X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2Ftypecheck%2FTcSimplify.lhs;h=ca9180fae91515621516cb14ad2b15f7b0351f6b;hb=20f50b2a3651ce7dacdcb86a83afb5c5d444cb0b;hp=e2737adef4d2f1f0b8a38278754517d37f204dd0;hpb=2269b0b4b06a110ad466b914037a763b4dca9190;p=ghc-hetmet.git diff --git a/ghc/compiler/typecheck/TcSimplify.lhs b/ghc/compiler/typecheck/TcSimplify.lhs index e2737ad..ca9180f 100644 --- a/ghc/compiler/typecheck/TcSimplify.lhs +++ b/ghc/compiler/typecheck/TcSimplify.lhs @@ -1,551 +1,949 @@ % -% (c) The GRASP/AQUA Project, Glasgow University, 1992-1996 +% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 % \section[TcSimplify]{TcSimplify} -\begin{code} -#include "HsVersions.h" + +\begin{code} module TcSimplify ( - tcSimplify, tcSimplifyAndCheck, - tcSimplifyTop, tcSimplifyThetas, tcSimplifyCheckThetas, tcSimplifyRank2, + tcSimplifyInfer, tcSimplifyInferCheck, + tcSimplifyCheck, tcSimplifyRestricted, + tcSimplifyToDicts, tcSimplifyIPs, tcSimplifyTop, + + tcSimplifyDeriv, tcSimplifyDefault, bindInstsOfLocalFuns ) where -IMP_Ubiq() +#include "HsVersions.h" -import HsSyn ( MonoBinds(..), HsExpr(..), InPat, OutPat, HsLit, - Match, HsBinds, HsType, ArithSeqInfo, Fixity, - GRHSsAndBinds, Stmt, DoOrListComp, Fake ) -import HsBinds ( andMonoBinds ) -import TcHsSyn ( SYN_IE(TcExpr), SYN_IE(TcMonoBinds), SYN_IE(TcDictBinds) ) +import {-# SOURCE #-} TcUnify( unifyTauTy ) -import TcMonad -import Inst ( lookupInst, lookupSimpleInst, - tyVarsOfInst, isTyVarDict, isDict, - matchesInst, instToId, instBindingRequired, - instCanBeGeneralised, newDictsAtLoc, - pprInst, - Inst(..), SYN_IE(LIE), zonkLIE, emptyLIE, pprLIE, pprLIEInFull, - plusLIE, unitLIE, consLIE, InstOrigin(..), - OverloadedLit ) -import TcEnv ( tcGetGlobalTyVars ) -import SpecEnv ( SpecEnv ) -import TcType ( TcIdOcc(..), SYN_IE(TcIdBndr), - SYN_IE(TcType), SYN_IE(TcTyVar), SYN_IE(TcTyVarSet), TcMaybe, tcInstType +import HsSyn ( MonoBinds(..), HsExpr(..), andMonoBinds, andMonoBindList ) +import TcHsSyn ( TcExpr, TcId, + TcMonoBinds, TcDictBinds ) -import Unify ( unifyTauTy ) -import Bag ( Bag, unitBag, listToBag, foldBag, filterBag, emptyBag, bagToList, - snocBag, consBag, unionBags, isEmptyBag ) -import Class ( GenClass, SYN_IE(Class), SYN_IE(ClassInstEnv), - isSuperClassOf, classSuperDictSelId, classInstEnv +import TcMonad +import Inst ( lookupInst, LookupInstResult(..), + tyVarsOfInst, predsOfInsts, predsOfInst, newDicts, + isDict, isClassDict, isLinearInst, linearInstType, + isStdClassTyVarDict, isMethodFor, isMethod, + instToId, tyVarsOfInsts, cloneDict, + ipNamesOfInsts, ipNamesOfInst, dictPred, + instBindingRequired, instCanBeGeneralised, + newDictsFromOld, newMethodAtLoc, + getDictClassTys, isTyVarDict, + instLoc, pprInst, zonkInst, tidyInsts, tidyMoreInsts, + Inst, LIE, pprInsts, pprInstsInFull, + mkLIE, lieToList ) -import Id ( GenId ) -import PrelInfo ( isNumericClass, isStandardClass, isCcallishClass ) - -import Maybes ( expectJust, firstJust, catMaybes, seqMaybe, maybeToBool ) -import Outputable ( PprStyle, Outputable(..){-instance * []-} ) -import PprType ( GenType, GenTyVar ) -import Pretty -import SrcLoc ( noSrcLoc ) -import Type ( GenType, SYN_IE(Type), SYN_IE(TauType), mkTyVarTy, getTyVar, eqSimpleTy, - getTyVar_maybe ) -import TysWiredIn ( intTy, unitTy ) -import TyVar ( GenTyVar, SYN_IE(GenTyVarSet), - elementOfTyVarSet, emptyTyVarSet, unionTyVarSets, - isEmptyTyVarSet, tyVarSetToList ) -import Unique ( Unique ) -import Util +import TcEnv ( tcGetGlobalTyVars, tcGetInstEnv, tcLookupGlobalId ) +import InstEnv ( lookupInstEnv, classInstEnv, InstLookupResult(..) ) +import TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, checkAmbiguity ) +import TcType ( TcTyVar, TcTyVarSet, ThetaType, + mkClassPred, isOverloadedTy, mkTyConApp, + mkTyVarTy, tcGetTyVar, isTyVarClassPred, mkTyVarTys, + tyVarsOfPred, isIPPred, inheritablePred, predHasFDs ) +import Id ( idType, mkUserLocal ) +import Var ( TyVar ) +import Name ( getOccName, getSrcLoc ) +import NameSet ( NameSet, mkNameSet, elemNameSet ) +import Class ( classBigSig ) +import FunDeps ( oclose, grow, improve, pprEquationDoc ) +import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass, + splitIdName, fstIdName, sndIdName ) + +import Subst ( mkTopTyVarSubst, substTheta, substTy ) +import TysWiredIn ( unitTy, pairTyCon ) +import VarSet +import FiniteMap +import Outputable +import ListSetOps ( equivClasses ) +import Util ( zipEqual ) +import List ( partition ) +import CmdLineOpts \end{code} %************************************************************************ %* * -\subsection[tcSimplify-main]{Main entry function} +\subsection{NOTES} %* * %************************************************************************ -* May modify the substitution to bind ambiguous type variables. + -------------------------------------- + Notes on quantification + -------------------------------------- -Specification -~~~~~~~~~~~~~ -(1) If an inst constrains only ``global'' type variables, (or none), - return it as a ``global'' inst. +Suppose we are about to do a generalisation step. +We have in our hand -OTHERWISE + G the environment + T the type of the RHS + C the constraints from that RHS -(2) Simplify it repeatedly (checking for (1) of course) until it is a dict - constraining only a type variable. +The game is to figure out -(3) If it constrains a ``local'' type variable, return it as a ``local'' inst. - Otherwise it must be ambiguous, so try to resolve the ambiguity. + 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 -\begin{code} -tcSimpl :: Bool -- True <=> simplify const insts - -> TcTyVarSet s -- ``Global'' type variables - -> TcTyVarSet s -- ``Local'' type variables - -- ASSERT: both these tyvar sets are already zonked - -> LIE s -- Given; these constrain only local tyvars - -> LIE s -- Wanted - -> TcM s (LIE s, -- Free - TcMonoBinds s, -- Bindings - LIE s) -- Remaining wanteds; no dups - -tcSimpl squash_consts global_tvs local_tvs givens wanteds - = -- ASSSERT: global_tvs and local_tvs are already zonked - -- Make sure the insts fixed points of the substitution - zonkLIE givens `thenNF_Tc` \ givens -> - zonkLIE wanteds `thenNF_Tc` \ wanteds -> - - -- Deal with duplicates and type constructors - elimTyCons - squash_consts (\tv -> tv `elementOfTyVarSet` global_tvs) - givens wanteds `thenTc` \ (globals, tycon_binds, locals_and_ambigs) -> - - -- Now disambiguate if necessary - let - ambigs = filterBag is_ambiguous locals_and_ambigs - in - if not (isEmptyBag ambigs) then - -- Some ambiguous dictionaries. We now disambiguate them, - -- which binds the offending type variables to suitable types in the - -- substitution, and then we retry the whole process. This - -- time there won't be any ambiguous ones. - -- There's no need to back-substitute on global and local tvs, - -- because the ambiguous type variables can't be in either. + forall Q. Cq => T - -- Why do we retry the whole process? Because binding a type variable - -- to a particular type might enable a short-cut simplification which - -- elimTyCons will have missed the first time. +and