (K (a,b), Eq b) => a -> a + +Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after +is a "bubble" that's a set of constraints + + Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY + +Hence another idea. To decide Q start with fv(T) and grow it +by transitive closure in Cq (no functional dependencies involved). +Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok. +The definitely-ambiguous can then float out, and get smashed at top level +(which squashes out the constants, like Eq (T a) above) + + + -------------------------------------- + Notes on principal types + -------------------------------------- + + class C a where + op :: a -> a + + f x = let g y = op (y::Int) in True Here the principal type of f is (forall a. a->a) but we'll produce the non-principal type f :: forall a. C Int => a -> a -Ambiguity -~~~~~~~~~ + -------------------------------------- + The need for forall's in constraints + -------------------------------------- + +[Exchange on Haskell Cafe 5/6 Dec 2000] + + class C t where op :: t -> Bool + instance C [t] where op x = True + + p y = (let f :: c -> Bool; f x = op (y >> return x) in f, y ++ []) + q y = (y ++ [], let f :: c -> Bool; f x = op (y >> return x) in f) + +The definitions of p and q differ only in the order of the components in +the pair on their right-hand sides. And yet: + + ghc and "Typing Haskell in Haskell" reject p, but accept q; + Hugs rejects q, but accepts p; + hbc rejects both p and q; + nhc98 ... (Malcolm, can you fill in the blank for us!). + +The type signature for f forces context reduction to take place, and +the results of this depend on whether or not the type of y is known, +which in turn depends on which component of the pair the type checker +analyzes first. + +Solution: if y::m a, float out the constraints + Monad m, forall c. C (m c) +When m is later unified with [], we can solve both constraints. + + + -------------------------------------- + Notes on implicit parameters + -------------------------------------- + +Question 1: can we "inherit" implicit parameters +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this: - instance C (T a) Int where ... - instance C (T a) Bool where ... + f x = (x::Int) + ?y + +where f is *not* a top-level binding. +From the RHS of f we'll get the constraint (?y::Int). +There are two types we might infer for f: + + f :: Int -> Int + +(so we get ?y from the context of f's definition), or + + f :: (?y::Int) => Int -> Int + +At first you might think the first was better, becuase then +?y behaves like a free variable of the definition, rather than +having to be passed at each call site. But of course, the WHOLE +IDEA is that ?y should be passed at each call site (that's what +dynamic binding means) so we'd better infer the second. + +BOTTOM LINE: when *inferring types* you *must* quantify +over implicit parameters. See the predicate isFreeWhenInferring. + + +Question 2: type signatures +~~~~~~~~~~~~~~~~~~~~~~~~~~~ +BUT WATCH OUT: When you supply a type signature, we can't force you +to quantify over implicit parameters. For example: + + (?x + 1) :: Int + +This is perfectly reasonable. We do not want to insist on + + (?x + 1) :: (?x::Int => Int) + +That would be silly. Here, the definition site *is* the occurrence site, +so the above strictures don't apply. Hence the difference between +tcSimplifyCheck (which *does* allow implicit paramters to be inherited) +and tcSimplifyCheckBind (which does not). + +What about when you supply a type signature for a binding? +Is it legal to give the following explicit, user type +signature to f, thus: + + f :: Int -> Int + f x = (x::Int) + ?y + +At first sight this seems reasonable, but it has the nasty property +that adding a type signature changes the dynamic semantics. +Consider this: + + (let f x = (x::Int) + ?y + in (f 3, f 3 with ?y=5)) with ?y = 6 + + returns (3+6, 3+5) +vs + (let f :: Int -> Int + f x = x + ?y + in (f 3, f 3 with ?y=5)) with ?y = 6 + + returns (3+6, 3+6) + +Indeed, simply inlining f (at the Haskell source level) would change the +dynamic semantics. + +Nevertheless, as Launchbury says (email Oct 01) we can't really give the +semantics for a Haskell program without knowing its typing, so if you +change the typing you may change the semantics. + +To make things consistent in all cases where we are *checking* against +a supplied signature (as opposed to inferring a type), we adopt the +rule: -and suppose we infer a context + a signature does not need to quantify over implicit params. - C (T x) y +[This represents a (rather marginal) change of policy since GHC 5.02, +which *required* an explicit signature to quantify over all implicit +params for the reasons mentioned above.] -from some expression, where x and y are type varibles, -and x is ambiguous, and y is being quantified over. -Should we complain, or should we generate the type +But that raises a new question. Consider - forall x y. C (T x) y => + Given (signature) ?x::Int + Wanted (inferred) ?x::Int, ?y::Bool -The idea is that at the call of the function we might -know that y is Int (say), so the "x" isn't really ambiguous. -Notice that we have to add "x" to the type variables over -which we generalise. +Clearly we want to discharge the ?x and float the ?y out. But +what is the criterion that distinguishes them? Clearly it isn't +what free type variables they have. The Right Thing seems to be +to float a constraint that + neither mentions any of the quantified type variables + nor any of the quantified implicit parameters -Something similar can happen even if C constrains only ambiguous -variables. Suppose we infer the context +See the predicate isFreeWhenChecking. - C [x] -where x is ambiguous. Then we could infer the type +Question 3: monomorphism +~~~~~~~~~~~~~~~~~~~~~~~~ +There's a nasty corner case when the monomorphism restriction bites: - forall x. C [x] => + z = (x::Int) + ?y -in the hope that at the call site there was an instance -decl such as +The argument above suggests that we *must* generalise +over the ?y parameter, to get + z :: (?y::Int) => Int, +but the monomorphism restriction says that we *must not*, giving + z :: Int. +Why does the momomorphism restriction say this? Because if you have - instance Num a => C [a] where ... + let z = x + ?y in z+z -and hence the default mechanism would resolve the "a". +you might not expect the addition to be done twice --- but it will if +we follow the argument of Question 2 and generalise over ?y. + + +Question 4: top level +~~~~~~~~~~~~~~~~~~~~~ +At the top level, monomorhism makes no sense at all. + + module Main where + main = let ?x = 5 in print foo + + foo = woggle 3 + + woggle :: (?x :: Int) => Int -> Int + woggle y = ?x + y + +We definitely don't want (foo :: Int) with a top-level implicit parameter +(?x::Int) becuase there is no way to bind it. + + +Possible choices +~~~~~~~~~~~~~~~~ +(A) Always generalise over implicit parameters + Bindings that fall under the monomorphism restriction can't + be generalised + + Consequences: + * Inlining remains valid + * No unexpected loss of sharing + * But simple bindings like + z = ?y + 1 + will be rejected, unless you add an explicit type signature + (to avoid the monomorphism restriction) + z :: (?y::Int) => Int + z = ?y + 1 + This seems unacceptable + +(B) Monomorphism restriction "wins" + Bindings that fall under the monomorphism restriction can't + be generalised + Always generalise over implicit parameters *except* for bindings + that fall under the monomorphism restriction + + Consequences + * Inlining isn't valid in general + * No unexpected loss of sharing + * Simple bindings like + z = ?y + 1 + accepted (get value of ?y from binding site) + +(C) Always generalise over implicit parameters + Bindings that fall under the monomorphism restriction can't + be generalised, EXCEPT for implicit parameters + Consequences + * Inlining remains valid + * Unexpected loss of sharing (from the extra generalisation) + * Simple bindings like + z = ?y + 1 + accepted (get value of ?y from occurrence sites) + + +Discussion +~~~~~~~~~~ +None of these choices seems very satisfactory. But at least we should +decide which we want to do. + +It's really not clear what is the Right Thing To Do. If you see + + z = (x::Int) + ?y + +would you expect the value of ?y to be got from the *occurrence sites* +of 'z', or from the valuue of ?y at the *definition* of 'z'? In the +case of function definitions, the answer is clearly the former, but +less so in the case of non-fucntion definitions. On the other hand, +if we say that we get the value of ?y from the definition site of 'z', +then inlining 'z' might change the semantics of the program. + +Choice (C) really says "the monomorphism restriction doesn't apply +to implicit parameters". Which is fine, but remember that every +innocent binding 'x = ...' that mentions an implicit parameter in +the RHS becomes a *function* of that parameter, called at each +use of 'x'. Now, the chances are that there are no intervening 'with' +clauses that bind ?y, so a decent compiler should common up all +those function calls. So I think I strongly favour (C). Indeed, +one could make a similar argument for abolishing the monomorphism +restriction altogether. + +BOTTOM LINE: we choose (B) at present. See tcSimplifyRestricted + + + +%************************************************************************ +%* * +\subsection{tcSimplifyInfer} +%* * +%************************************************************************ + +tcSimplify is called when we *inferring* a type. Here's the overall game plan: + + 1. Compute Q = grow( fvs(T), C ) + + 2. Partition C based on Q into Ct and Cq. Notice that ambiguous + predicates will end up in Ct; we deal with them at the top level + + 3. Try improvement, using functional dependencies + + 4. If Step 3 did any unification, repeat from step 1 + (Unification can change the result of 'grow'.) + +Note: we don't reduce dictionaries in step 2. For example, if we have +Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different +after step 2. However note that we may therefore quantify over more +type variables than we absolutely have to. + +For the guts, we need a loop, that alternates context reduction and +improvement with unification. E.g. Suppose we have + + class C x y | x->y where ... + +and tcSimplify is called with: + (C Int a, C Int b) +Then improvement unifies a with b, giving + (C Int a, C Int a) + +If we need to unify anything, we rattle round the whole thing all over +again. \begin{code} -module TcSimplify ( - tcSimplify, tcSimplifyAndCheck, - tcSimplifyTop, tcSimplifyThetas, tcSimplifyCheckThetas, - bindInstsOfLocalFuns - ) where +tcSimplifyInfer + :: SDoc + -> TcTyVarSet -- fv(T); type vars + -> [Inst] -- Wanted + -> TcM ([TcTyVar], -- Tyvars to quantify (zonked) + TcDictBinds, -- Bindings + [TcId]) -- Dict Ids that must be bound here (zonked) + -- Any free (escaping) Insts are tossed into the environment +\end{code} -#include "HsVersions.h" -import CmdLineOpts ( opt_MaxContextReductionDepth, opt_GlasgowExts ) -import HsSyn ( MonoBinds(..), HsExpr(..), andMonoBinds, andMonoBindList ) -import TcHsSyn ( TcExpr, TcId, - TcMonoBinds, TcDictBinds - ) +\begin{code} +tcSimplifyInfer doc tau_tvs wanted_lie + = inferLoop doc (varSetElems tau_tvs) + wanted_lie `thenM` \ (qtvs, frees, binds, irreds) -> + + extendLIEs frees `thenM_` + returnM (qtvs, binds, map instToId irreds) + +inferLoop doc tau_tvs wanteds + = -- Step 1 + zonkTcTyVarsAndFV tau_tvs `thenM` \ tau_tvs' -> + mappM zonkInst wanteds `thenM` \ wanteds' -> + tcGetGlobalTyVars `thenM` \ gbl_tvs -> + let + preds = fdPredsOfInsts wanteds' + qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs -import TcMonad -import Inst ( lookupInst, lookupSimpleInst, LookupInstResult(..), - tyVarsOfInst, - isDict, isStdClassTyVarDict, isMethodFor, - instToId, instBindingRequired, instCanBeGeneralised, - newDictFromOld, - instLoc, getDictClassTys, - pprInst, zonkInst, tidyInst, tidyInsts, - Inst, LIE, pprInsts, pprInstsInFull, mkLIE, emptyLIE, - plusLIE, pprOrigin - ) -import TcEnv ( tcGetGlobalTyVars ) -import TcType ( TcType, TcTyVarSet, typeToTcType ) -import TcUnify ( unifyTauTy ) -import Id ( idType ) -import VarSet ( mkVarSet ) - -import Bag ( bagToList ) -import Class ( Class, ClassInstEnv, classBigSig, classInstEnv ) -import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass ) - -import Type ( Type, ThetaType, TauType, mkTyVarTy, getTyVar, - isTyVarTy, substTopTheta, splitSigmaTy, tyVarsOfTypes - ) -import PprType ( pprConstraint ) -import TysWiredIn ( unitTy ) -import VarSet -import VarEnv ( zipVarEnv ) -import FiniteMap -import BasicTypes ( TopLevelFlag(..) ) -import CmdLineOpts ( opt_GlasgowExts ) -import Outputable -import Util -import List ( partition ) + try_me inst + | isFreeWhenInferring qtvs inst = Free + | isClassDict inst = DontReduceUnlessConstant -- Dicts + | otherwise = ReduceMe NoSCs -- Lits and Methods + in + traceTc (text "infloop" <+> vcat [ppr tau_tvs', ppr wanteds', ppr preds, + ppr (grow preds tau_tvs'), ppr qtvs]) `thenM_` + -- Step 2 + reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) -> + + -- Step 3 + if no_improvement then + returnM (varSetElems qtvs, frees, binds, irreds) + else + -- If improvement did some unification, we go round again. There + -- are two subtleties: + -- a) We start again with irreds, not wanteds + -- Using an instance decl might have introduced a fresh type variable + -- which might have been unified, so we'd get an infinite loop + -- if we started again with wanteds! See example [LOOP] + -- + -- b) It's also essential to re-process frees, because unification + -- might mean that a type variable that looked free isn't now. + -- + -- Hence the (irreds ++ frees) + + -- However, NOTICE that when we are done, we might have some bindings, but + -- the final qtvs might be empty. See [NO TYVARS] below. + + inferLoop doc tau_tvs (irreds ++ frees) `thenM` \ (qtvs1, frees1, binds1, irreds1) -> + returnM (qtvs1, frees1, binds `unionBags` binds1, irreds1) +\end{code} + +Example [LOOP] + + class If b t e r | b t e -> r + instance If T t e t + instance If F t e e + class Lte a b c | a b -> c where lte :: a -> b -> c + instance Lte Z b T + instance (Lte a b l,If l b a c) => Max a b c + +Wanted: Max Z (S x) y + +Then we'll reduce using the Max instance to: + (Lte Z (S x) l, If l (S x) Z y) +and improve by binding l->T, after which we can do some reduction +on both the Lte and If constraints. What we *can't* do is start again +with (Max Z (S x) y)! + +[NO TYVARS] + + class Y a b | a -> b where + y :: a -> X b + + instance Y [[a]] a where + y ((x:_):_) = X x + + k :: X a -> X a -> X a + + g :: Num a => [X a] -> [X a] + g xs = h xs + where + h ys = ys ++ map (k (y [[0]])) xs + +The excitement comes when simplifying the bindings for h. Initially +try to simplify {y @ [[t1]] t2, 0 @ t1}, with initial qtvs = {t2}. +From this we get t1:=:t2, but also various bindings. We can't forget +the bindings (because of [LOOP]), but in fact t1 is what g is +polymorphic in. + +The net effect of [NO TYVARS] + +\begin{code} +isFreeWhenInferring :: TyVarSet -> Inst -> Bool +isFreeWhenInferring qtvs inst + = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars + && isInheritableInst inst -- And no implicit parameter involved + -- (see "Notes on implicit parameters") + +isFreeWhenChecking :: TyVarSet -- Quantified tyvars + -> NameSet -- Quantified implicit parameters + -> Inst -> Bool +isFreeWhenChecking qtvs ips inst + = isFreeWrtTyVars qtvs inst + && isFreeWrtIPs ips inst + +isFreeWrtTyVars qtvs inst = not (tyVarsOfInst inst `intersectsVarSet` qtvs) +isFreeWrtIPs ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst)) \end{code} %************************************************************************ %* * -\subsection[tcSimplify-main]{Main entry function} +\subsection{tcSimplifyCheck} %* * %************************************************************************ -The main wrapper is @tcSimplify@. It just calls @tcSimpl@, but with -the ``don't-squash-consts'' flag set depending on top-level ness. For -top level defns we *do* squash constants, so that they stay local to a -single defn. This makes things which are inlined more likely to be -exportable, because their constants are "inside". Later passes will -float them out if poss, after inlinings are sorted out. +@tcSimplifyCheck@ is used when we know exactly the set of variables +we are going to quantify over. For example, a class or instance declaration. \begin{code} -tcSimplify - :: SDoc - -> TopLevelFlag - -> TcTyVarSet -- ``Local'' type variables - -- ASSERT: this tyvar set is already zonked - -> LIE -- Wanted - -> TcM s (LIE, -- Free - TcDictBinds, -- Bindings - LIE) -- Remaining wanteds; no dups +tcSimplifyCheck + :: SDoc + -> [TcTyVar] -- Quantify over these + -> [Inst] -- Given + -> [Inst] -- Wanted + -> TcM TcDictBinds -- Bindings + +-- tcSimplifyCheck is used when checking expression type signatures, +-- class decls, instance decls etc. +-- +-- NB: tcSimplifyCheck does not consult the +-- global type variables in the environment; so you don't +-- need to worry about setting them before calling tcSimplifyCheck +tcSimplifyCheck doc qtvs givens wanted_lie + = ASSERT( all isSkolemTyVar qtvs ) + do { (qtvs', frees, binds) <- tcSimplCheck doc get_qtvs AddSCs givens wanted_lie + ; extendLIEs frees + ; return binds } + where +-- get_qtvs = zonkTcTyVarsAndFV qtvs + get_qtvs = return (mkVarSet qtvs) -- All skolems + + +-- tcSimplifyInferCheck is used when we know the constraints we are to simplify +-- against, but we don't know the type variables over which we are going to quantify. +-- This happens when we have a type signature for a mutually recursive group +tcSimplifyInferCheck + :: SDoc + -> TcTyVarSet -- fv(T) + -> [Inst] -- Given + -> [Inst] -- Wanted + -> TcM ([TcTyVar], -- Variables over which to quantify + TcDictBinds) -- Bindings + +tcSimplifyInferCheck doc tau_tvs givens wanted_lie + = do { (qtvs', frees, binds) <- tcSimplCheck doc get_qtvs AddSCs givens wanted_lie + ; extendLIEs frees + ; return (qtvs', binds) } + where + -- Figure out which type variables to quantify over + -- You might think it should just be the signature tyvars, + -- but in bizarre cases you can get extra ones + -- f :: forall a. Num a => a -> a + -- f x = fst (g (x, head [])) + 1 + -- g a b = (b,a) + -- Here we infer g :: forall a b. a -> b -> (b,a) + -- We don't want g to be monomorphic in b just because + -- f isn't quantified over b. + all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens) + + get_qtvs = zonkTcTyVarsAndFV all_tvs `thenM` \ all_tvs' -> + tcGetGlobalTyVars `thenM` \ gbl_tvs -> + let + qtvs = all_tvs' `minusVarSet` gbl_tvs + -- We could close gbl_tvs, but its not necessary for + -- soundness, and it'll only affect which tyvars, not which + -- dictionaries, we quantify over + in + returnM qtvs +\end{code} -tcSimplify str top_lvl local_tvs wanted_lie - | isEmptyVarSet local_tvs - = returnTc (wanted_lie, EmptyMonoBinds, emptyLIE) +Here is the workhorse function for all three wrappers. - | otherwise - = reduceContext str try_me [] wanteds `thenTc` \ (binds, frees, irreds) -> +\begin{code} +tcSimplCheck doc get_qtvs want_scs givens wanted_lie + = do { (qtvs, frees, binds, irreds) <- check_loop givens wanted_lie + + -- Complain about any irreducible ones + ; if not (null irreds) + then do { givens' <- mappM zonkInst given_dicts_and_ips + ; groupErrs (addNoInstanceErrs (Just doc) givens') irreds } + else return () + + ; returnM (qtvs, frees, binds) } + where + given_dicts_and_ips = filter (not . isMethod) givens + -- For error reporting, filter out methods, which are + -- only added to the given set as an optimisation + + ip_set = mkNameSet (ipNamesOfInsts givens) + + check_loop givens wanteds + = -- Step 1 + mappM zonkInst givens `thenM` \ givens' -> + mappM zonkInst wanteds `thenM` \ wanteds' -> + get_qtvs `thenM` \ qtvs' -> + + -- Step 2 + let + -- When checking against a given signature we always reduce + -- until we find a match against something given, or can't reduce + try_me inst | isFreeWhenChecking qtvs' ip_set inst = Free + | otherwise = ReduceMe want_scs + in + reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) -> + + -- Step 3 + if no_improvement then + returnM (varSetElems qtvs', frees, binds, irreds) + else + check_loop givens' (irreds ++ frees) `thenM` \ (qtvs', frees1, binds1, irreds1) -> + returnM (qtvs', frees1, binds `unionBags` binds1, irreds1) +\end{code} + + +%************************************************************************ +%* * + tcSimplifySuperClasses +%* * +%************************************************************************ + +Note [SUPERCLASS-LOOP 1] +~~~~~~~~~~~~~~~~~~~~~~~~ +We have to be very, very careful when generating superclasses, lest we +accidentally build a loop. Here's an example: + + class S a + + class S a => C a where { opc :: a -> a } + class S b => D b where { opd :: b -> b } + + instance C Int where + opc = opd + + instance D Int where + opd = opc + +From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int} +Simplifying, we may well get: + $dfCInt = :C ds1 (opd dd) + dd = $dfDInt + ds1 = $p1 dd +Notice that we spot that we can extract ds1 from dd. + +Alas! Alack! We can do the same for (instance D Int): + + $dfDInt = :D ds2 (opc dc) + dc = $dfCInt + ds2 = $p1 dc + +And now we've defined the superclass in terms of itself. + +Solution: never generate a superclass selectors at all when +satisfying the superclass context of an instance declaration. + +Two more nasty cases are in + tcrun021 + tcrun033 + +\begin{code} +tcSimplifySuperClasses qtvs givens sc_wanteds + = ASSERT( all isSkolemTyVar qtvs ) + do { (_, frees, binds1) <- tcSimplCheck doc get_qtvs NoSCs givens sc_wanteds + ; binds2 <- tc_simplify_top doc False NoSCs frees + ; return (binds1 `unionBags` binds2) } + where + get_qtvs = return (mkVarSet qtvs) + doc = ptext SLIT("instance declaration superclass context") +\end{code} + + +%************************************************************************ +%* * +\subsection{tcSimplifyRestricted} +%* * +%************************************************************************ + +tcSimplifyRestricted infers which type variables to quantify for a +group of restricted bindings. This isn't trivial. + +Eg1: id = \x -> x + We want to quantify over a to get id :: forall a. a->a + +Eg2: eq = (==) + We do not want to quantify over a, because there's an Eq a + constraint, so we get eq :: a->a->Bool (notice no forall) + +So, assume: + RHS has type 'tau', whose free tyvars are tau_tvs + RHS has constraints 'wanteds' + +Plan A (simple) + Quantify over (tau_tvs \ ftvs(wanteds)) + This is bad. The constraints may contain (Monad (ST s)) + where we have instance Monad (ST s) where... + so there's no need to be monomorphic in s! + + Also the constraint might be a method constraint, + whose type mentions a perfectly innocent tyvar: + op :: Num a => a -> b -> a + Here, b is unconstrained. A good example would be + foo = op (3::Int) + We want to infer the polymorphic type + foo :: forall b. b -> b + + +Plan B (cunning, used for a long time up to and including GHC 6.2) + Step 1: Simplify the constraints as much as possible (to deal + with Plan A's problem). Then set + qtvs = tau_tvs \ ftvs( simplify( wanteds ) ) + + Step 2: Now simplify again, treating the constraint as 'free' if + it does not mention qtvs, and trying to reduce it otherwise. + The reasons for this is to maximise sharing. + + This fails for a very subtle reason. Suppose that in the Step 2 + a constraint (Foo (Succ Zero) (Succ Zero) b) gets thrown upstairs as 'free'. + In the Step 1 this constraint might have been simplified, perhaps to + (Foo Zero Zero b), AND THEN THAT MIGHT BE IMPROVED, to bind 'b' to 'T'. + This won't happen in Step 2... but that in turn might prevent some other + constraint (Baz [a] b) being simplified (e.g. via instance Baz [a] T where {..