float the constraints Ct further outwards. - disambiguateDicts ambigs `thenTc_` - tcSimpl squash_consts global_tvs local_tvs givens wanteds +Here are the things that *must* be true: - else - -- No ambiguous dictionaries. Just bash on with the results - -- of the elimTyCons + (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 - -- Check for non-generalisable insts - let - locals = locals_and_ambigs -- ambigs is empty - cant_generalise = filterBag (not . instCanBeGeneralised) locals - in - checkTc (isEmptyBag cant_generalise) - (genCantGenErr cant_generalise) `thenTc_` +(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! - -- Deal with superclass relationships - elimSCs givens locals `thenNF_Tc` \ (sc_binds, locals2) -> +Example: class H x y | x->y where ... - -- Finished - returnTc (globals, sc_binds `AndMonoBinds` tycon_binds, locals2) - where - is_ambiguous (Dict _ _ ty _ _) - = not (getTyVar "is_ambiguous" ty `elementOfTyVarSet` local_tvs) -\end{code} + fv(G) = {a} C = {H a b, H c d} + T = c -> b -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. + (A) Q intersect {a} is empty + (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d} -\begin{code} -tcSimplify - :: TcTyVarSet s -- ``Local'' type variables - -> LIE s -- Wanted - -> TcM s (LIE s, -- Free - TcDictBinds s, -- Bindings - LIE s) -- Remaining wanteds; no dups - -tcSimplify local_tvs wanteds - = tcGetGlobalTyVars `thenNF_Tc` \ global_tvs -> - tcSimpl False global_tvs local_tvs emptyBag wanteds -\end{code} + 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 + +(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. + + + +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. + +So here's the plan. We WARN about probable ambiguity if + + fv(Cq) is not a subset of oclose(fv(T) union fv(G), C) + +(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. + +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 + +Only if we union {a} from G with {b} from T before using oclose, +do we see that c is fixed. + +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. + + +Can we ever be *certain* about ambiguity? Yes: if there's a constraint + + c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY + +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 + + + -------------------------------------- + Notes on implicit parameters + -------------------------------------- + +Question 1: can we "inherit" implicit parameters +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider this: + + 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: + + a signature does not need to quantify over implicit params. + +[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.] + +But that raises a new question. Consider + + Given (signature) ?x::Int + Wanted (inferred) ?x::Int, ?y::Bool + +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 + +See the predicate isFreeWhenChecking. + + +Question 3: monomorphism +~~~~~~~~~~~~~~~~~~~~~~~~ +There's a nasty corner case when the monomorphism restriction bites: + + z = (x::Int) + ?y + +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 + + let z = x + ?y in z+z + +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. + + + +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. -@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. \begin{code} -tcSimplifyAndCheck - :: TcTyVarSet s -- ``Local'' type variables; ASSERT is fixpoint - -> LIE s -- Given - -> LIE s -- Wanted - -> TcM s (LIE s, -- Free - TcDictBinds s) -- Bindings - -tcSimplifyAndCheck local_tvs givens wanteds - = tcGetGlobalTyVars `thenNF_Tc` \ global_tvs -> - tcSimpl False global_tvs local_tvs - givens wanteds `thenTc` \ (free_insts, binds, wanteds') -> - checkTc (isEmptyBag wanteds') - (reduceErr wanteds') `thenTc_` - returnTc (free_insts, binds) +tcSimplifyInfer + :: SDoc + -> TcTyVarSet -- fv(T); type vars + -> LIE -- Wanted + -> TcM ([TcTyVar], -- Tyvars to quantify (zonked) + LIE, -- Free + TcDictBinds, -- Bindings + [TcId]) -- Dict Ids that must be bound here (zonked) \end{code} -@tcSimplifyRank2@ checks that the argument of a rank-2 polymorphic function -is not overloaded. \begin{code} -tcSimplifyRank2 :: TcTyVarSet s -- ``Local'' type variables; ASSERT is fixpoint - -> LIE s -- Given - -> TcM s (LIE s, -- Free - TcDictBinds s) -- Bindings +tcSimplifyInfer doc tau_tvs wanted_lie + = inferLoop doc (varSetElems tau_tvs) + (lieToList wanted_lie) `thenTc` \ (qtvs, frees, binds, irreds) -> + -- Check for non-generalisable insts + mapTc_ addCantGenErr (filter (not . instCanBeGeneralised) irreds) `thenTc_` -tcSimplifyRank2 local_tvs givens - = zonkLIE givens `thenNF_Tc` \ givens' -> - elimTyCons True - (\tv -> not (tv `elementOfTyVarSet` local_tvs)) - -- This predicate claims that all - -- any non-local tyvars are global, - -- thereby postponing dealing with - -- ambiguity until the enclosing Gen - emptyLIE givens' `thenTc` \ (free, dict_binds, wanteds) -> + returnTc (qtvs, mkLIE frees, binds, map instToId irreds) - checkTc (isEmptyBag wanteds) (reduceErr wanteds) `thenTc_` +inferLoop doc tau_tvs wanteds + = -- Step 1 + zonkTcTyVarsAndFV tau_tvs `thenNF_Tc` \ tau_tvs' -> + mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' -> + tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs -> + let + preds = predsOfInsts wanteds' + qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs + + try_me inst + | isFreeWhenInferring qtvs inst = Free + | isClassDict inst = DontReduceUnlessConstant -- Dicts + | otherwise = ReduceMe -- Lits and Methods + in + -- Step 2 + reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) -> - returnTc (free, dict_binds) + -- Step 3 + if no_improvement then + returnTc (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) `thenTc` \ (qtvs1, frees1, binds1, irreds1) -> + returnTc (qtvs1, frees1, binds `AndMonoBinds` binds1, irreds1) \end{code} -@tcSimplifyTop@ deals with constant @Insts@, using the standard simplification -mechansim with the extra flag to say ``beat out constant insts''. +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} -tcSimplifyTop :: LIE s -> TcM s (TcDictBinds s) -tcSimplifyTop dicts - = tcSimpl True emptyTyVarSet emptyTyVarSet emptyBag dicts `thenTc` \ (_, binds, _) -> - returnTc binds +isFreeWhenInferring :: TyVarSet -> Inst -> Bool +isFreeWhenInferring qtvs inst + = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars + && all inheritablePred (predsOfInst 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[elimTyCons]{@elimTyCons@} +\subsection{tcSimplifyCheck} %* * %************************************************************************ +@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} -elimTyCons :: Bool -- True <=> Simplify const insts - -> (TcTyVar s -> Bool) -- Free tyvar predicate - -> LIE s -- Given - -> LIE s -- Wanted - -> TcM s (LIE s, -- Free - TcDictBinds s, -- Bindings - LIE s -- Remaining wanteds; no dups; - -- dicts only (no Methods) - ) +tcSimplifyCheck + :: SDoc + -> [TcTyVar] -- Quantify over these + -> [Inst] -- Given + -> LIE -- Wanted + -> TcM (LIE, -- Free + TcDictBinds) -- Bindings + +-- tcSimplifyCheck is used when checking expression type signatures, +-- class decls, instance decls etc. +-- Note that we psss isFree (not isFreeAndInheritable) to tcSimplCheck +-- It's important that we can float out non-inheritable predicates +-- Example: (?x :: Int) is ok! +tcSimplifyCheck doc qtvs givens wanted_lie + = tcSimplCheck doc get_qtvs + givens wanted_lie `thenTc` \ (qtvs', frees, binds) -> + returnTc (frees, binds) + where + get_qtvs = zonkTcTyVarsAndFV qtvs + + +-- 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 + -> LIE -- Wanted + -> TcM ([TcTyVar], -- Variables over which to quantify + LIE, -- Free + TcDictBinds) -- Bindings + +tcSimplifyInferCheck doc tau_tvs givens wanted_lie + = tcSimplCheck doc get_qtvs givens wanted_lie + 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 `thenNF_Tc` \ all_tvs' -> + tcGetGlobalTyVars `thenNF_Tc` \ 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 + returnNF_Tc qtvs \end{code} -The bindings returned may mention any or all of ``givens'', so the -order in which the generated binds are put together is {\em tricky}. -Case~4 of @try@ is the general case to see. - -When we do @eTC givens (wanted:wanteds)@ [some details omitted], we... +Here is the workhorse function for all three wrappers. - (1) first look up @wanted@; this gives us one binding to heave in: - wanted = rhs - - (2) step (1) also gave us some @simpler_wanteds@; we simplify - these and get some (simpler-wanted-)bindings {\em that must be - in scope} for the @wanted=rhs@ binding above! - - (3) we simplify the remaining @wanteds@ (recursive call), giving - us yet more bindings. +\begin{code} +tcSimplCheck doc get_qtvs givens wanted_lie + = check_loop givens (lieToList wanted_lie) `thenTc` \ (qtvs, frees, binds, irreds) -> -The final arrangement of the {\em non-recursive} bindings is + -- Complain about any irreducible ones + complainCheck doc givens irreds `thenNF_Tc_` - let in - let wanted = rhs in - let ... + -- Done + returnTc (qtvs, mkLIE frees, binds) -\begin{code} -elimTyCons squash_consts is_free_tv givens wanteds - = eTC givens (bagToList wanteds) `thenTc` \ (_, free, binds, irreds) -> - returnTc (free,binds,irreds) where --- eTC :: LIE s -> [Inst s] --- -> TcM s (LIE s, LIE s, TcDictBinds s, LIE s) - - eTC givens [] = returnTc (givens, emptyBag, EmptyMonoBinds, emptyBag) - - eTC givens (wanted:wanteds) - -- Case 0: same as an existing inst - | maybeToBool maybe_equiv - = eTC givens wanteds `thenTc` \ (givens1, frees, binds, irreds) -> - let - -- Create a new binding iff it's needed - this = expectJust "eTC" maybe_equiv - new_binds | instBindingRequired wanted = (VarMonoBind (instToId wanted) (HsVar (instToId this))) - `AndMonoBinds` binds - | otherwise = binds - in - returnTc (givens1, frees, new_binds, irreds) - - -- Case 1: constrains no type variables at all - -- In this case we have a quick go to see if it has an - -- instance which requires no inputs (ie a constant); if so we use - -- it; if not, we give up on the instance and just heave it out the - -- top in the free result - | isEmptyTyVarSet tvs_of_wanted - = simplify_it squash_consts {- If squash_consts is false, - simplify only if trival -} - givens wanted wanteds - - -- Case 2: constrains free vars only, so fling it out the top in free_ids - | all is_free_tv (tyVarSetToList tvs_of_wanted) - = eTC (wanted `consBag` givens) wanteds `thenTc` \ (givens1, frees, binds, irreds) -> - returnTc (givens1, wanted `consBag` frees, binds, irreds) - - -- Case 3: is a dict constraining only a tyvar, - -- so return it as part of the "wanteds" result - | isTyVarDict wanted - = eTC (wanted `consBag` givens) wanteds `thenTc` \ (givens1, frees, binds, irreds) -> - returnTc (givens1, frees, binds, wanted `consBag` irreds) - - -- Case 4: is not a simple dict, so look up in instance environment - | otherwise - = simplify_it True {- Simplify even if not trivial -} - givens wanted wanteds - where - tvs_of_wanted = tyVarsOfInst wanted - - -- Look for something in "givens" that matches "wanted" - Just the_equiv = maybe_equiv - maybe_equiv = foldBag seqMaybe try Nothing givens - try given | wanted `matchesInst` given = Just given - | otherwise = Nothing - - - simplify_it simplify_always givens wanted wanteds - -- Recover immediately on no-such-instance errors - = recoverTc (returnTc (wanted `consBag` givens, emptyLIE, EmptyMonoBinds, emptyLIE)) - (simplify_one simplify_always givens wanted) - `thenTc` \ (givens1, frees1, binds1, irreds1) -> - eTC givens1 wanteds `thenTc` \ (givens2, frees2, binds2, irreds2) -> - returnTc (givens2, frees1 `plusLIE` frees2, - binds1 `AndMonoBinds` binds2, - irreds1 `plusLIE` irreds2) - - - simplify_one simplify_always givens wanted - | not (instBindingRequired wanted) - = -- No binding required for this chap, so squash right away - lookupInst wanted `thenTc` \ (simpler_wanteds, _) -> - eTC givens simpler_wanteds `thenTc` \ (givens1, frees1, binds1, irreds1) -> - returnTc (wanted `consBag` givens1, frees1, binds1, irreds1) - - | otherwise - = -- An binding is required for this inst - lookupInst wanted `thenTc` \ (simpler_wanteds, bind@(VarMonoBind _ rhs)) -> - - if (not_var rhs && not simplify_always) then - -- Ho ho! It isn't trivial to simplify "wanted", - -- because the rhs isn't a simple variable. Unless the flag - -- simplify_always is set, just give up now and - -- just fling it out the top. - returnTc (wanted `consLIE` givens, unitLIE wanted, EmptyMonoBinds, emptyLIE) - else - -- Aha! Either it's easy, or simplify_always is True - -- so we must do it right here. - eTC givens simpler_wanteds `thenTc` \ (givens1, frees1, binds1, irreds1) -> - returnTc (wanted `consLIE` givens1, frees1, - binds1 `AndMonoBinds` bind, - irreds1) - - not_var :: TcExpr s -> Bool - not_var (HsVar _) = False - not_var other = True + ip_set = mkNameSet (ipNamesOfInsts givens) + + check_loop givens wanteds + = -- Step 1 + mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' -> + mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' -> + get_qtvs `thenNF_Tc` \ 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 + in + reduceContext doc try_me givens' wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) -> + + -- Step 3 + if no_improvement then + returnTc (varSetElems qtvs', frees, binds, irreds) + else + check_loop givens' (irreds ++ frees) `thenTc` \ (qtvs', frees1, binds1, irreds1) -> + returnTc (qtvs', frees1, binds `AndMonoBinds` binds1, irreds1) \end{code} %************************************************************************ %* * -\subsection[elimSCs]{@elimSCs@} +\subsection{tcSimplifyRestricted} %* * %************************************************************************ \begin{code} -elimSCs :: LIE s -- Given; no dups - -> LIE s -- Wanted; no dups; all dictionaries, all - -- constraining just a type variable - -> NF_TcM s (TcDictBinds s, -- Bindings - LIE s) -- Minimal wanted set - -elimSCs givens wanteds - = -- Sort the wanteds so that subclasses occur before superclasses - elimSCs_help - (filterBag isDict givens) -- Filter out non-dictionaries - (sortSC