}) + and that in turn breaks the invariant that no constraints are quantified over. + + Test typecheck/should_compile/tc177 (which failed in GHC 6.2) demonstrates + the problem. + + +Plan C (brutal) + Step 1: Simplify the constraints as much as possible (to deal + with Plan A's problem). Then set + qtvs = tau_tvs \ ftvs( simplify( wanteds ) ) + Return the bindings from Step 1. + + +A note about Plan C (arising from "bug" reported by George Russel March 2004) +Consider this: + + instance (HasBinary ty IO) => HasCodedValue ty + + foo :: HasCodedValue a => String -> IO a + + doDecodeIO :: HasCodedValue a => () -> () -> IO a + doDecodeIO codedValue view + = let { act = foo "foo" } in act + +You might think this should work becuase the call to foo gives rise to a constraint +(HasCodedValue t), which can be satisfied by the type sig for doDecodeIO. But the +restricted binding act = ... calls tcSimplifyRestricted, and PlanC simplifies the +constraint using the (rather bogus) instance declaration, and now we are stuffed. + +I claim this is not really a bug -- but it bit Sergey as well as George. So here's +plan D + + +Plan D (a variant of plan B) + Step 1: Simplify the constraints as much as possible (to deal + with Plan A's problem), BUT DO NO IMPROVEMENT. Then set + qtvs = tau_tvs \ ftvs( simplify( wanteds ) ) + + Step 2: Now simplify again, treating the constraint as 'free' if + it does not mention qtvs, and trying to reduce it otherwise. + + The point here is that it's generally OK to have too few qtvs; that is, + to make the thing more monomorphic than it could be. We don't want to + do that in the common cases, but in wierd cases it's ok: the programmer + can always add a signature. + + Too few qtvs => too many wanteds, which is what happens if you do less + improvement. + + +\begin{code} +tcSimplifyRestricted -- Used for restricted binding groups + -- i.e. ones subject to the monomorphism restriction + :: SDoc + -> TopLevelFlag + -> [Name] -- Things bound in this group + -> TcTyVarSet -- Free in the type of the RHSs + -> [Inst] -- Free in the RHSs + -> TcM ([TcTyVar], -- Tyvars to quantify (zonked) + TcDictBinds) -- Bindings + -- tcSimpifyRestricted returns no constraints to + -- quantify over; by definition there are none. + -- They are all thrown back in the LIE + +tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds + -- Zonk everything in sight + = mappM zonkInst wanteds `thenM` \ wanteds' -> + zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenM` \ tau_tvs' -> + tcGetGlobalTyVars `thenM` \ gbl_tvs' -> + + -- 'reduceMe': Reduce as far as we can. Don't stop at + -- dicts; the idea is to get rid of as many type + -- variables as possible, and we don't want to stop + -- at (say) Monad (ST s), because that reduces + -- immediately, with no constraint on s. + -- + -- BUT do no improvement! See Plan D above + reduceContextWithoutImprovement + doc reduceMe wanteds' `thenM` \ (_frees, _binds, constrained_dicts) -> - -- Check for non-generalisable insts + -- Next, figure out the tyvars we will quantify over let - cant_generalise = filter (not . instCanBeGeneralised) irreds + constrained_tvs = tyVarsOfInsts constrained_dicts + qtvs = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs') + `minusVarSet` constrained_tvs in - checkTc (null cant_generalise) - (genCantGenErr cant_generalise) `thenTc_` - - -- Check for ambiguous insts. - -- You might think these can't happen (I did) because an ambiguous - -- inst like (Eq a) will get tossed out with "frees", and eventually - -- dealt with by tcSimplifyTop. - -- But we can get stuck with - -- C a b - -- where "a" is one of the local_tvs, but "b" is unconstrained. - -- Then we must yell about the ambiguous b - -- But we must only do so if "b" really is unconstrained; so - -- we must grab the global tyvars to answer that question - tcGetGlobalTyVars `thenNF_Tc` \ global_tvs -> + traceTc (text "tcSimplifyRestricted" <+> vcat [ + pprInsts wanteds, pprInsts _frees, pprInsts constrained_dicts, + ppr _binds, + ppr constrained_tvs, ppr tau_tvs', ppr qtvs ]) `thenM_` + + -- The first step may have squashed more methods than + -- necessary, so try again, this time more gently, knowing the exact + -- set of type variables to quantify over. + -- + -- We quantify only over constraints that are captured by qtvs; + -- these will just be a subset of non-dicts. This in contrast + -- to normal inference (using isFreeWhenInferring) in which we quantify over + -- all *non-inheritable* constraints too. This implements choice + -- (B) under "implicit parameter and monomorphism" above. + -- + -- Remember that we may need to do *some* simplification, to + -- (for example) squash {Monad (ST s)} into {}. It's not enough + -- just to float all constraints + -- + -- At top level, we *do* squash methods becuase we want to + -- expose implicit parameters to the test that follows let - avail_tvs = local_tvs `unionVarSet` global_tvs - (irreds', bad_guys) = partition (isEmptyVarSet . ambig_tv_fn) irreds - ambig_tv_fn dict = tyVarsOfInst dict `minusVarSet` avail_tvs + is_nested_group = isNotTopLevel top_lvl + try_me inst | isFreeWrtTyVars qtvs inst, + (is_nested_group || isDict inst) = Free + | otherwise = ReduceMe AddSCs in - addAmbigErrs ambig_tv_fn bad_guys `thenNF_Tc_` + reduceContextWithoutImprovement + doc try_me wanteds' `thenM` \ (frees, binds, irreds) -> + ASSERT( null irreds ) + + -- See "Notes on implicit parameters, Question 4: top level" + if is_nested_group then + extendLIEs frees `thenM_` + returnM (varSetElems qtvs, binds) + else + let + (non_ips, bad_ips) = partition isClassDict frees + in + addTopIPErrs bndrs bad_ips `thenM_` + extendLIEs non_ips `thenM_` + returnM (varSetElems qtvs, binds) +\end{code} + + +%************************************************************************ +%* * +\subsection{tcSimplifyToDicts} +%* * +%************************************************************************ + +On the LHS of transformation rules we only simplify methods and constants, +getting dictionaries. We want to keep all of them unsimplified, to serve +as the available stuff for the RHS of the rule. + +The same thing is used for specialise pragmas. Consider + + f :: Num a => a -> a + {-# SPECIALISE f :: Int -> Int #-} + f = ... + +The type checker generates a binding like: + + f_spec = (f :: Int -> Int) + +and we want to end up with + + f_spec = _inline_me_ (f Int dNumInt) + +But that means that we must simplify the Method for f to (f Int dNumInt)! +So tcSimplifyToDicts squeezes out all Methods. +IMPORTANT NOTE: we *don't* want to do superclass commoning up. Consider - -- Finished - returnTc (mkLIE frees, binds, mkLIE irreds') + fromIntegral :: (Integral a, Num b) => a -> b + {-# RULES "foo" fromIntegral = id :: Int -> Int #-} + +Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont* +want to get + + forall dIntegralInt. + fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int + +because the scsel will mess up RULE matching. Instead we want + + forall dIntegralInt, dNumInt. + fromIntegral Int Int dIntegralInt dNumInt = id Int + +Hence "WithoutSCs" + +\begin{code} +tcSimplifyToDicts :: [Inst] -> TcM (TcDictBinds) +tcSimplifyToDicts wanteds + = simpleReduceLoop doc try_me wanteds `thenM` \ (frees, binds, irreds) -> + -- Since try_me doesn't look at types, we don't need to + -- do any zonking, so it's safe to call reduceContext directly + ASSERT( null frees ) + extendLIEs irreds `thenM_` + returnM binds + + where + doc = text "tcSimplifyToDicts" + + -- Reduce methods and lits only; stop as soon as we get a dictionary + try_me inst | isDict inst = KeepDictWithoutSCs -- See notes above re "WithoutSCs" + | otherwise = ReduceMe NoSCs +\end{code} + + + +tcSimplifyBracket is used when simplifying the constraints arising from +a Template Haskell bracket [| ... |]. We want to check that there aren't +any constraints that can't be satisfied (e.g. Show Foo, where Foo has no +Show instance), but we aren't otherwise interested in the results. +Nor do we care about ambiguous dictionaries etc. We will type check +this bracket again at its usage site. + +\begin{code} +tcSimplifyBracket :: [Inst] -> TcM () +tcSimplifyBracket wanteds + = simpleReduceLoop doc reduceMe wanteds `thenM_` + returnM () where - wanteds = bagToList wanted_lie - - try_me inst - -- Does not constrain a local tyvar - | isEmptyVarSet (tyVarsOfInst inst `intersectVarSet` local_tvs) - = -- if is_top_level then - -- FreeIfTautological -- Special case for inference on - -- -- top-level defns - -- else - Free - - -- We're infering (not checking) the type, and - -- the inst constrains a local type variable - | isDict inst = DontReduce -- Dicts - | otherwise = ReduceMe AddToIrreds -- Lits and Methods + doc = text "tcSimplifyBracket" \end{code} -@tcSimplifyAndCheck@ is similar to the above, except that it checks -that there is an empty wanted-set at the end. It may still return -some of constant insts, which have to be resolved finally at the end. + +%************************************************************************ +%* * +\subsection{Filtering at a dynamic binding} +%* * +%************************************************************************ + +When we have + let ?x = R in B + +we must discharge all the ?x constraints from B. We also do an improvement +step; if we have ?x::t1 and ?x::t2 we must unify t1, t2. + +Actually, the constraints from B might improve the types in ?x. For example + + f :: (?x::Int) => Char -> Char + let ?x = 3 in f 'c' + +then the constraint (?x::Int) arising from the call to f will +force the binding for ?x to be of type Int. + +\begin{code} +tcSimplifyIPs :: [Inst] -- The implicit parameters bound here + -> [Inst] -- Wanted + -> TcM TcDictBinds +tcSimplifyIPs given_ips wanteds + = simpl_loop given_ips wanteds `thenM` \ (frees, binds) -> + extendLIEs frees `thenM_` + returnM binds + where + doc = text "tcSimplifyIPs" <+> ppr given_ips + ip_set = mkNameSet (ipNamesOfInsts given_ips) + + -- Simplify any methods that mention the implicit parameter + try_me inst | isFreeWrtIPs ip_set inst = Free + | otherwise = ReduceMe NoSCs + + simpl_loop givens wanteds + = mappM zonkInst givens `thenM` \ givens' -> + mappM zonkInst wanteds `thenM` \ wanteds' -> + + reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) -> + + if no_improvement then + ASSERT( null irreds ) + returnM (frees, binds) + else + simpl_loop givens' (irreds ++ frees) `thenM` \ (frees1, binds1) -> + returnM (frees1, binds `unionBags` binds1) +\end{code} + + +%************************************************************************ +%* * +\subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@} +%* * +%************************************************************************ + +When doing a binding group, we may have @Insts@ of local functions. +For example, we might have... +\begin{verbatim} +let f x = x + 1 -- orig local function (overloaded) + f.1 = f Int -- two instances of f + f.2 = f Float + in + (f.1 5, f.2 6.7) +\end{verbatim} +The point is: we must drop the bindings for @f.1@ and @f.2@ here, +where @f@ is in scope; those @Insts@ must certainly not be passed +upwards towards the top-level. If the @Insts@ were binding-ified up +there, they would have unresolvable references to @f@. + +We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@. +For each method @Inst@ in the @init_lie@ that mentions one of the +@Ids@, we create a binding. We return the remaining @Insts@ (in an +@LIE@), as well as the @HsBinds@ generated. \begin{code} -tcSimplifyAndCheck - :: SDoc - -> TcTyVarSet -- ``Local'' type variables - -- ASSERT: this tyvar set is already zonked - -> LIE -- Given; constrain only local tyvars - -> LIE -- Wanted - -> TcM s (LIE, -- Free - TcDictBinds) -- Bindings - -tcSimplifyAndCheck str local_tvs given_lie wanted_lie - | isEmptyVarSet local_tvs - -- This can happen quite legitimately; for example in - -- instance Num Int where ... - = returnTc (wanted_lie, EmptyMonoBinds) +bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcDictBinds +-- Simlifies only MethodInsts, and generate only bindings of form +-- fm = f tys dicts +-- We're careful not to even generate bindings of the form +-- d1 = d2 +-- You'd think that'd be fine, but it interacts with what is +-- arguably a bug in Match.tidyEqnInfo (see notes there) + +bindInstsOfLocalFuns wanteds local_ids + | null overloaded_ids + -- Common case + = extendLIEs wanteds `thenM_` + returnM emptyLHsBinds | otherwise - = reduceContext str try_me givens wanteds `thenTc` \ (binds, frees, irreds) -> - - -- Complain about any irreducible ones - mapNF_Tc complain irreds `thenNF_Tc_` - - -- Done - returnTc (mkLIE frees, binds) + = simpleReduceLoop doc try_me for_me `thenM` \ (frees, binds, irreds) -> + ASSERT( null irreds ) + extendLIEs not_for_me `thenM_` + extendLIEs frees `thenM_` + returnM binds where - givens = bagToList given_lie - wanteds = bagToList wanted_lie - given_dicts = filter isDict givens - - try_me inst - -- Does not constrain a local tyvar - | isEmptyVarSet (tyVarsOfInst inst `intersectVarSet` local_tvs) - = Free - - -- When checking against a given signature we always reduce - -- until we find a match against something given, or can't reduce - | otherwise - = ReduceMe AddToIrreds - - complain dict = mapNF_Tc zonkInst givens `thenNF_Tc` \ givens -> - addNoInstanceErr str given_dicts dict + doc = text "bindInsts" <+> ppr local_ids + overloaded_ids = filter is_overloaded local_ids + is_overloaded id = isOverloadedTy (idType id) + (for_me, not_for_me) = partition (isMethodFor overloaded_set) wanteds + + overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them + -- so it's worth building a set, so that + -- lookup (in isMethodFor) is faster + try_me inst | isMethod inst = ReduceMe NoSCs + | otherwise = Free \end{code} @@ -300,95 +1311,209 @@ tcSimplifyAndCheck str local_tvs given_lie wanted_lie The main control over context reduction is here \begin{code} -data WhatToDo - = ReduceMe -- Try to reduce this - NoInstanceAction -- What to do if there's no such instance +data WhatToDo + = ReduceMe WantSCs -- Try to reduce this + -- If there's no instance, behave exactly like + -- DontReduce: add the inst to the irreductible ones, + -- but don't produce an error message of any kind. + -- It might be quite legitimate such as (Eq a)! + + | KeepDictWithoutSCs -- Return as irreducible; don't add its superclasses + -- Rather specialised: see notes with