wanteds) - -elimSCs_help :: LIE s -- Given; no dups - -> [Inst s] -- Wanted; no dups; - -> NF_TcM s (TcDictBinds s, -- Bindings - LIE s) -- Minimal wanted set - -elimSCs_help given [] = returnNF_Tc (EmptyMonoBinds, emptyLIE) - -elimSCs_help givens (wanted:wanteds) - = trySC givens wanted `thenNF_Tc` \ (givens1, binds1, irreds1) -> - elimSCs_help givens1 wanteds `thenNF_Tc` \ (binds2, irreds2) -> - returnNF_Tc (binds1 `AndMonoBinds` binds2, irreds1 `plusLIE` irreds2) - - -trySC :: LIE s -- Givens - -> Inst s -- Wanted - -> NF_TcM s (LIE s, -- New givens, - TcDictBinds s, -- Bindings - LIE s) -- Irreducible wanted set - -trySC givens wanted@(Dict _ wanted_class wanted_ty wanted_orig loc) - | not (maybeToBool maybe_best_subclass_chain) - = -- No superclass relationship - returnNF_Tc ((wanted `consLIE` givens), EmptyMonoBinds, unitLIE wanted) +tcSimplifyRestricted -- Used for restricted binding groups + -- i.e. ones subject to the monomorphism restriction + :: SDoc + -> TcTyVarSet -- Free in the type of the RHSs + -> LIE -- Free in the RHSs + -> TcM ([TcTyVar], -- Tyvars to quantify (zonked) + LIE, -- Free + TcDictBinds) -- Bindings + +tcSimplifyRestricted doc tau_tvs wanted_lie + = -- First squash out all methods, to find the constrained tyvars + -- We can't just take the free vars of wanted_lie because that'll + -- have methods that may incidentally mention entirely unconstrained variables + -- e.g. a call to f :: Eq a => a -> b -> b + -- Here, b is unconstrained. A good example would be + -- foo = f (3::Int) + -- We want to infer the polymorphic type + -- foo :: forall b. b -> b + let + wanteds = lieToList wanted_lie + try_me inst = 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. + in + simpleReduceLoop doc try_me wanteds `thenTc` \ (_, _, constrained_dicts) -> - | otherwise - = -- There's a subclass relationship with a "given" - -- Build intermediate dictionaries + -- Next, figure out the tyvars we will quantify over + zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenNF_Tc` \ tau_tvs' -> + tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs -> let - theta = [ (clas, wanted_ty) | clas <- reverse classes ] - -- The reverse is because the list comes back in the "wrong" order I think + constrained_tvs = tyVarsOfInsts constrained_dicts + qtvs = (tau_tvs' `minusVarSet` oclose (predsOfInsts constrained_dicts) gbl_tvs) + `minusVarSet` constrained_tvs in - newDictsAtLoc wanted_orig loc theta `thenNF_Tc` \ (intermediates, _) -> - -- Create bindings for the wanted dictionary and the intermediates. - -- Later binds may depend on earlier ones, so each new binding is pushed - -- on the front of the accumulating parameter list of bindings + -- The first step may have squashed more methods than + -- necessary, so try again, this time 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 + mapNF_Tc zonkInst (lieToList wanted_lie) `thenNF_Tc` \ wanteds' -> let - mk_bind (dict,clas) dict_sub@(Dict _ dict_sub_class ty _ _) - = ((dict_sub, dict_sub_class), - (VarMonoBind (instToId dict) - (DictApp (TyApp (HsVar (RealId (classSuperDictSelId dict_sub_class - clas))) - [ty]) - [instToId dict_sub]))) - (_, new_binds) = mapAccumR mk_bind (wanted,wanted_class) (given : intermediates) + try_me inst | isFreeWrtTyVars qtvs inst = Free + | otherwise = ReduceMe in - returnNF_Tc (wanted `consLIE` givens `plusLIE` listToBag intermediates, - andMonoBinds new_binds, - emptyLIE) + reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) -> + ASSERT( no_improvement ) + ASSERT( null irreds ) + -- No need to loop because simpleReduceLoop will have + -- already done any improvement necessary - where - maybe_best_subclass_chain = foldBag choose_best find_subclass_chain Nothing givens - Just (given, classes, _) = maybe_best_subclass_chain + returnTc (varSetElems qtvs, mkLIE frees, 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: - choose_best c1@(Just (_,_,n1)) c2@(Just (_,_,n2)) | n1 <= n2 = c1 - | otherwise = c2 - choose_best Nothing c2 = c2 - choose_best c1 Nothing = c1 + f_spec = (f :: Int -> Int) - find_subclass_chain given@(Dict _ given_class given_ty _ _) - | wanted_ty `eqSimpleTy` given_ty - = case (wanted_class `isSuperClassOf` given_class) of +and we want to end up with - Just classes -> Just (given, - classes, - length classes) + f_spec = _inline_me_ (f Int dNumInt) - Nothing -> Nothing +But that means that we must simplify the Method for f to (f Int dNumInt)! +So tcSimplifyToDicts squeezes out all Methods. - | otherwise = Nothing +IMPORTANT NOTE: we *don't* want to do superclass commoning up. Consider + fromIntegral :: (Integral a, Num b) => a -> b + {-# RULES "foo" fromIntegral = id :: Int -> Int #-} -sortSC :: LIE s -- Expected to be all dicts (no MethodIds), all of - -- which constrain type variables - -> [Inst s] -- Sorted with subclasses before superclasses +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 matching. Instead we want + + forall dIntegralInt, dNumInt. + fromIntegral Int Int dIntegralInt dNumInt = id Int + +Hence "DontReduce NoSCs" + +\begin{code} +tcSimplifyToDicts :: LIE -> TcM ([Inst], TcDictBinds) +tcSimplifyToDicts wanted_lie + = simpleReduceLoop doc try_me wanteds `thenTc` \ (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 ) + returnTc (irreds, binds) -sortSC dicts = sortLt lt (bagToList dicts) where - (Dict _ c1 ty1 _ _) `lt` (Dict _ c2 ty2 _ _) - = maybeToBool (c2 `isSuperClassOf` c1) - -- The ice is a bit thin here because this "lt" isn't a total order - -- But it *is* transitive, so it works ok + doc = text "tcSimplifyToDicts" + wanteds = lieToList wanted_lie + + -- Reduce methods and lits only; stop as soon as we get a dictionary + try_me inst | isDict inst = DontReduce NoSCs + | otherwise = ReduceMe \end{code} %************************************************************************ %* * -\subsection[simple]{@Simple@ versions} +\subsection{Filtering at a dynamic binding} %* * %************************************************************************ -Much simpler versions when there are no bindings to make! +When we have + let ?x = R in B -@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. +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. -\begin{code} -tcSimplifyThetas :: (Class -> ClassInstEnv) -- How to find the ClassInstEnv - -> [(Class, TauType)] -- Given - -> [(Class, TauType)] -- Wanted - -> TcM s [(Class, TauType)] +Actually, the constraints from B might improve the types in ?x. For example + f :: (?x::Int) => Char -> Char + let ?x = 3 in f 'c' -tcSimplifyThetas inst_mapper given wanted - = elimTyConsSimple inst_mapper wanted `thenTc` \ wanted1 -> - returnTc (elimSCsSimple given wanted1) -\end{code} - -@tcSimplifyCheckThetas@ just checks class-type constraints, essentially; -used with \tr{default} declarations. We are only interested in -whether it worked or not. +then the constraint (?x::Int) arising from the call to f will +force the binding for ?x to be of type Int. \begin{code} -tcSimplifyCheckThetas :: [(Class, TauType)] -- Simplify this to nothing at all - -> TcM s () +tcSimplifyIPs :: [Inst] -- The implicit parameters bound here + -> LIE + -> TcM (LIE, TcDictBinds) +tcSimplifyIPs given_ips wanted_lie + = simpl_loop given_ips wanteds `thenTc` \ (frees, binds) -> + returnTc (mkLIE frees, binds) + where + doc = text "tcSimplifyIPs" <+> ppr given_ips + wanteds = lieToList wanted_lie + ip_set = mkNameSet (ipNamesOfInsts given_ips) -tcSimplifyCheckThetas theta - = elimTyConsSimple classInstEnv theta `thenTc` \ theta1 -> - ASSERT( null theta1 ) - returnTc () -\end{code} + -- Simplify any methods that mention the implicit parameter + try_me inst | isFreeWrtIPs ip_set inst = Free + | otherwise = ReduceMe + simpl_loop givens wanteds + = mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' -> + mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' -> -\begin{code} -elimTyConsSimple :: (Class -> ClassInstEnv) - -> [(Class,Type)] - -> TcM s [(Class,Type)] -elimTyConsSimple inst_mapper theta - = elim theta - where - elim [] = returnTc [] - elim ((clas,ty):rest) = elim_one clas ty `thenTc` \ r1 -> - elim rest `thenTc` \ r2 -> - returnTc (r1++r2) - - elim_one clas ty - = case getTyVar_maybe ty of - - Just tv -> returnTc [(clas,ty)] - - otherwise -> recoverTc (returnTc []) $ - lookupSimpleInst (inst_mapper clas) clas ty `thenTc` \ theta -> - elim theta - -elimSCsSimple :: [(Class,Type)] -- Given - -> [(Class,Type)] -- Wanted - -> [(Class,Type)] -- Subset of wanted; no dups, no subclass relnships - -elimSCsSimple givens [] = [] -elimSCsSimple givens (c_t@(clas,ty) : rest) - | any (`subsumes` c_t) givens || - any (`subsumes` c_t) rest -- (clas,ty) is old hat - = elimSCsSimple givens rest - | otherwise -- (clas,ty) is new - = c_t : elimSCsSimple (c_t : givens) rest - where - rest' = elimSCsSimple rest - (c1,t1) `subsumes` (c2,t2) = t1 `eqSimpleTy` t2 && - (c1 == c2 || maybeToBool (c2 `isSuperClassOf` c1)) --- We deal with duplicates here ^^^^^^^^ --- It's a simple place to do it, although it's done in elimTyCons in the --- full-blown version of the simpifier. + reduceContext doc try_me givens' wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) -> + + if no_improvement then + ASSERT( null irreds ) + returnTc (frees, binds) + else + simpl_loop givens' (irreds ++ frees) `thenTc` \ (frees1, binds1) -> + returnTc (frees1, binds `AndMonoBinds` binds1) \end{code} + %************************************************************************ %* * \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@} @@ -572,53 +970,630 @@ For each method @Inst@ in the @init_lie@ that mentions one of the @LIE@), as well as the @HsBinds@ generated. \begin{code} -bindInstsOfLocalFuns :: LIE s -> [TcIdBndr s] -> TcM s (LIE s, TcMonoBinds s) +bindInstsOfLocalFuns :: LIE -> [TcId] -> TcM (LIE, TcMonoBinds) bindInstsOfLocalFuns init_lie local_ids - = foldrTc bind_inst (emptyBag, EmptyMonoBinds) (bagToList init_lie) + | null overloaded_ids + -- Common case + = returnTc (init_lie, EmptyMonoBinds) + + | otherwise + = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) -> + ASSERT( null irreds ) + returnTc (mkLIE frees, binds) + where + doc = text "bindInsts" <+> ppr local_ids + wanteds = lieToList init_lie + overloaded_ids = filter is_overloaded local_ids + is_overloaded id = isOverloadedTy (idType id) + + 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 | isMethodFor overloaded_set inst = ReduceMe + | otherwise = Free +\end{code} + + +%************************************************************************ +%* * +\subsection{Data types for the reduction mechanism} +%* * +%************************************************************************ + +The main control over context reduction is here + +\begin{code} +data WhatToDo + = ReduceMe -- 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)! + + | DontReduce WantSCs -- Return as irreducible + + | DontReduceUnlessConstant -- Return as irreducible unless it can + -- be reduced to a constant in one step + + | Free -- Return as free + +reduceMe :: Inst -> WhatToDo +reduceMe inst = ReduceMe + +data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses + -- of a predicate when adding it to the avails +\end{code} + + + +\begin{code} +type Avails = FiniteMap Inst Avail + +data Avail + = 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) + + | NoRhs -- Used for Insts like (CCallable f) + -- where no witness is required. + + | Rhs -- Used when there is a RHS + TcExpr -- 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 + [TcExpr] -- 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 NoRhs = text "" +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 + +The loop startes +\begin{code} +extractResults :: Avails + -> [Inst] -- Wanted + -> NF_TcM (TcDictBinds, -- Bindings + [Inst], -- Irreducible ones + [Inst]) -- Free ones + +extractResults avails wanteds + = go avails EmptyMonoBinds [] [] wanteds + where + go avails binds irreds frees [] + = returnNF_Tc (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 NoRhs -> 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 (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 (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) + + Just (Linear n split_inst avail) + -> split n (instToId split_inst) avail w `thenNF_Tc` \ (binds', (rhs:rhss), irreds') -> + go (addToFM avails w (LinRhss rhss)) + (binds `AndMonoBinds` addBind binds' w rhs) + (irreds' ++ irreds) frees (split_inst:ws) + + + add_given avails w + | instBindingRequired w = addToFM avails w (Given (instToId w) True) + | otherwise = addToFM avails w NoRhs + -- NB: make sure that CCallable/CReturnable use NoRhs rather + -- than Given, else we end up with bogus bindings. + + 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 -> Avail -> Inst + -> NF_TcM (TcDictBinds, [TcExpr], [Inst]) +-- (split n split_id avail 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) +split n split_id avail wanted + = go n + where + ty = linearInstType wanted + pair_ty = mkTyConApp pairTyCon [ty,ty] + id = instToId wanted + occ = getOccName id + loc = getSrcLoc id + + go 1 = case avail of + Given id _ -> returnNF_Tc (EmptyMonoBinds, [HsVar id], []) + Irred -> cloneDict wanted `thenNF_Tc` \ w' -> + returnNF_Tc (EmptyMonoBinds, [HsVar (instToId w')], [w']) + + go n = go ((n+1) `div` 2) `thenNF_Tc` \ (binds1, rhss, irred) -> + expand n rhss `thenNF_Tc` \ (binds2, rhss') -> + returnNF_Tc (binds1 `AndMonoBinds` binds2, rhss', irred) + + -- (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) `thenNF_Tc` \ (binds', rhss') -> + returnNF_Tc (binds', head rhss : rhss') + where + go rhss = mapAndUnzipNF_Tc do_one rhss `thenNF_Tc` \ (binds', rhss') -> + returnNF_Tc (andMonoBindList binds', concat rhss') + + do_one rhs = tcGetUnique `thenNF_Tc` \ uniq -> + tcLookupGlobalId fstIdName `thenNF_Tc` \ fst_id -> + tcLookupGlobalId sndIdName `thenNF_Tc` \ snd_id -> + let + x = mkUserLocal occ uniq pair_ty loc + in + returnNF_Tc (VarMonoBind x (mk_app split_id rhs), + [mk_fs_app fst_id ty x, mk_fs_app snd_id ty x]) + +mk_fs_app id ty var = HsVar id `TyApp` [ty,ty] `HsApp` HsVar var + +mk_app id rhs = HsApp (HsVar id) rhs + +addBind binds inst rhs = binds `AndMonoBinds` VarMonoBind (instToId inst) rhs +\end{code} + + +%************************************************************************ +%* * +\subsection[reduce]{@reduce@} +%* * +%************************************************************************ + +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 + = mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' -> + reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) -> + if no_improvement then + returnTc (frees, binds, irreds) + else + simpleReduceLoop doc try_me (irreds ++ frees) `thenTc` \ (frees1, binds1, irreds1) -> + returnTc (frees1, binds `AndMonoBinds` binds1, irreds1) +\end{code} + + + +\begin{code} +reduceContext :: SDoc + -> (Inst -> WhatToDo) + -> [Inst] -- Given + -> [Inst] -- Wanted + -> NF_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 "----------------------", + doc, + text "given" <+> ppr givens, + text "wanted" <+> ppr wanteds, + text "----------------------" + ])) `thenNF_Tc_` + + -- Build the Avail mapping from "givens" + foldlNF_Tc addGiven emptyFM givens `thenNF_Tc` \ init_state -> + + -- Do the real work + reduceList (0,[]) try_me wanteds init_state `thenNF_Tc` \ avails -> + + -- Do improvement, using everything in avails + -- In particular, avails includes all superclasses of everything + tcImprove avails `thenTc` \ no_improvement -> + + extractResults avails wanteds `thenNF_Tc` \ (binds, irreds, frees) -> + + traceTc (text "reduceContext end" <+> (vcat [ + text "----------------------", + doc, + text "given" <+> ppr givens, + text "wanted" <+> ppr wanteds, + text "----", + text "avails" <+> pprAvails avails, + text "frees" <+> ppr frees, + text "no_improvement =" <+> ppr no_improvement, + text "----------------------" + ])) `thenNF_Tc_` + + returnTc (no_improvement, frees, binds, irreds) + +tcImprove avails + = tcGetInstEnv `thenTc` \ inst_env -> + let + preds = [ (pred, pp_loc) + | inst <- keysFM avails, + let pp_loc = pprInstLoc (instLoc inst), + pred <- predsOfInst inst, + predHasFDs pred + ] + -- 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 + eqns = improve (classInstEnv inst_env) preds + in + if null eqns then + returnTc True + else + traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns)) `thenNF_Tc_` + mapTc_ unify eqns `thenTc_` + returnTc False + where + unify ((qtvs, t1, t2), doc) + = tcAddErrCtxt doc $ + tcInstTyVars (varSetElems qtvs) `thenNF_Tc` \ (_, _, tenv) -> + unifyTauTy (substTy tenv t1) (substTy tenv t2) +\end{code} + +The main context-reduction function is @reduce@. Here's its game plan. + +\begin{code} +reduceList :: (Int,[Inst]) -- Stack (for err msgs) + -- along with its depth + -> (Inst -> WhatToDo) + -> [Inst] + -> Avails + -> TcM Avails +\end{code} + +@reduce@ is passed + try_me: given an inst, this function returns + Reduce reduce this + DontReduce return this in "irreds" + Free return this in "frees" + + wanteds: The list of insts to reduce + state: An accumulating parameter of type Avails + that contains the state of the algorithm + + 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. + +\begin{code} +reduceList (n,stack) try_me wanteds state + | n > opt_MaxContextReductionDepth + = failWithTc (reduceDepthErr n stack) + + | otherwise + = +#ifdef DEBUG + (if n > 8 then + pprTrace "Jeepers! ReduceContext:" (reduceDepthMsg n 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 ws state' + + -- Base case: we're done! +reduce stack try_me wanted state + -- It's the same as an existing inst, or a superclass thereof + | Just avail <- isAvailable state wanted + = if isLinearInst wanted then + addLinearAvailable state avail wanted `thenNF_Tc` \ (state', wanteds') -> + reduceList stack try_me wanteds' state' + else + returnTc state -- No op for non-linear things + + | otherwise + = case try_me wanted of { + + DontReduce want_scs -> addIrred want_scs state wanted + + ; 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 + + ; ReduceMe -> -- It should be reduced + lookupInst wanted `thenNF_Tc` \ lookup_result -> + case lookup_result of + GenInst wanteds' rhs -> reduceList stack try_me wanteds' state `thenTc` \ state' -> + addWanted state' wanted rhs wanteds' + SimpleInst rhs -> addWanted state wanted rhs [] + + NoInstance -> -- No such instance! + -- Add it and its superclasses + addIrred AddSCs state wanted + + } + where + try_simple do_this_otherwise + = lookupInst wanted `thenNF_Tc` \ lookup_result -> + case lookup_result of + SimpleInst rhs -> addWanted state wanted rhs [] + other -> do_this_otherwise state wanted +\end{code} + + +\begin{code} +------------------------- +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 + +addLinearAvailable :: Avails -> Avail -> Inst -> NF_TcM (Avails, [Inst]) +addLinearAvailable avails avail wanted + | need_split avail + = tcLookupGlobalId splitIdName `thenNF_Tc` \ split_id -> + newMethodAtLoc (instLoc wanted) split_id + [linearInstType wanted] `thenNF_Tc` \ (split_inst,_) -> + returnNF_Tc (addToFM avails wanted (Linear 2 split_inst avail), [split_inst]) + + | otherwise + = returnNF_Tc (addToFM avails wanted avail', []) + where + avail' = case avail of + Given id _ -> Given id True + Linear n i a -> Linear (n+1) i a + + need_split Irred = True + need_split (Given _ used) = used + need_split (Linear _ _ _) = False + +------------------------- +addFree :: Avails -> Inst -> NF_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 + -- df::Floating a + -- dn::Num a + -- but a is not bound here, then we *don't* want to derive + -- dn from df here lest we lose sharing. + -- +addFree avails free = returnNF_Tc (addToFM avails free IsFree) + +addWanted :: Avails -> Inst -> TcExpr -> [Inst] -> NF_TcM Avails +addWanted avails wanted rhs_expr wanteds +-- Do *not* add superclasses as well. Here's an example of why not +-- class Eq a => Foo a b +-- instance Eq a => Foo [a] a +-- If we are reducing +-- (Foo [t] t) +-- we'll first deduce that it holds (via the instance decl). We +-- must not then overwrite the Eq t constraint with a superclass selection! +-- ToDo: this isn't entirely unsatisfactory, because +-- we may also lose some entirely-legitimate sharing this way + + = ASSERT( not (wanted `elemFM` avails) ) + returnNF_Tc (addToFM avails wanted avail) where - bind_inst inst@(Method uniq (TcId id) tys _ _ orig loc) (insts, binds) - | id `is_elem` local_ids - = lookupInst inst `thenTc` \ (dict_insts, bind) -> - returnTc (listToBag dict_insts `plusLIE` insts, - bind `AndMonoBinds` binds) + avail | instBindingRequired wanted = Rhs rhs_expr wanteds + | otherwise = ASSERT( null wanteds ) NoRhs + +addGiven :: Avails -> Inst -> NF_TcM Avails +addGiven state given = addAvailAndSCs state given (Given (instToId given) False) + +addIrred :: WantSCs -> Avails -> Inst -> NF_TcM Avails +addIrred NoSCs state irred = returnNF_Tc (addToFM state irred Irred) +addIrred AddSCs state irred = addAvailAndSCs state irred Irred - bind_inst some_other_inst (insts, binds) - -- Either not a method, or a method instance for an id not in local_ids - = returnTc (some_other_inst `consBag` insts, binds) +addAvailAndSCs :: Avails -> Inst -> Avail -> NF_TcM Avails +addAvailAndSCs avails wanted avail + = add_scs (addToFM avails wanted avail) wanted - is_elem = isIn "bindInstsOfLocalFuns" +add_scs :: Avails -> Inst -> NF_TcM Avails + -- Add all the superclasses of the Inst to Avails + -- Invariant: the Inst is already in Avails. + +add_scs avails dict + | not (isClassDict dict) + = returnNF_Tc avails + + | otherwise -- It is a dictionary + = newDictsFromOld dict sc_theta' `thenNF_Tc` \ sc_dicts -> + foldlNF_Tc add_sc avails (zipEqual "add_scs" sc_dicts sc_sels) + where + (clas, tys) = getDictClassTys dict + (tyvars, sc_theta, sc_sels, _) = classBigSig clas + sc_theta' = substTheta (mkTopTyVarSubst tyvars tys) sc_theta + + add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses + = case lookupFM avails sc_dict of + Just (Given _ _) -> returnNF_Tc avails -- See Note [SUPER] below + other -> addAvailAndSCs avails sc_dict avail + where + sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys) [instToId dict] + avail = Rhs sc_sel_rhs [dict] \end{code} +Note [SUPER]. We have to be careful here. If we are *given* d1:Ord a, +and want to deduce (d2:C [a]) where + + class Ord a => C a where + instance Ord a => C [a] where ... + +Then we'll use the instance decl to deduce C [a] and then add the +superclasses of C [a] to avails. But we must not overwrite the binding +for d1:Ord a (which is given) with a superclass selection or we'll just +build a loop! Hence looking for Given. Crudely, Given is cheaper +than a selection. + %************************************************************************ %* * -\section[Disambig]{Disambiguation of overloading} +\section{tcSimplifyTop: defaulting} %* * %************************************************************************ +@tcSimplifyTop@ is called once per module to simplify all the constant +and ambiguous Insts. + +We need to be careful of one case. Suppose we have + + instance Num a => Num (Foo a b) where ... + +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?? + +It's OK: the final zonking stage should zap y to (), which is fine. + + +\begin{code} +tcSimplifyTop :: LIE -> TcM TcDictBinds +tcSimplifyTop wanted_lie + = simpleReduceLoop (text "tcSimplTop") reduceMe wanteds `thenTc` \ (frees, binds, 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:_) = not (non_std_tyvars `intersectsVarSet` tyVarsOfInst d) + 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 + -- This group includes both non-existent instances + -- e.g. Num (IO a) and Eq (Int -> Int) + -- and ambiguous dictionaries + -- e.g. Num a + addTopAmbigErrs bad_guys `thenNF_Tc_` + + returnTc (binds `andMonoBinds` andMonoBindList binds_ambig) + where + wanteds = lieToList wanted_lie + + d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2 + +get_tv d = case getDictClassTys d of + (clas, [ty]) -> tcGetTyVar "tcSimplify" ty +get_clas d = case getDictClassTys d of + (clas, [ty]) -> 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. @@ -627,25 +1602,6 @@ 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. -IMPORTANT: @disambiguate@ assumes that its argument dictionaries -constrain only a simple type variable. - -\begin{code} -type SimpleDictInfo s = (Inst s, Class, TcTyVar s) - -disambiguateDicts :: LIE s -> TcM s () - -disambiguateDicts insts - = mapTc disambigOne inst_infos `thenTc` \ binds_lists -> - returnTc () - where - inst_infos = equivClasses cmp_tyvars (map mk_inst_info (bagToList insts)) - (_,_,tv1) `cmp_tyvars` (_,_,tv2) = tv1 `cmp` tv2 - - mk_inst_info dict@(Dict _ clas ty _ _) - = (dict, clas, getTyVar "disambiguateDicts" ty) -\end{code} - @disambigOne@ assumes that its arguments dictionaries constrain all the same type variable. @@ -659,10 +1615,15 @@ Since we're not using the result of @foo@, the result if (presumably) @void@. \begin{code} -disambigOne :: [SimpleDictInfo s] -> TcM s () - -disambigOne dict_infos - | any isNumericClass classes && all isStandardClass classes +disambigGroup :: [Inst] -- All standard classes of form (C a) + -> 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 @@ -673,68 +1634,335 @@ disambigOne dict_infos tcGetDefaultTys `thenNF_Tc` \ default_tys -> let try_default [] -- No defaults work, so fail - = failTc (ambigErr dicts) + = failTc try_default (default_ty : default_tys) - = tryTc (try_default default_tys) $ -- If default_ty fails, we try + = tryTc_ (try_default default_tys) $ -- If default_ty fails, we try -- default_tys instead - tcSimplifyCheckThetas thetas `thenTc` \ _ -> + tcSimplifyDefault theta `thenTc` \ _ -> returnTc default_ty where - thetas = classes `zip` repeat default_ty + theta = [mkClassPred clas [default_ty] | clas <- classes] in -- See if any default works, and if so bind the type variable to it - try_default default_tys `thenTc` \ chosen_default_ty -> - tcInstType [] chosen_default_ty `thenNF_Tc` \ chosen_default_tc_ty -> -- Tiresome! - unifyTauTy chosen_default_tc_ty (mkTyVarTy tyvar) - - | all isCcallishClass classes - = -- Default CCall stuff to (); we don't even both to check that () is an - -- instance of CCallable/CReturnable, because we know it is. - unifyTauTy (mkTyVarTy tyvar) unitTy - + -- If not, add an AmbigErr + recoverTc (addAmbigErrs 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 + unifyTauTy chosen_default_ty (mkTyVarTy tyvar) `thenTc_` + simpleReduceLoop (text "disambig" <+> ppr dicts) + reduceMe dicts `thenTc` \ (frees, binds, ambigs) -> + WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs ) + warnDefault dicts chosen_default_ty `thenTc_` + 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 - = failTc (ambigErr dicts) + = addAmbigErrs dicts `thenNF_Tc_` + returnTc EmptyMonoBinds where - (_,_,tyvar) = head dict_infos -- Should be non-empty - dicts = [dict | (dict,_,_) <- dict_infos] - classes = [clas | (_,clas,_) <- dict_infos] - + tyvar = get_tv (head dicts) -- Should be non-empty + classes = map get_clas dicts \end{code} +[Aside - why the defaulting mechanism is turned off when + dealing with arguments and results to ccalls. +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.) -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? +The interaction between the defaulting mechanism for numeric +values and CC & CR can be a bit puzzling to the user at times. +For example, + + x <- _ccall_ f + if (x /= 0) then + _ccall_ g x + else + return () + +What type has 'x' got here? That depends on the default list +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. + +To try to minimise the potential for surprises here, the +defaulting mechanism is turned off in the presence of +CCallable and CReturnable. + +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} -genCantGenErr insts sty -- Can't generalise these Insts - = hang (ptext SLIT("Cannot generalise these overloadings (in a _ccall_):")) - 4 (vcat (map (ppr sty) (bagToList insts))) +tcSimplifyDeriv :: [TyVar] + -> ThetaType -- Wanted + -> TcM ThetaType -- Needed + +tcSimplifyDeriv tyvars theta + = tcInstTyVars tyvars `thenNF_Tc` \ (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 DataDeclOrigin (substTheta tenv