tcSimplifyToDicts - | DontReduce -- Return as irreducible + | DontReduceUnlessConstant -- Return as irreducible unless it can + -- be reduced to a constant in one step | Free -- Return as free - | FreeIfTautological -- Return as free iff it's tautological; - -- if not, return as irreducible - -- The FreeIfTautological case is to allow the possibility - -- of generating functions with types like - -- f :: C Int => Int -> Int - -- Here, the C Int isn't a tautology presumably because Int - -- isn't an instance of C in this module; but perhaps it will - -- be at f's call site(s). Haskell doesn't allow this at - -- present. - -data NoInstanceAction - = Stop -- Fail; no error message - -- (Only used when tautology checking.) - - | AddToIrreds -- Just add the inst to the irreductible ones; don't - -- produce an error message of any kind. - -- It might be quite legitimate such as (Eq a)! +reduceMe :: Inst -> WhatToDo +reduceMe inst = ReduceMe AddSCs + +data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses + -- of a predicate when adding it to the avails + -- The reason for this flag is entirely the super-class loop problem + -- Note [SUPER-CLASS LOOP 1] \end{code} \begin{code} -type RedState s - = (Avails s, -- What's available - [Inst], -- Insts for which try_me returned Free - [Inst] -- Insts for which try_me returned DontReduce - ) - -type Avails s = FiniteMap Inst Avail +type Avails = FiniteMap Inst Avail +emptyAvails = emptyFM data Avail - = Avail - TcId -- The "main Id"; that is, the Id for the Inst that - -- caused this avail to be put into the finite map in the first place - -- It is this Id that is bound to the RHS. - - RHS -- The RHS: an expression whose value is that Inst. - -- The main Id should be bound to this RHS - - [TcId] -- Extra Ids that must all be bound to the main Id. - -- At the end we generate a list of bindings - -- { i1 = main_id; i2 = main_id; i3 = main_id; ... } - -data RHS - = NoRhs -- Used for irreducible dictionaries, - -- which are going to be lambda bound, or for those that are - -- suppplied as "given" when checking againgst a signature. - -- - -- NoRhs is also used for Insts like (CCallable f) - -- where no witness is required. - - | Rhs -- Used when there is a RHS - TcExpr - Bool -- True => the RHS simply selects a superclass dictionary - -- from a subclass dictionary. - -- False => not so. - -- This is useful info, because superclass selection - -- is cheaper than building the dictionary using its dfun, - -- and we can sometimes replace the latter with the former - - | PassiveScSel -- Used for as-yet-unactivated RHSs. For example suppose we have - -- an (Ord t) dictionary; then we put an (Eq t) entry in - -- the finite map, with an PassiveScSel. Then if the - -- the (Eq t) binding is ever *needed* we make it an Rhs - TcExpr - [Inst] -- List of Insts that are free in the RHS. - -- If the main Id is subsequently needed, we toss this list into - -- the needed-inst pool so that we make sure their bindings - -- will actually be produced. - -- - -- Invariant: these Insts are already in the finite mapping - - -pprAvails avails = vcat (map pp (eltsFM avails)) - where - pp (Avail main_id rhs ids) - = ppr main_id <> colon <+> brackets (ppr ids) <+> pprRhs rhs + = IsFree -- Used for free Insts + | Irred -- Used for irreducible dictionaries, + -- which are going to be lambda bound + + | Given TcId -- Used for dictionaries for which we have a binding + -- e.g. those "given" in a signature + Bool -- True <=> actually consumed (splittable IPs only) + + | Rhs -- Used when there is a RHS + (LHsExpr TcId) -- The RHS + [Inst] -- Insts free in the RHS; we need these too + + | Linear -- Splittable Insts only. + Int -- The Int is always 2 or more; indicates how + -- many copies are required + Inst -- The splitter + Avail -- Where the "master copy" is + + | LinRhss -- Splittable Insts only; this is used only internally + -- by extractResults, where a Linear + -- is turned into an LinRhss + [LHsExpr TcId] -- A supply of suitable RHSs + +pprAvails avails = vcat [sep [ppr inst, nest 2 (equals <+> pprAvail avail)] + | (inst,avail) <- fmToList avails ] + +instance Outputable Avail where + ppr = pprAvail + +pprAvail IsFree = text "Free" +pprAvail Irred = text "Irred" +pprAvail (Given x b) = text "Given" <+> ppr x <+> + if b then text "(used)" else empty +pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs) +pprAvail (Linear n i a) = text "Linear" <+> ppr n <+> braces (ppr i) <+> ppr a +pprAvail (LinRhss rhss) = text "LinRhss" <+> ppr rhss +\end{code} + +Extracting the bindings from a bunch of Avails. +The bindings do *not* come back sorted in dependency order. +We assume that they'll be wrapped in a big Rec, so that the +dependency analyser can sort them out later -pprRhs NoRhs = text "" -pprRhs (Rhs rhs b) = ppr rhs -pprRhs (PassiveScSel rhs is) = text "passive" <+> ppr rhs +The loop startes +\begin{code} +extractResults :: Avails + -> [Inst] -- Wanted + -> TcM (TcDictBinds, -- Bindings + [Inst], -- Irreducible ones + [Inst]) -- Free ones + +extractResults avails wanteds + = go avails emptyBag [] [] wanteds + where + go avails binds irreds frees [] + = returnM (binds, irreds, frees) + + go avails binds irreds frees (w:ws) + = case lookupFM avails w of + Nothing -> pprTrace "Urk: extractResults" (ppr w) $ + go avails binds irreds frees ws + + Just IsFree -> go (add_free avails w) binds irreds (w:frees) ws + Just Irred -> go (add_given avails w) binds (w:irreds) frees ws + + Just (Given id _) -> go avails new_binds irreds frees ws + where + new_binds | id == instToId w = binds + | otherwise = addBind binds w (L (instSpan w) (HsVar id)) + -- The sought Id can be one of the givens, via a superclass chain + -- and then we definitely don't want to generate an x=x binding! + + Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds frees (ws' ++ ws) + where + new_binds = addBind binds w rhs + + Just (Linear n split_inst avail) -- Transform Linear --> LinRhss + -> get_root irreds frees avail w `thenM` \ (irreds', frees', root_id) -> + split n (instToId split_inst) root_id w `thenM` \ (binds', rhss) -> + go (addToFM avails w (LinRhss rhss)) + (binds `unionBags` binds') + irreds' frees' (split_inst : w : ws) + + Just (LinRhss (rhs:rhss)) -- Consume one of the Rhss + -> go new_avails new_binds irreds frees ws + where + new_binds = addBind binds w rhs + new_avails = addToFM avails w (LinRhss rhss) + + get_root irreds frees (Given id _) w = returnM (irreds, frees, id) + get_root irreds frees Irred w = cloneDict w `thenM` \ w' -> + returnM (w':irreds, frees, instToId w') + get_root irreds frees IsFree w = cloneDict w `thenM` \ w' -> + returnM (irreds, w':frees, instToId w') + + add_given avails w = addToFM avails w (Given (instToId w) True) + + add_free avails w | isMethod w = avails + | otherwise = add_given avails w + -- NB: Hack alert! + -- Do *not* replace Free by Given if it's a method. + -- The following situation shows why this is bad: + -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b + -- From an application (truncate f i) we get + -- t1 = truncate at f + -- t2 = t1 at i + -- If we have also have a second occurrence of truncate, we get + -- t3 = truncate at f + -- t4 = t3 at i + -- When simplifying with i,f free, we might still notice that + -- t1=t3; but alas, the binding for t2 (which mentions t1) + -- will continue to float out! + -- (split n i a) returns: n rhss + -- auxiliary bindings + -- 1 or 0 insts to add to irreds + + +split :: Int -> TcId -> TcId -> Inst + -> TcM (TcDictBinds, [LHsExpr TcId]) +-- (split n split_id root_id wanted) returns +-- * a list of 'n' expressions, all of which witness 'avail' +-- * a bunch of auxiliary bindings to support these expressions +-- * one or zero insts needed to witness the whole lot +-- (maybe be zero if the initial Inst is a Given) +-- +-- NB: 'wanted' is just a template + +split n split_id root_id wanted + = go n + where + ty = linearInstType wanted + pair_ty = mkTyConApp pairTyCon [ty,ty] + id = instToId wanted + occ = getOccName id + loc = getSrcLoc id + span = instSpan wanted + + go 1 = returnM (emptyBag, [L span $ HsVar root_id]) + + go n = go ((n+1) `div` 2) `thenM` \ (binds1, rhss) -> + expand n rhss `thenM` \ (binds2, rhss') -> + returnM (binds1 `unionBags` binds2, rhss') + + -- (expand n rhss) + -- Given ((n+1)/2) rhss, make n rhss, using auxiliary bindings + -- e.g. expand 3 [rhs1, rhs2] + -- = ( { x = split rhs1 }, + -- [fst x, snd x, rhs2] ) + expand n rhss + | n `rem` 2 == 0 = go rhss -- n is even + | otherwise = go (tail rhss) `thenM` \ (binds', rhss') -> + returnM (binds', head rhss : rhss') + where + go rhss = mapAndUnzipM do_one rhss `thenM` \ (binds', rhss') -> + returnM (listToBag binds', concat rhss') + + do_one rhs = newUnique `thenM` \ uniq -> + tcLookupId fstName `thenM` \ fst_id -> + tcLookupId sndName `thenM` \ snd_id -> + let + x = mkUserLocal occ uniq pair_ty loc + in + returnM (L span (VarBind x (mk_app span split_id rhs)), + [mk_fs_app span fst_id ty x, mk_fs_app span snd_id ty x]) + +mk_fs_app span id ty var = L span (HsVar id) `mkHsTyApp` [ty,ty] `mkHsApp` (L span (HsVar var)) + +mk_app span id rhs = L span (HsApp (L span (HsVar id)) rhs) + +addBind binds inst rhs = binds `unionBags` unitBag (L (instLocSrcSpan (instLoc inst)) + (VarBind (instToId inst) rhs)) +instSpan wanted = instLocSrcSpan (instLoc wanted) \end{code} @@ -398,66 +1523,145 @@ pprRhs (PassiveScSel rhs is) = text "passive" <+> ppr rhs %* * %************************************************************************ -The main entry point for context reduction is @reduceContext@: +When the "what to do" predicate doesn't depend on the quantified type variables, +matters are easier. We don't need to do any zonking, unless the improvement step +does something, in which case we zonk before iterating. + +The "given" set is always empty. + +\begin{code} +simpleReduceLoop :: SDoc + -> (Inst -> WhatToDo) -- What to do, *not* based on the quantified type variables + -> [Inst] -- Wanted + -> TcM ([Inst], -- Free + TcDictBinds, + [Inst]) -- Irreducible + +simpleReduceLoop doc try_me wanteds + = mappM zonkInst wanteds `thenM` \ wanteds' -> + reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) -> + if no_improvement then + returnM (frees, binds, irreds) + else + simpleReduceLoop doc try_me (irreds ++ frees) `thenM` \ (frees1, binds1, irreds1) -> + returnM (frees1, binds `unionBags` binds1, irreds1) +\end{code} + + \begin{code} -reduceContext :: SDoc -> (Inst -> WhatToDo) - -> [Inst] -- Given - -> [Inst] -- Wanted - -> TcM s (TcDictBinds, - [Inst], -- Free - [Inst]) -- Irreducible - -reduceContext str try_me givens wanteds - = -- Zonking first - mapNF_Tc zonkInst givens `thenNF_Tc` \ givens -> - mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds -> - -{- - pprTrace "reduceContext" (vcat [ +reduceContext :: SDoc + -> (Inst -> WhatToDo) + -> [Inst] -- Given + -> [Inst] -- Wanted + -> TcM (Bool, -- True <=> improve step did no unification + [Inst], -- Free + TcDictBinds, -- Dictionary bindings + [Inst]) -- Irreducible + +reduceContext doc try_me givens wanteds + = + traceTc (text "reduceContext" <+> (vcat [ text "----------------------", - str, + doc, text "given" <+> ppr givens, text "wanted" <+> ppr wanteds, text "----------------------" - ]) $ --} + ])) `thenM_` + -- Build the Avail mapping from "givens" - foldlNF_Tc addGiven emptyFM givens `thenNF_Tc` \ avails -> + foldlM addGiven emptyAvails givens `thenM` \ init_state -> -- Do the real work - reduceList (0,[]) try_me wanteds (avails, [], []) `thenTc` \ (avails, frees, irreds) -> + reduceList (0,[]) try_me wanteds init_state `thenM` \ avails -> - -- Extract the bindings from avails - let - binds = foldFM add_bind EmptyMonoBinds avails - - add_bind _ (Avail main_id rhs ids) binds - = foldr add_synonym (add_rhs_bind rhs binds) ids - where - add_rhs_bind (Rhs rhs _) binds = binds `AndMonoBinds` VarMonoBind main_id rhs - add_rhs_bind other binds = binds - - -- Add the trivial {x = y} bindings - -- The main Id can end up in the list when it's first added passively - -- and then activated, so we have to filter it out. A bit of a hack. - add_synonym id binds - | id /= main_id = binds `AndMonoBinds` VarMonoBind id (HsVar main_id) - | otherwise = binds - in -{- - pprTrace ("reduceContext end") (vcat [ + -- Do improvement, using everything in avails + -- In particular, avails includes all superclasses of everything + tcImprove avails `thenM` \ no_improvement -> + + extractResults avails wanteds `thenM` \ (binds, irreds, frees) -> + + traceTc (text "reduceContext end" <+> (vcat [ text "----------------------", - str, + doc, text "given" <+> ppr givens, text "wanted" <+> ppr wanteds, - text "----", + text "----", + text "avails" <+> pprAvails avails, + text "frees" <+> ppr frees, + text "no_improvement =" <+> ppr no_improvement, + text "----------------------" + ])) `thenM_` + + returnM (no_improvement, frees, binds, irreds) + +-- reduceContextWithoutImprovement differs from reduceContext +-- (a) no improvement +-- (b) 'givens' is assumed empty +reduceContextWithoutImprovement doc try_me wanteds + = + traceTc (text "reduceContextWithoutImprovement" <+> (vcat [ + text "----------------------", + doc, + text "wanted" <+> ppr wanteds, + text "----------------------" + ])) `thenM_` + + -- Do the real work + reduceList (0,[]) try_me wanteds emptyAvails `thenM` \ avails -> + extractResults avails wanteds `thenM` \ (binds, irreds, frees) -> + + traceTc (text "reduceContextWithoutImprovement end" <+> (vcat [ + text "----------------------", + doc, + text "wanted" <+> ppr wanteds, + text "----", text "avails" <+> pprAvails avails, - text "irreds" <+> ppr irreds, + text "frees" <+> ppr frees, text "----------------------" - ]) $ --} - returnTc (binds, frees, irreds) + ])) `thenM_` + + returnM (frees, binds, irreds) + +tcImprove :: Avails -> TcM Bool -- False <=> no change +-- Perform improvement using all the predicates in Avails +tcImprove avails + = tcGetInstEnvs `thenM` \ inst_envs -> + let + preds = [ (pred, pp_loc) + | (inst, avail) <- fmToList avails, + pred <- get_preds inst avail, + let pp_loc = pprInstLoc (instLoc inst) + ] + -- Avails has all the superclasses etc (good) + -- It also has all the intermediates of the deduction (good) + -- It does not have duplicates (good) + -- NB that (?x::t1) and (?x::t2) will be held separately in avails + -- so that improve will see them separate + + -- For free Methods, we want to take predicates from their context, + -- but for Methods that have been squished their context will already + -- be in Avails, and we don't want duplicates. Hence this rather + -- horrid get_preds function + get_preds inst IsFree = fdPredsOfInst inst + get_preds inst other | isDict inst = [dictPred inst] + | otherwise = [] + + eqns = improve get_insts preds + get_insts clas = classInstances inst_envs clas + in + if null eqns then + returnM True + else + traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns)) `thenM_` + mappM_ unify eqns `thenM_` + returnM False + where + unify ((qtvs, pairs), doc) + = addErrCtxt doc $ + tcInstTyVars (varSetElems qtvs) `thenM` \ (_, _, tenv) -> + mapM_ (unif_pr tenv) pairs + unif_pr tenv (ty1,ty2) = unifyType (substTy tenv ty1) (substTy tenv ty2) \end{code} The main context-reduction function is @reduce@. Here's its game plan. @@ -467,8 +1671,8 @@ reduceList :: (Int,[Inst]) -- Stack (for err msgs) -- along with its depth -> (Inst -> WhatToDo) -> [Inst] - -> RedState s - -> TcM s (RedState s) + -> Avails + -> TcM Avails \end{code} @reduce@ is passed @@ -478,12 +1682,12 @@ reduceList :: (Int,[Inst]) -- Stack (for err msgs) Free return this in "frees" wanteds: The list of insts to reduce - state: An accumulating parameter of type RedState + state: An accumulating parameter of type Avails that contains the state of the algorithm - - It returns a RedState. -The (n,stack) pair is just used for error reporting. + It returns a Avails. + +The (n,stack) pair is just used for error reporting. n is always the depth of the stack. The stack is the stack of Insts being reduced: to produce X I had to produce Y, to produce Y I had to produce Z, and so on. @@ -497,429 +1701,408 @@ reduceList (n,stack) try_me wanteds state = #ifdef DEBUG (if n > 8 then - pprTrace "Jeepers! ReduceContext:" (reduceDepthMsg n stack) + pprTrace "Interesting! Context reduction stack deeper than 8:" + (int n $$ ifPprDebug (nest 2 (pprStack stack))) else (\x->x)) #endif go wanteds state where - go [] state = returnTc state - go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenTc` \ state' -> + go [] state = returnM state + go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenM` \ state' -> go ws state' -- Base case: we're done! -reduce stack try_me wanted state@(avails, frees, irreds) +reduce stack try_me wanted avails -- It's the same as an existing inst, or a superclass thereof - | wanted `elemFM` avails - = returnTc (activate avails wanted, frees, irreds) + | Just avail <- isAvailable avails wanted + = if isLinearInst wanted then + addLinearAvailable avails avail wanted `thenM` \ (avails', wanteds') -> + reduceList stack try_me wanteds' avails' + else + returnM avails -- No op for non-linear things | otherwise = case try_me wanted of { - ReduceMe no_instance_action -> -- It should be reduced - lookupInst wanted `thenNF_Tc` \ lookup_result -> - case lookup_result of - GenInst wanteds' rhs -> use_instance wanteds' rhs - SimpleInst rhs -> use_instance [] rhs - - NoInstance -> -- No such instance! - case no_instance_action of - Stop -> failTc - AddToIrreds -> add_to_irreds - ; - Free -> -- It's free and this isn't a top-level binding, so just chuck it upstairs - -- First, see if the inst can be reduced to a constant in one step - lookupInst wanted `thenNF_Tc` \ lookup_result -> - case lookup_result of - SimpleInst rhs -> use_instance [] rhs - other -> add_to_frees - - - - ; - FreeIfTautological -> -- It's free and this is a top level binding, so - -- check whether it's a tautology or not - tryTc_ - add_to_irreds -- If tautology trial fails, add to irreds + KeepDictWithoutSCs -> addIrred NoSCs avails wanted - -- If tautology succeeds, just add to frees - (reduce stack try_me_taut wanted (avails, [], []) `thenTc_` - returnTc (avails, wanted:frees, irreds)) + ; DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced) + -- First, see if the inst can be reduced to a constant in one step + try_simple (addIrred AddSCs) -- Assume want superclasses + ; Free -> -- It's free so just chuck it upstairs + -- First, see if the inst can be reduced to a constant in one step + try_simple addFree - ; - DontReduce -> -- It's irreducible (or at least should not be reduced) - -- See if the inst can be reduced to a constant in one step - lookupInst wanted `thenNF_Tc` \ lookup_result -> + ; ReduceMe want_scs -> -- It should be reduced + lookupInst wanted `thenM` \ lookup_result -> case lookup_result of - SimpleInst rhs -> use_instance [] rhs - other -> add_to_irreds + GenInst wanteds' rhs -> addIrred NoSCs avails wanted `thenM` \ avails1 -> + reduceList stack try_me wanteds' avails1 `thenM` \ avails2 -> + addWanted want_scs avails2 wanted rhs wanteds' + -- Experiment with temporarily doing addIrred *before* the reduceList, + -- which has the effect of adding the thing we are trying + -- to prove to the database before trying to prove the things it + -- needs. See note [RECURSIVE DICTIONARIES] + -- NB: we must not do an addWanted before, because that adds the + -- superclasses too, and thaat can lead to a spurious loop; see + -- the examples in [SUPERCLASS-LOOP] + -- So we do an addIrred before, and then overwrite it afterwards with addWanted + + SimpleInst rhs -> addWanted want_scs avails wanted rhs [] + + NoInstance -> -- No such instance! + -- Add it and its superclasses + addIrred want_scs avails wanted } where - -- The three main actions - add_to_frees = let - avails' = addFree avails wanted - -- Add the thing to the avails set so any identical Insts - -- will be commoned up with it right here - in - returnTc (avails', wanted:frees, irreds) - - add_to_irreds = addGiven avails wanted `thenNF_Tc` \ avails' -> - returnTc (avails', frees, wanted:irreds) - - use_instance wanteds' rhs = addWanted avails wanted rhs `thenNF_Tc` \ avails' -> - reduceList stack try_me wanteds' (avails', frees, irreds) - - - -- The try-me to use when trying to identify tautologies - -- It blunders on reducing as much as possible - try_me_taut inst = ReduceMe Stop -- No error recovery + try_simple do_this_otherwise + = lookupInst wanted `thenM` \ lookup_result -> + case lookup_result of + SimpleInst rhs -> addWanted AddSCs avails wanted rhs [] + other -> do_this_otherwise avails wanted \end{code} \begin{code} -activate :: Avails s -> Inst -> Avails s - -- Activate the binding for Inst, ensuring that a binding for the - -- wanted Inst will be generated. - -- (Activate its parent if necessary, recursively). - -- Precondition: the Inst is in Avails already - -activate avails wanted - | not (instBindingRequired wanted) - = avails +------------------------- +isAvailable :: Avails -> Inst -> Maybe Avail +isAvailable avails wanted = lookupFM avails wanted + -- NB 1: the Ord instance of Inst compares by the class/type info + -- *not* by unique. So + -- d1::C Int == d2::C Int - | otherwise - = case lookupFM avails wanted of +addLinearAvailable :: Avails -> Avail -> Inst -> TcM (Avails, [Inst]) +addLinearAvailable avails avail wanted + -- avails currently maps [wanted -> avail] + -- Extend avails to reflect a neeed for an extra copy of avail - Just (Avail main_id (PassiveScSel rhs insts) ids) -> - foldl activate avails' insts -- Activate anything it needs - where - avails' = addToFM avails wanted avail' - avail' = Avail main_id (Rhs rhs True) (wanted_id : ids) -- Activate it + | Just avail' <- split_avail avail + = returnM (addToFM avails wanted avail', []) - Just (Avail main_id other_rhs ids) -> -- Just add to the synonyms list - addToFM avails wanted (Avail main_id other_rhs (wanted_id : ids)) + | otherwise + = tcLookupId splitName `thenM` \ split_id -> + tcInstClassOp (instLoc wanted) split_id + [linearInstType wanted] `thenM` \ split_inst -> + returnM (addToFM avails wanted (Linear 2 split_inst avail), [split_inst]) - Nothing -> panic "activate" where - wanted_id = instToId wanted - -addWanted avails wanted rhs_expr - = ASSERT( not (wanted `elemFM` avails) ) - returnNF_Tc (addToFM avails wanted avail) - -- NB: we don't add the thing's superclasses too! - -- Why not? Because addWanted is used when we've successfully used an - -- instance decl to reduce something; e.g. - -- d:Ord [a] = dfunOrd (d1:Eq [a]) (d2:Ord a) - -- Note that we pass the superclasses to the dfun, so they will be "wanted". - -- If we put the superclasses of "d" in avails, then we might end up - -- expressing "d1" in terms of "d", which would be a disaster. - where - avail = Avail (instToId wanted) rhs [] - - rhs | instBindingRequired wanted = Rhs rhs_expr False -- Not superclass selection - | otherwise = NoRhs - -addFree :: Avails s -> Inst -> (Avails s) + split_avail :: Avail -> Maybe Avail + -- (Just av) if there's a modified version of avail that + -- we can use to replace avail in avails + -- Nothing if there isn't, so we need to create a Linear + split_avail (Linear n i a) = Just (Linear (n+1) i a) + split_avail (Given id used) | not used = Just (Given id True) + | otherwise = Nothing + split_avail Irred = Nothing + split_avail IsFree = Nothing + split_avail other = pprPanic "addLinearAvailable" (ppr avail $$ ppr wanted $$ ppr avails) + +------------------------- +addFree :: Avails -> Inst -> TcM Avails -- When an Inst is tossed upstairs as 'free' we nevertheless add it -- to avails, so that any other equal Insts will be commoned up right -- here rather than also being tossed upstairs. This is really just -- an optimisation, and perhaps it is more trouble that it is worth, -- as the following comments show! -- - -- NB1: do *not* add superclasses. If we have + -- NB: do *not* add superclasses. If we have -- df::Floating a -- dn::Num a - -- but a is not bound here, then we *don't* want to derive + -- but a is not bound here, then we *don't* want to derive -- dn from df here lest we lose sharing. -- - -- NB2: do *not* add the Inst to avails at all if it's a method. - -- The following situation shows why this is bad: - -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b - -- From an application (truncate f i) we get - -- t1 = truncate at f - -- t2 = t1 at i - -- If we have also have a secon occurrence of truncate, we get - -- t3 = truncate at f - -- t4 = t3 at i - -- When simplifying with i,f free, we might still notice that - -- t1=t3; but alas, the binding for t2 (which mentions t1) - -- will continue to float out! - -- Solution: never put methods in avail till they are captured - -- in which case addFree isn't used -addFree avails free - | isDict free = addToFM avails free (Avail (instToId free) NoRhs []) - | otherwise = avails - -addGiven :: Avails s -> Inst -> NF_TcM s (Avails s) -addGiven avails given - = -- ASSERT( not (given `elemFM` avails) ) - -- This assertion isn't necessarily true. It's permitted - -- to given a redundant context in a type signature (eg (Ord a, Eq a) => ...) - -- and when typechecking instance decls we generate redundant "givens" too. - addAvail avails given avail - where - avail = Avail (instToId given) NoRhs [] - -addAvail avails wanted avail - = addSuperClasses (addToFM avails wanted avail) wanted - -addSuperClasses :: Avails s -> Inst -> NF_TcM s (Avails s) - -- Add all the superclasses of the Inst to