theta) `thenNF_Tc` \ wanteds -> + simpleReduceLoop doc reduceMe wanteds `thenTc` \ (frees, _, irreds) -> + ASSERT( null frees ) -- reduceMe never returns Free + + doptsTc Opt_AllowUndecidableInstances `thenNF_Tc` \ undecidable_ok -> + let + tv_set = mkVarSet tvs + simpl_theta = map dictPred irreds -- reduceMe squashes all non-dicts + + check_pred pred + | isEmptyVarSet pred_tyvars -- Things like (Eq T) should be rejected + = addErrTc (noInstErr pred) + + | not undecidable_ok && not (isTyVarClassPred pred) + -- Check that the returned dictionaries are all of form (C a b) + -- (where a, b are type variables). + -- We allow this 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) + = addErrTc (noInstErr pred) + + | not (pred_tyvars `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. + = addErrTc (badDerivedPred pred) + + | otherwise + = returnNF_Tc () + where + pred_tyvars = tyVarsOfPred pred + + rev_env = mkTopTyVarSubst tvs (mkTyVarTys tyvars) + -- This reverse-mapping is a Royal Pain, + -- but the result should mention TyVars not TcTyVars + in + + mapNF_Tc check_pred simpl_theta `thenNF_Tc_` + checkAmbiguity tvs simpl_theta tv_set `thenTc_` + returnTc (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} -ambigErr dicts sty - = sep [text "Ambiguous context" <+> pprLIE sty lie, - nest 4 (pprLIEInFull sty lie) - ] +tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it + -> TcM () + +tcSimplifyDefault theta + = newDicts DataDeclOrigin theta `thenNF_Tc` \ wanteds -> + simpleReduceLoop doc reduceMe wanteds `thenTc` \ (frees, _, irreds) -> + ASSERT( null frees ) -- try_me never returns Free + mapNF_Tc (addErrTc . noInstErr) irreds `thenNF_Tc_` + if null irreds then + returnTc () + else + failTc where - lie = listToBag dicts -- Yuk + doc = ptext SLIT("default declaration") \end{code} -@reduceErr@ complains if we can't express required dictionaries in -terms of the signature. + +%************************************************************************ +%* * +\section{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} -reduceErr lie sty - = sep [text "Context" <+> pprLIE sty lie, - nest 4 (text "required by inferred type, but missing on a type signature"), - nest 4 (pprLIEInFull sty lie) - ] -\end{code} +groupInsts :: [Inst] -> [[Inst]] +-- Group together insts with the same origin +-- We want to report them together in error messages +groupInsts [] = [] +groupInsts (inst:insts) = (inst:friends) : groupInsts 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 + + +addTopAmbigErrs dicts + = mapNF_Tc (addTopInstanceErrs tidy_env) (groupInsts no_insts) `thenNF_Tc_` + mapNF_Tc (addTopIPErrs tidy_env) (groupInsts bad_ips) `thenNF_Tc_` + mapNF_Tc (addAmbigErr tidy_env) ambigs `thenNF_Tc_` + returnNF_Tc () + where + fixed_tvs = oclose (predsOfInsts tidy_dicts) emptyVarSet + (tidy_env, tidy_dicts) = tidyInsts dicts + (bad_ips, non_ips) = partition is_ip tidy_dicts + (no_insts, ambigs) = partition no_inst non_ips + is_ip d = any isIPPred (predsOfInst d) + no_inst d = not (isTyVarDict d) || tyVarsOfInst d `subVarSet` fixed_tvs + +plural [x] = empty +plural xs = char 's' + +addTopIPErrs tidy_env tidy_dicts + = addInstErrTcM (instLoc (head tidy_dicts)) + (tidy_env, + ptext SLIT("Unbound implicit parameter") <> plural tidy_dicts <+> pprInsts tidy_dicts) + +-- Used for top-level irreducibles +addTopInstanceErrs tidy_env tidy_dicts + = addInstErrTcM (instLoc (head tidy_dicts)) + (tidy_env, + ptext SLIT("No instance") <> plural tidy_dicts <+> + ptext SLIT("for") <+> pprInsts tidy_dicts) + +addAmbigErrs dicts + = mapNF_Tc (addAmbigErr tidy_env) tidy_dicts + where + (tidy_env, tidy_dicts) = tidyInsts dicts +addAmbigErr tidy_env tidy_dict + = addInstErrTcM (instLoc tidy_dict) + (tidy_env, + sep [text "Ambiguous type variable(s)" <+> pprQuotedList ambig_tvs, + nest 4 (text "in the constraint" <+> quotes (pprInst tidy_dict))]) + where + ambig_tvs = varSetElems (tyVarsOfInst tidy_dict) + +warnDefault dicts default_ty + = doptsTc Opt_WarnTypeDefaults `thenTc` \ warn_flag -> + tcAddSrcLoc (get_loc (head dicts)) (warnTc warn_flag warn_msg) + where + -- Tidy them first + (_, tidy_dicts) = tidyInsts dicts + get_loc i = case instLoc i of { (_,loc,_) -> loc } + warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+> + quotes (ppr default_ty), + pprInstsInFull tidy_dicts] + +complainCheck doc givens irreds + = mapNF_Tc zonkInst given_dicts_and_ips `thenNF_Tc` \ givens' -> + mapNF_Tc (addNoInstanceErrs doc givens') (groupInsts irreds) `thenNF_Tc_` + returnNF_Tc () + where + given_dicts_and_ips = filter (not . isMethod) givens + -- Filter out methods, which are only added to + -- the given set as an optimisation + +addNoInstanceErrs what_doc givens dicts + = getDOptsTc `thenNF_Tc` \ dflags -> + tcGetInstEnv `thenNF_Tc` \ inst_env -> + let + (tidy_env1, tidy_givens) = tidyInsts givens + (tidy_env2, tidy_dicts) = tidyMoreInsts tidy_env1 dicts + + doc = vcat [sep [herald <+> pprInsts tidy_dicts, + nest 4 $ ptext SLIT("from the context") <+> pprInsts tidy_givens], + ambig_doc, + ptext SLIT("Probable fix:"), + nest 4 fix1, + nest 4 fix2] + + herald = ptext SLIT("Could not") <+> unambig_doc <+> ptext SLIT("deduce") + unambig_doc | ambig_overlap = ptext SLIT("unambiguously") + | otherwise = empty + + -- The error message when we don't find a suitable instance + -- is complicated by the fact that sometimes this is because + -- there is no instance, and sometimes it's because there are + -- too many instances (overlap). See the comments in TcEnv.lhs + -- with the InstEnv stuff. + + ambig_doc + | not ambig_overlap = empty + | otherwise + = vcat [ptext SLIT("The choice of (overlapping) instance declaration"), + nest 4 (ptext SLIT("depends on the instantiation of") <+> + quotes (pprWithCommas ppr (varSetElems (tyVarsOfInsts tidy_dicts))))] + + fix1 = sep [ptext SLIT("Add") <+> pprInsts tidy_dicts, + ptext SLIT("to the") <+> what_doc] + + fix2 | null instance_dicts + = empty + | otherwise + = ptext SLIT("Or add an instance declaration for") <+> pprInsts instance_dicts + + instance_dicts = [d | d <- tidy_dicts, isClassDict d, not (isTyVarDict d)] + -- Insts for which it is worth suggesting an adding an instance declaration + -- Exclude implicit parameters, and tyvar dicts + + -- Checks for the ambiguous case when we have overlapping instances + ambig_overlap = any ambig_overlap1 dicts + ambig_overlap1 dict + | isClassDict dict + = case lookupInstEnv dflags inst_env clas tys of + NoMatch ambig -> ambig + other -> False + | otherwise = False + where + (clas,tys) = getDictClassTys dict + in + addInstErrTcM (instLoc (head dicts)) (tidy_env2, doc) + +-- Used for the ...Thetas variants; all top level +noInstErr pred = ptext SLIT("No instance for") <+> quotes (ppr pred) + +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)] + +reduceDepthMsg n stack = nest 4 (pprInstsInFull stack) + +----------------------------------------------- +addCantGenErr inst + = addErrTc (sep [ptext SLIT("Cannot generalise these overloadings (in a _ccall_):"), + nest 4 (ppr inst <+> pprInstLoc (instLoc inst))]) +\end{code}