Avails - -- Invariant: the Inst is already in Avails. +addFree avails free = returnM (addToFM avails free IsFree) -addSuperClasses avails dict - | not (isDict dict) - = returnNF_Tc avails +addWanted :: WantSCs -> Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails +addWanted want_scs avails wanted rhs_expr wanteds + = addAvailAndSCs want_scs avails wanted avail + where + avail = Rhs rhs_expr wanteds - | otherwise -- It is a dictionary - = foldlNF_Tc add_sc avails (zipEqual "addSuperClasses" sc_theta' sc_sels) +addGiven :: Avails -> Inst -> TcM Avails +addGiven avails given = addAvailAndSCs AddSCs avails given (Given (instToId given) False) + -- Always add superclasses for 'givens' + -- + -- No ASSERT( not (given `elemFM` avails) ) because in an instance + -- decl for Ord t we can add both Ord t and Eq t as 'givens', + -- so the assert isn't true + +addIrred :: WantSCs -> Avails -> Inst -> TcM Avails +addIrred want_scs avails irred = ASSERT2( not (irred `elemFM` avails), ppr irred $$ ppr avails ) + addAvailAndSCs want_scs avails irred Irred + +addAvailAndSCs :: WantSCs -> Avails -> Inst -> Avail -> TcM Avails +addAvailAndSCs want_scs avails inst avail + | not (isClassDict inst) = return avails_with_inst + | NoSCs <- want_scs = return avails_with_inst + | otherwise = do { traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps]) + ; addSCs is_loop avails_with_inst inst } + where + avails_with_inst = addToFM avails inst avail + + is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys + -- Note: this compares by *type*, not by Unique + deps = findAllDeps (unitVarSet (instToId inst)) avail + dep_tys = map idType (varSetElems deps) + + findAllDeps :: IdSet -> Avail -> IdSet + -- Find all the Insts that this one depends on + -- See Note [SUPERCLASS-LOOP 2] + -- Watch out, though. Since the avails may contain loops + -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far + findAllDeps so_far (Rhs _ kids) = foldl find_all so_far kids + findAllDeps so_far other = so_far + + find_all :: IdSet -> Inst -> IdSet + find_all so_far kid + | kid_id `elemVarSet` so_far = so_far + | Just avail <- lookupFM avails kid = findAllDeps so_far' avail + | otherwise = so_far' + where + so_far' = extendVarSet so_far kid_id -- Add the new kid to so_far + kid_id = instToId kid + +addSCs :: (TcPredType -> Bool) -> Avails -> Inst -> TcM Avails + -- Add all the superclasses of the Inst to Avails + -- The first param says "dont do this because the original thing + -- depends on this one, so you'd build a loop" + -- Invariant: the Inst is already in Avails. + +addSCs is_loop avails dict + = do { sc_dicts <- newDictsAtLoc (instLoc dict) sc_theta' + ; foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels) } where (clas, tys) = getDictClassTys dict - - (tyvars, sc_theta, sc_sels, _, _) = classBigSig clas - sc_theta' = substTopTheta (zipVarEnv tyvars tys) sc_theta - - add_sc avails ((super_clas, super_tys), sc_sel) - = newDictFromOld dict super_clas super_tys `thenNF_Tc` \ super_dict -> - let - sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys) - [instToId dict] - in - case lookupFM avails super_dict of - - Just (Avail main_id (Rhs rhs False {- not sc selection -}) ids) -> - -- Already there, but not as a superclass selector - -- No need to look at its superclasses; since it's there - -- already they must be already in avails - -- However, we must remember to activate the dictionary - -- from which it is (now) generated - returnNF_Tc (activate avails' dict) - where - avails' = addToFM avails super_dict avail - avail = Avail main_id (Rhs sc_sel_rhs True) ids -- Superclass selection - - Just (Avail _ _ _) -> returnNF_Tc avails - -- Already there; no need to do anything - - Nothing -> - -- Not there at all, so add it, and its superclasses - addAvail avails super_dict avail - where - avail = Avail (instToId super_dict) - (PassiveScSel sc_sel_rhs [dict]) - [] + (tyvars, sc_theta, sc_sels, _) = classBigSig clas + sc_theta' = substTheta (zipTopTvSubst tyvars tys) sc_theta + + add_sc avails (sc_dict, sc_sel) + | is_loop (dictPred sc_dict) = return avails -- See Note [SUPERCLASS-LOOP 2] + | is_given sc_dict = return avails + | otherwise = addSCs is_loop avails' sc_dict + where + sc_sel_rhs = mkHsDictApp (mkHsTyApp (L (instSpan dict) (HsVar sc_sel)) tys) [instToId dict] + avails' = addToFM avails sc_dict (Rhs sc_sel_rhs [dict]) + + is_given :: Inst -> Bool + is_given sc_dict = case lookupFM avails sc_dict of + Just (Given _ _) -> True -- Given is cheaper than superclass selection + other -> False \end{code} -%************************************************************************ -%* * -\subsection[simple]{@Simple@ versions} -%* * -%************************************************************************ - -Much simpler versions when there are no bindings to make! - -@tcSimplifyThetas@ simplifies class-type constraints formed by -@deriving@ declarations and when specialising instances. We are -only interested in the simplified bunch of class/type constraints. - -It simplifies to constraints of the form (C a b c) where -a,b,c are type variables. This is required for the context of -instance declarations. - -\begin{code} -tcSimplifyThetas :: (Class -> ClassInstEnv) -- How to find the ClassInstEnv - -> ThetaType -- Wanted - -> TcM s ThetaType -- Needed - -tcSimplifyThetas inst_mapper wanteds - = reduceSimple inst_mapper [] wanteds `thenNF_Tc` \ irreds -> - let - -- For multi-param Haskell, check that the returned dictionaries - -- don't have any of the form (C Int Bool) for which - -- we expect an instance here - -- For Haskell 98, check that all the constraints are of the form C a, - -- where a is a type variable - bad_guys | opt_GlasgowExts = [ct | ct@(clas,tys) <- irreds, - isEmptyVarSet (tyVarsOfTypes tys)] - | otherwise = [ct | ct@(clas,tys) <- irreds, - not (all isTyVarTy tys)] - in - if null bad_guys then - returnTc irreds - else - mapNF_Tc addNoInstErr bad_guys `thenNF_Tc_` - failTc -\end{code} +Note [SUPERCLASS-LOOP 2] +~~~~~~~~~~~~~~~~~~~~~~~~ +But the above isn't enough. Suppose we are *given* d1:Ord a, +and want to deduce (d2:C [a]) where -@tcSimplifyCheckThetas@ just checks class-type constraints, essentially; -used with \tr{default} declarations. We are only interested in -whether it worked or not. + class Ord a => C a where + instance Ord [a] => C [a] where ... -\begin{code} -tcSimplifyCheckThetas :: ThetaType -- Given - -> ThetaType -- Wanted - -> TcM s () +Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the +superclasses of C [a] to avails. But we must not overwrite the binding +for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just +build a loop! -tcSimplifyCheckThetas givens wanteds - = reduceSimple classInstEnv givens wanteds `thenNF_Tc` \ irreds -> - if null irreds then - returnTc () - else - mapNF_Tc addNoInstErr irreds `thenNF_Tc_` - failTc -\end{code} +Here's another variant, immortalised in tcrun020 + class Monad m => C1 m + class C1 m => C2 m x + instance C2 Maybe Bool +For the instance decl we need to build (C1 Maybe), and it's no good if +we run around and add (C2 Maybe Bool) and its superclasses to the avails +before we search for C1 Maybe. +Here's another example + class Eq b => Foo a b + instance Eq a => Foo [a] a +If we are reducing + (Foo [t] t) -\begin{code} -type AvailsSimple = FiniteMap (Class, [TauType]) Bool - -- True => irreducible - -- False => given, or can be derived from a given or from an irreducible - -reduceSimple :: (Class -> ClassInstEnv) - -> ThetaType -- Given - -> ThetaType -- Wanted - -> NF_TcM s ThetaType -- Irreducible - -reduceSimple inst_mapper givens wanteds - = reduce_simple (0,[]) inst_mapper givens_fm wanteds `thenNF_Tc` \ givens_fm' -> - returnNF_Tc [ct | (ct,True) <- fmToList givens_fm'] - where - givens_fm = foldl addNonIrred emptyFM givens +we'll first deduce that it holds (via the instance decl). We must not +then overwrite the Eq t constraint with a superclass selection! -reduce_simple :: (Int,ThetaType) -- Stack - -> (Class -> ClassInstEnv) - -> AvailsSimple - -> ThetaType - -> NF_TcM s AvailsSimple +At first I had a gross hack, whereby I simply did not add superclass constraints +in addWanted, though I did for addGiven and addIrred. This was sub-optimal, +becuase it lost legitimate superclass sharing, and it still didn't do the job: +I found a very obscure program (now tcrun021) in which improvement meant the +simplifier got two bites a the cherry... so something seemed to be an Irred +first time, but reducible next time. -reduce_simple (n,stack) inst_mapper avails wanteds - = go avails wanteds - where - go avails [] = returnNF_Tc avails - go avails (w:ws) = reduce_simple_help (n+1,w:stack) inst_mapper avails w `thenNF_Tc` \ avails' -> - go avails' ws +Now we implement the Right Solution, which is to check for loops directly +when adding superclasses. It's a bit like the occurs check in unification. -reduce_simple_help stack inst_mapper givens wanted@(clas,tys) - | wanted `elemFM` givens - = returnNF_Tc givens - | otherwise - = lookupSimpleInst (inst_mapper clas) clas tys `thenNF_Tc` \ maybe_theta -> +Note [RECURSIVE DICTIONARIES] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider + data D r = ZeroD | SuccD (r (D r)); + + instance (Eq (r (D r))) => Eq (D r) where + ZeroD == ZeroD = True + (SuccD a) == (SuccD b) = a == b + _ == _ = False; + + equalDC :: D [] -> D [] -> Bool; + equalDC = (==); - case maybe_theta of - Nothing -> returnNF_Tc (addIrred givens wanted) - Just theta -> reduce_simple stack inst_mapper (addNonIrred givens wanted) theta +We need to prove (Eq (D [])). Here's how we go: -addIrred :: AvailsSimple -> (Class, [TauType]) -> AvailsSimple -addIrred givens ct - = addSCs (addToFM givens ct True) ct + d1 : Eq (D []) -addNonIrred :: AvailsSimple -> (Class, [TauType]) -> AvailsSimple -addNonIrred givens ct - = addSCs (addToFM givens ct False) ct +by instance decl, holds if + d2 : Eq [D []] + where d1 = dfEqD d2 -addSCs givens ct@(clas,tys) - = foldl add givens sc_theta - where - (tyvars, sc_theta_tmpl, _, _, _) = classBigSig clas - sc_theta = substTopTheta (zipVarEnv tyvars tys) sc_theta_tmpl +by instance decl of Eq, holds if + d3 : D [] + where d2 = dfEqList d3 + d1 = dfEqD d2 - add givens ct = case lookupFM givens ct of - Nothing -> -- Add it and its superclasses - addSCs (addToFM givens ct False) ct +But now we can "tie the knot" to give - Just True -> -- Set its flag to False; superclasses already done - addToFM givens ct False + d3 = d1 + d2 = dfEqList d3 + d1 = dfEqD d2 - Just False -> -- Already done - givens - -\end{code} +and it'll even run! The trick is to put the thing we are trying to prove +(in this case Eq (D []) into the database before trying to prove its +contributing clauses. + %************************************************************************ %* * -\subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@} +\section{tcSimplifyTop: defaulting} %* * %************************************************************************ -When doing a binding group, we may have @Insts@ of local functions. -For example, we might have... -\begin{verbatim} -let f x = x + 1 -- orig local function (overloaded) - f.1 = f Int -- two instances of f - f.2 = f Float - in - (f.1 5, f.2 6.7) -\end{verbatim} -The point is: we must drop the bindings for @f.1@ and @f.2@ here, -where @f@ is in scope; those @Insts@ must certainly not be passed -upwards towards the top-level. If the @Insts@ were binding-ified up -there, they would have unresolvable references to @f@. -We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@. -For each method @Inst@ in the @init_lie@ that mentions one of the -@Ids@, we create a binding. We return the remaining @Insts@ (in an -@LIE@), as well as the @HsBinds@ generated. +@tcSimplifyTop@ is called once per module to simplify all the constant +and ambiguous Insts. -\begin{code} -bindInstsOfLocalFuns :: LIE -> [TcId] -> TcM s (LIE, TcMonoBinds) +We need to be careful of one case. Suppose we have -bindInstsOfLocalFuns init_lie local_ids - | null overloaded_ids || null lie_for_here - -- Common case - = returnTc (init_lie, EmptyMonoBinds) + instance Num a => Num (Foo a b) where ... - | otherwise - = reduceContext (text "bindInsts" <+> ppr local_ids) - try_me [] lie_for_here `thenTc` \ (binds, frees, irreds) -> - ASSERT( null irreds ) - returnTc (mkLIE frees `plusLIE` mkLIE lie_not_for_here, binds) - where - overloaded_ids = filter is_overloaded local_ids - is_overloaded id = case splitSigmaTy (idType id) of - (_, theta, _) -> not (null theta) +and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify +to (Num x), and default x to Int. But what about y?? - overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them - -- so it's worth building a set, so that - -- lookup (in isMethodFor) is faster +It's OK: the final zonking stage should zap y to (), which is fine. - -- No sense in repeatedly zonking lots of - -- constant constraints so filter them out here - (lie_for_here, lie_not_for_here) = partition (isMethodFor overloaded_set) - (bagToList init_lie) - try_me inst | isMethodFor overloaded_set inst = ReduceMe AddToIrreds - | otherwise = Free -\end{code} +\begin{code} +tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds +tcSimplifyTop wanteds + = tc_simplify_top doc False {- Not interactive loop -} AddSCs wanteds + where + doc = text "tcSimplifyTop" + +tcSimplifyInteractive wanteds + = tc_simplify_top doc True {- Interactive loop -} AddSCs wanteds + where + doc = text "tcSimplifyTop" + +-- The TcLclEnv should be valid here, solely to improve +-- error message generation for the monomorphism restriction +tc_simplify_top doc is_interactive want_scs wanteds + = do { lcl_env <- getLclEnv + ; traceTc (text "tcSimplifyTop" <+> ppr (lclEnvElts lcl_env)) + + ; let try_me inst = ReduceMe want_scs + ; (frees, binds, irreds) <- simpleReduceLoop doc try_me wanteds + + ; let + -- First get rid of implicit parameters + (non_ips, bad_ips) = partition isClassDict irreds + + -- All the non-tv or multi-param ones are definite errors + (unary_tv_dicts, non_tvs) = partition is_unary_tyvar_dict non_ips + bad_tyvars = unionVarSets (map tyVarsOfInst non_tvs) -%************************************************************************ -%* * -\section[Disambig]{Disambiguation of overloading} -%* * -%************************************************************************ + -- Group by type variable + tv_groups = equivClasses cmp_by_tyvar unary_tv_dicts + + -- Pick the ones which its worth trying to disambiguate + -- namely, the ones whose type variable isn't bound + -- up with one of the non-tyvar classes + (default_gps, non_default_gps) = partition defaultable_group tv_groups + defaultable_group ds + = not (bad_tyvars `intersectsVarSet` tyVarsOfInst (head ds)) + && defaultable_classes (map get_clas ds) + defaultable_classes clss + | is_interactive = any isInteractiveClass clss + | otherwise = all isStandardClass clss && any isNumericClass clss + + isInteractiveClass cls = isNumericClass cls + || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey]) + -- In interactive mode, we default Show a to Show () + -- to avoid graututious errors on "show []" + + -- Collect together all the bad guys + bad_guys = non_tvs ++ concat non_default_gps + (ambigs, no_insts) = partition isTyVarDict bad_guys + -- If the dict has no type constructors involved, it must be ambiguous, + -- except I suppose that another error with fundeps maybe should have + -- constrained those type variables + + -- Report definite errors + ; ASSERT( null frees ) + groupErrs (addNoInstanceErrs Nothing []) no_insts + ; strangeTopIPErrs bad_ips + + -- Deal with ambiguity errors, but only if + -- if there has not been an error so far: + -- errors often give rise to spurious ambiguous Insts. + -- For example: + -- f = (*) -- Monomorphic + -- g :: Num a => a -> a + -- g x = f x x + -- Here, we get a complaint when checking the type signature for g, + -- that g isn't polymorphic enough; but then we get another one when + -- dealing with the (Num a) context arising from f's definition; + -- we try to unify a with Int (to default it), but find that it's + -- already been unified with the rigid variable from g's type sig + ; binds_ambig <- ifErrsM (returnM []) $ + do { -- Complain about the ones that don't fall under + -- the Haskell rules for disambiguation + -- This group includes both non-existent instances + -- e.g. Num (IO a) and Eq (Int -> Int) + -- and ambiguous dictionaries + -- e.g. Num a + addTopAmbigErrs ambigs + + -- Disambiguate the ones that look feasible + ; mappM disambigGroup default_gps } + + ; return (binds `unionBags` unionManyBags binds_ambig) } + +---------------------------------- +d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2 + +is_unary_tyvar_dict :: Inst -> Bool -- Dicts of form (C a) + -- Invariant: argument is a ClassDict, not IP or method +is_unary_tyvar_dict d = case getDictClassTys d of + (_, [ty]) -> tcIsTyVarTy ty + other -> False + +get_tv d = case getDictClassTys d of + (clas, [ty]) -> tcGetTyVar "tcSimplify" ty +get_clas d = case getDictClassTys d of + (clas, _) -> clas +\end{code} If a dictionary constrains a type variable which is -\begin{itemize} -\item -not mentioned in the environment -\item -and not mentioned in the type of the expression -\end{itemize} + * not mentioned in the environment + * and not mentioned in the type of the expression then it is ambiguous. No further information will arise to instantiate the type variable; nor will it be generalised and turned into an extra parameter to a function. It is an error for this to occur, except that Haskell provided for certain rules to be applied in the special case of numeric types. - Specifically, if -\begin{itemize} -\item -at least one of its classes is a numeric class, and -\item -all of its classes are numeric or standard -\end{itemize} + * at least one of its classes is a numeric class, and + * all of its classes are numeric or standard then the type variable can be defaulted to the first type in the default-type list which is an instance of all the offending classes. @@ -928,58 +2111,6 @@ dictionaries and either resolves them (producing bindings) or complains. It works by splitting the dictionary list by type variable, and using @disambigOne@ to do the real business. - -@tcSimplifyTop@ is called once per module to simplify -all the constant and ambiguous Insts. - -\begin{code} -tcSimplifyTop :: LIE -> TcM s TcDictBinds -tcSimplifyTop wanted_lie - = reduceContext (text "tcSimplTop") try_me [] wanteds `thenTc` \ (binds1, frees, irreds) -> - ASSERT( null frees ) - - let - -- All the non-std ones are definite errors - (stds, non_stds) = partition isStdClassTyVarDict irreds - - - -- Group by type variable - std_groups = equivClasses cmp_by_tyvar stds - - -- Pick the ones which its worth trying to disambiguate - (std_oks, std_bads) = partition worth_a_try std_groups - -- Have a try at disambiguation - -- if the type variable isn't bound - -- up with one of the non-standard classes - worth_a_try group@(d:_) = isEmptyVarSet (tyVarsOfInst d `intersectVarSet` non_std_tyvars) - non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds) - - -- Collect together all the bad guys - bad_guys = non_stds ++ concat std_bads - in - - -- Disambiguate the ones that look feasible - mapTc disambigGroup std_oks `thenTc` \ binds_ambig -> - - -- And complain about the ones that don't - mapNF_Tc complain bad_guys `thenNF_Tc_` - - returnTc (binds1 `andMonoBinds` andMonoBindList binds_ambig) - where - wanteds = bagToList wanted_lie - try_me inst = ReduceMe AddToIrreds - - d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2 - - complain d | isEmptyVarSet (tyVarsOfInst d) = addTopInstanceErr d - | otherwise = addAmbigErr tyVarsOfInst d - -get_tv d = case getDictClassTys d of - (clas, [ty]) -> getTyVar "tcSimplifyTop" ty -get_clas d = case getDictClassTys d of - (clas, [ty]) -> clas -\end{code} - @disambigOne@ assumes that its arguments dictionaries constrain all the same type variable. @@ -994,14 +2125,9 @@ Since we're not using the result of @foo@, the result if (presumably) \begin{code} disambigGroup :: [Inst] -- All standard classes of form (C a) - -> TcM s TcDictBinds + -> TcM TcDictBinds disambigGroup dicts - | any isNumericClass classes -- Guaranteed all standard classes - -- see comment at the end of function for reasons as to - -- why the defaulting mechanism doesn't apply to groups that - -- include CCallable or CReturnable dicts. - && not (any isCcallishClass classes) = -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT -- SO, TRY DEFAULT TYPES IN ORDER @@ -1009,50 +2135,50 @@ disambigGroup dicts -- default list which can satisfy all the ambiguous classes. -- For example, if Real a is reqd, but the only type in the -- default list is Int. - tcGetDefaultTys `thenNF_Tc` \ default_tys -> + get_default_tys `thenM` \ default_tys -> let try_default [] -- No defaults work, so fail - = failTc + = failM try_default (default_ty : default_tys) - = tryTc_ (try_default default_tys) $ -- If default_ty fails, we try + = tryTcLIE_ (try_default default_tys) $ -- If default_ty fails, we try -- default_tys instead - tcSimplifyCheckThetas [] thetas `thenTc` \ _ -> - returnTc default_ty + tcSimplifyDefault theta `thenM` \ _ -> + returnM default_ty where - thetas = classes `zip` repeat [default_ty] - in - -- See if any default works, and if so bind the type variable to it - -- If not, add an AmbigErr - recoverTc (complain dicts `thenNF_Tc_` returnTc EmptyMonoBinds) $ - - try_default default_tys `thenTc` \ chosen_default_ty -> - - -- Bind the type variable and reduce the context, for real this time - let - chosen_default_tc_ty = typeToTcType chosen_default_ty -- Tiresome! + theta = [mkClassPred clas [default_ty] | clas <- classes] in - unifyTauTy chosen_default_tc_ty (mkTyVarTy tyvar) `thenTc_` - reduceContext (text "disambig" <+> ppr dicts) - try_me [] dicts `thenTc` \ (binds, frees, ambigs) -> - ASSERT( null frees && null ambigs ) - returnTc binds - - | all isCreturnableClass classes - = -- Default CCall stuff to (); we don't even both to check that () is an - -- instance of CReturnable, because we know it is. - unifyTauTy (mkTyVarTy tyvar) unitTy `thenTc_` - returnTc EmptyMonoBinds - - | otherwise -- No defaults - = complain dicts `thenNF_Tc_` - returnTc EmptyMonoBinds - + -- See if any default works + tryM (try_default default_tys) `thenM` \ mb_ty -> + case mb_ty of + Left _ -> bomb_out + Right chosen_default_ty -> choose_default chosen_default_ty where - complain = addAmbigErrs tyVarsOfInst - try_me inst = ReduceMe AddToIrreds -- This reduce should not fail - tyvar = get_tv (head dicts) -- Should be non-empty - classes = map get_clas dicts + tyvar = get_tv (head dicts) -- Should be non-empty + classes = map get_clas dicts + + choose_default default_ty -- Commit to tyvar = default_ty + = -- Bind the type variable + unifyType default_ty (mkTyVarTy tyvar) `thenM_` + -- and reduce the context, for real this time + simpleReduceLoop (text "disambig" <+> ppr dicts) + reduceMe dicts `thenM` \ (frees, binds, ambigs) -> + WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs ) + warnDefault dicts default_ty `thenM_` + returnM binds + + bomb_out = addTopAmbigErrs dicts `thenM_` + returnM emptyBag + +get_default_tys + = do { mb_defaults <- getDefaultTys + ; case mb_defaults of + Just tys -> return tys + Nothing -> -- No use-supplied default; + -- use [Integer, Double] + do { integer_ty <- tcMetaTy integerTyConName + ; checkWiredInTyCon doubleTyCon + ; return [integer_ty, doubleTy] } } \end{code} [Aside - why the defaulting mechanism is turned off when @@ -1060,10 +2186,8 @@ disambigGroup dicts When typechecking _ccall_s, TcExpr ensures that the external function is only passed arguments (and in the other direction, -results) of a restricted set of 'native' types. This is -implemented via the help of the pseudo-type classes, -@CReturnable@ (CR) and @CCallable@ (CC.) - +results) of a restricted set of 'native' types. + The interaction between the defaulting mechanism for numeric values and CC & CR can be a bit puzzling to the user at times. For example, @@ -1079,74 +2203,317 @@ in operation, if it is equal to Haskell 98's default-default of (Integer, Double), 'x' has type Double, since Integer is not an instance of CR. If the default list is equal to Haskell 1.4's default-default of (Int, Double), 'x' has type -Int. +Int. + +End of aside] + + +%************************************************************************ +%* * +\subsection[simple]{@Simple@ versions} +%* * +%************************************************************************ + +Much simpler versions when there are no bindings to make! + +@tcSimplifyThetas@ simplifies class-type constraints formed by +@deriving@ declarations and when specialising instances. We are +only interested in the simplified bunch of class/type constraints. + +It simplifies to constraints of the form (C a b c) where +a,b,c are type variables. This is required for the context of +instance declarations. + +\begin{code} +tcSimplifyDeriv :: [TyVar] + -> ThetaType -- Wanted + -> TcM ThetaType -- Needed + +tcSimplifyDeriv tyvars theta + = tcInstTyVars tyvars `thenM` \ (tvs, _, tenv) -> + -- The main loop may do unification, and that may crash if + -- it doesn't see a TcTyVar, so we have to instantiate. Sigh + -- ToDo: what if two of them do get unified? + newDicts DerivOrigin (substTheta tenv theta) `thenM` \ wanteds -> + simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) -> + ASSERT( null frees ) -- reduceMe never returns Free + + doptM Opt_AllowUndecidableInstances `thenM` \ undecidable_ok -> + let + tv_set = mkVarSet tvs + + (bad_insts, ok_insts) = partition is_bad_inst irreds + is_bad_inst dict + = let pred = dictPred dict -- reduceMe squashes all non-dicts + in isEmptyVarSet (tyVarsOfPred pred) + -- Things like (Eq T) are bad + || (not undecidable_ok && not (isTyVarClassPred pred)) + -- The returned dictionaries should be of form (C a b) + -- (where a, b are type variables). + -- We allow non-tyvar dicts if we had -fallow-undecidable-instances, + -- but note that risks non-termination in the 'deriving' context-inference + -- fixpoint loop. It is useful for situations like + -- data Min h a = E | M a (h a) + -- which gives the instance decl + -- instance (Eq a, Eq (h a)) => Eq (Min h a) + + simpl_theta = map dictPred ok_insts + weird_preds = [pred | pred <- simpl_theta + , not (tyVarsOfPred pred `subVarSet` tv_set)] + -- Check for a bizarre corner case, when the derived instance decl should + -- have form instance C a b => D (T a) where ... + -- Note that 'b' isn't a parameter of T. This gives rise to all sorts + -- of problems; in particular, it's hard to compare solutions for + -- equality when finding the fixpoint. So I just rule it out for now. + + rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars) + -- This reverse-mapping is a Royal Pain, + -- but the result should mention TyVars not TcTyVars + in + + addNoInstanceErrs Nothing [] bad_insts `thenM_` + mapM_ (addErrTc . badDerivedPred) weird_preds `thenM_` + checkAmbiguity tvs simpl_theta tv_set `thenM_` + returnM (substTheta rev_env simpl_theta) + where + doc = ptext SLIT("deriving classes for a data type") +\end{code} + +@tcSimplifyDefault@ just checks class-type constraints, essentially; +used with \tr{default} declarations. We are only interested in +whether it worked or not. + +\begin{code} +tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it + -> TcM () + +tcSimplifyDefault theta + = newDicts DefaultOrigin theta `thenM` \ wanteds -> + simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) -> + ASSERT( null frees ) -- try_me never returns Free + addNoInstanceErrs Nothing [] irreds `thenM_` + if null irreds then + returnM () + else + failM + where + doc = ptext SLIT("default declaration") +\end{code} -To try to minimise the potential for surprises here, the -defaulting mechanism is turned off in the presence of -CCallable and CReturnable. -] +%************************************************************************ +%* * +\section{Errors and contexts} +%* * +%************************************************************************ -Errors and contexts -~~~~~~~~~~~~~~~~~~~ ToDo: for these error messages, should we note the location as coming from the insts, or just whatever seems to be around in the monad just now? \begin{code} -genCantGenErr insts -- Can't generalise these Insts - = sep [ptext SLIT("Cannot generalise these overloadings (in a _ccall_):"), - nest 4 (pprInstsInFull insts) - ] - -addAmbigErrs ambig_tv_fn dicts = mapNF_Tc (addAmbigErr ambig_tv_fn) dicts - -addAmbigErr ambig_tv_fn dict - = tcAddSrcLoc (instLoc dict) $ - addErrTcM (tidy_env, - sep [text "Ambiguous type variable(s)" <+> - hsep (punctuate comma (map (quotes . ppr) ambig_tvs)), - nest 4 (text "in the constraint" <+> quotes (pprInst tidy_dict)), - nest 4 (pprOrigin dict)]) +groupErrs :: ([Inst] -> TcM ()) -- Deal with one group + -> [Inst] -- The offending Insts + -> TcM () +-- Group together insts with the same origin +-- We want to report them together in error messages + +groupErrs report_err [] + = returnM () +groupErrs report_err (inst:insts) + = do_one (inst:friends) `thenM_` + groupErrs report_err others + + where + -- (It may seem a bit crude to compare the error messages, + -- but it makes sure that we combine just what the user sees, + -- and it avoids need equality on InstLocs.) + (friends, others) = partition is_friend insts + loc_msg = showSDoc (pprInstLoc (instLoc inst)) + is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg + do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts) + -- Add location and context information derived from the Insts + +-- Add the "arising from..." part to a message about bunch of dicts +addInstLoc :: [Inst] -> Message -> Message +addInstLoc insts msg = msg $$ nest 2 (pprInstLoc (instLoc (head insts))) + +addTopIPErrs :: [Name] -> [Inst] -> TcM () +addTopIPErrs bndrs [] + = return () +addTopIPErrs bndrs ips + = addErrTcM (tidy_env, mk_msg tidy_ips) + where + (tidy_env, tidy_ips) = tidyInsts ips + mk_msg ips = vcat [sep [ptext SLIT("Implicit parameters escape from"), + nest 2 (ptext SLIT("the monomorphic top-level binding(s) of") + <+> pprBinders bndrs <> colon)], + nest 2 (vcat (map ppr_ip ips)), + monomorphism_fix] + ppr_ip ip = pprPred (dictPred ip) <+> pprInstLoc (instLoc ip) + +strangeTopIPErrs :: [Inst] -> TcM () +strangeTopIPErrs dicts -- Strange, becuase addTopIPErrs should have caught them all + = groupErrs report tidy_dicts + where + (tidy_env, tidy_dicts) = tidyInsts dicts + report dicts = addErrTcM (tidy_env, mk_msg dicts) + mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <> + plural tidy_dicts <+> pprDictsTheta tidy_dicts) + +addNoInstanceErrs :: Maybe SDoc -- Nothing => top level + -- Just d => d describes the construct + -> [Inst] -- What is given by the context or type sig + -> [Inst] -- What is wanted + -> TcM () +addNoInstanceErrs mb_what givens [] + = returnM () +addNoInstanceErrs mb_what givens dicts + = -- Some of the dicts are here because there is no instances + -- and some because there are too many instances (overlap) + tcGetInstEnvs `thenM` \ inst_envs -> + let + (tidy_env1, tidy_givens) = tidyInsts givens + (tidy_env2, tidy_dicts) = tidyMoreInsts tidy_env1 dicts + + -- Run through the dicts, generating a message for each + -- overlapping one, but simply accumulating all the + -- no-instance ones so they can be reported as a group + (overlap_doc, no_inst_dicts) = foldl check_overlap (empty, []) tidy_dicts + check_overlap (overlap_doc, no_inst_dicts) dict + | not (isClassDict dict) = (overlap_doc, dict : no_inst_dicts) + | otherwise + = case lookupInstEnv inst_envs clas tys of + -- The case of exactly one match and no unifiers means + -- a successful lookup. That can't happen here, becuase + -- dicts only end up here if they didn't match in Inst.lookupInst +#ifdef DEBUG + ([m],[]) -> pprPanic "addNoInstanceErrs" (ppr dict) +#endif + ([], _) -> (overlap_doc, dict : no_inst_dicts) -- No match + res -> (mk_overlap_msg dict res $$ overlap_doc, no_inst_dicts) + where + (clas,tys) = getDictClassTys dict + in + + -- Now generate a good message for the no-instance bunch + mk_probable_fix tidy_env2 no_inst_dicts `thenM` \ (tidy_env3, probable_fix) -> + let + no_inst_doc | null no_inst_dicts = empty + | otherwise = vcat [addInstLoc no_inst_dicts heading, probable_fix] + heading | null givens = ptext SLIT("No instance") <> plural no_inst_dicts <+> + ptext SLIT("for") <+> pprDictsTheta no_inst_dicts + | otherwise = sep [ptext SLIT("Could not deduce") <+> pprDictsTheta no_inst_dicts, + nest 2 $ ptext SLIT("from the context") <+> pprDictsTheta tidy_givens] + in + -- And emit both the non-instance and overlap messages + addErrTcM (tidy_env3, no_inst_doc $$ overlap_doc) where - ambig_tvs = varSetElems (ambig_tv_fn tidy_dict) - (tidy_env, tidy_dict) = tidyInst emptyTidyEnv dict - --- Used for top-level irreducibles -addTopInstanceErr dict - = tcAddSrcLoc (instLoc dict) $ - addErrTcM (tidy_env, - sep [ptext SLIT("No instance for") <+> quotes (pprInst tidy_dict), - nest 4 $ pprOrigin dict]) + mk_overlap_msg dict (matches, unifiers) + = vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for") + <+> pprPred (dictPred dict))), + sep [ptext SLIT("Matching instances") <> colon, + nest 2 (vcat [pprInstances ispecs, pprInstances unifiers])], + ASSERT( not (null matches) ) + if not (isSingleton matches) + then -- Two or more matches + empty + else -- One match, plus some unifiers + ASSERT( not (null unifiers) ) + parens (vcat [ptext SLIT("The choice depends on the instantiation of") <+> + quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))), + ptext SLIT("Use -fallow-incoherent-instances to use the first choice above")])] + where + ispecs = [ispec | (_, ispec) <- matches] + + mk_probable_fix tidy_env dicts + = returnM (tidy_env, sep [ptext SLIT("Possible fix:"), nest 2 (vcat fixes)]) + where + fixes = add_ors (fix1 ++ fix2) + + fix1 = case mb_what of + Nothing -> [] -- Top level + Just what -> -- Nested (type signatures, instance decls) + [ sep [ ptext SLIT("add") <+> pprDictsTheta dicts, + ptext SLIT("to the") <+> what] ] + + fix2 | null instance_dicts = [] + | otherwise = [ ptext SLIT("add an instance declaration for") + <+> pprDictsTheta instance_dicts ] + instance_dicts = [d | d <- dicts, isClassDict d, not (isTyVarDict d)] + -- Insts for which it is worth suggesting an adding an instance declaration + -- Exclude implicit parameters, and tyvar dicts + + add_ors :: [SDoc] -> [SDoc] -- The empty case should not happen + add_ors [] = [ptext SLIT("[No suggested fixes]")] -- Strange + add_ors (f1:fs) = f1 : map (ptext SLIT("or") <+>) fs + +addTopAmbigErrs dicts +-- Divide into groups that share a common set of ambiguous tyvars + = mapM report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts]) where - (tidy_env, tidy_dict) = tidyInst emptyTidyEnv dict - -addNoInstanceErr str givens dict - = tcAddSrcLoc (instLoc dict) $ - addErrTcM (tidy_env, - sep [sep [ptext SLIT("Could not deduce") <+> quotes (pprInst tidy_dict), - nest 4 $ parens $ pprOrigin dict], - nest 4 $ ptext SLIT("from the context:") <+> pprInsts tidy_givens] - $$ - ptext SLIT("Probable cause:") <+> - vcat [sep [ptext SLIT("missing") <+> quotes (pprInst tidy_dict), - ptext SLIT("in") <+> str], - if all_tyvars then empty else - ptext SLIT("or missing instance declaration for") <+> quotes (pprInst tidy_dict)] - ) + (tidy_env, tidy_dicts) = tidyInsts dicts + + tvs_of :: Inst -> [TcTyVar] + tvs_of d = varSetElems (tyVarsOfInst d) + cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2 + + report :: [(Inst,[TcTyVar])] -> TcM () + report pairs@((inst,tvs) : _) -- The pairs share a common set of ambiguous tyvars + = mkMonomorphismMsg tidy_env tvs `thenM` \ (tidy_env, mono_msg) -> + setSrcSpan (instLocSrcSpan (instLoc inst)) $ + -- the location of the first one will do for the err message + addErrTcM (tidy_env, msg $$ mono_msg) + where + dicts = map fst pairs + msg = sep [text "Ambiguous type variable" <> plural tvs <+> + pprQuotedList tvs <+> in_msg, + nest 2 (pprDictsInFull dicts)] + in_msg = text "in the constraint" <> plural dicts <> colon + + +mkMonomorphismMsg :: TidyEnv -> [TcTyVar] -> TcM (TidyEnv, Message) +-- There's an error with these Insts; if they have free type variables +-- it's probably caused by the monomorphism restriction. +-- Try to identify the offending variable +-- ASSUMPTION: the Insts are fully zonked +mkMonomorphismMsg tidy_env inst_tvs + = findGlobals (mkVarSet inst_tvs) tidy_env `thenM` \ (tidy_env, docs) -> + returnM (tidy_env, mk_msg docs) + where + mk_msg [] = ptext SLIT("Probable fix: add a type signature that fixes these type variable(s)") + -- This happens in things like + -- f x = show (read "foo") + -- whre monomorphism doesn't play any role + mk_msg docs = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"), + nest 2 (vcat docs), + monomorphism_fix + ] +monomorphism_fix :: SDoc +monomorphism_fix = ptext SLIT("Probable fix:") <+> + (ptext SLIT("give these definition(s) an explicit type signature") + $$ ptext SLIT("or use -fno-monomorphism-restriction")) + +warnDefault dicts default_ty + = doptM Opt_WarnTypeDefaults `thenM` \ warn_flag -> + addInstCtxt (instLoc (head dicts)) (warnTc warn_flag warn_msg) where - all_tyvars = all isTyVarTy tys - (_, tys) = getDictClassTys dict - (tidy_env, tidy_dict:tidy_givens) = tidyInsts emptyTidyEnv (dict:givens) + -- Tidy them first + (_, tidy_dicts) = tidyInsts dicts + warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+> + quotes (ppr default_ty), + pprDictsInFull tidy_dicts] -- Used for the ...Thetas variants; all top level -addNoInstErr (c,ts) - = addErrTc (ptext SLIT("No instance for") <+> quotes (pprConstraint c ts)) +badDerivedPred pred + = vcat [ptext SLIT("Can't derive instances where the instance context mentions"), + ptext SLIT("type variables that are not data type parameters"), + nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)] reduceDepthErr n stack = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n, ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"), - nest 4 (pprInstsInFull stack)] + nest 4 (pprStack stack)] -reduceDepthMsg n stack = nest 4 (pprInstsInFull stack) +pprStack stack = vcat (map pprInstInFull stack) \end{code}