%
-% (c) The GRASP/AQUA Project, Glasgow University, 1992-1995
+% (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,
+
+ tcSimplifyThetas, tcSimplifyCheckThetas,
bindInstsOfLocalFuns
) where
-IMPORT_Trace -- ToDo: rm (debugging)
-import Outputable
-import Pretty
-
-import TcMonad -- typechecking monadic machinery
-import TcMonadFns ( newDicts, applyTcSubstAndExpectTyVars )
-import AbsSyn -- the stuff being typechecked
+#include "HsVersions.h"
-import AbsUniType ( isSuperClassOf, getTyVar, eqTyVar, ltTyVar,
- instantiateThetaTy, isFunType, getUniDataTyCon,
- getSuperDictSelId, InstTyEnv(..)
- IF_ATTACK_PRAGMAS(COMMA isTyVarTy COMMA pprUniType)
- IF_ATTACK_PRAGMAS(COMMA assocMaybe)
+import HsSyn ( MonoBinds(..), HsExpr(..), andMonoBinds, andMonoBindList )
+import TcHsSyn ( TcExpr, TcId,
+ TcMonoBinds, TcDictBinds
)
-import UniType ( UniType(..) ) -- ******* CHEATING ************
-import Disambig ( disambiguateDicts )
-import Errors ( reduceErr, genCantGenErr, Error(..) )
-import Id ( mkInstId )
-import Inst ( extractTyVarsFromInst, isTyVarDict, matchesInst,
+
+import TcMonad
+import Inst ( lookupInst, lookupSimpleInst, LookupInstResult(..),
+ tyVarsOfInst, predsOfInsts, predsOfInst,
+ isDict, isClassDict, instName,
+ isStdClassTyVarDict, isMethodFor,
+ instToId, tyVarsOfInsts,
instBindingRequired, instCanBeGeneralised,
- Inst(..), -- We import the CONCRETE type, because
- -- TcSimplify is allowed to see the rep
- -- of Insts
- InstOrigin, OverloadedLit, InstTemplate
+ newDictsFromOld, instMentionsIPs,
+ getDictClassTys, isTyVarDict,
+ instLoc, pprInst, zonkInst, tidyInsts, tidyMoreInsts,
+ Inst, LIE, pprInsts, pprInstsInFull,
+ mkLIE, lieToList
)
-import InstEnv
-import LIE
-import ListSetOps ( minusList )
-import Maybes ( catMaybes, maybeToBool, Maybe(..) )
-import Util
+import TcEnv ( tcGetGlobalTyVars, tcGetInstEnv )
+import InstEnv ( lookupInstEnv, classInstEnv, InstLookupResult(..) )
+
+import TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, unifyTauTy )
+import TcType ( ThetaType, PredType, mkClassPred, isOverloadedTy,
+ mkTyVarTy, tcGetTyVar, isTyVarClassPred,
+ tyVarsOfPred, getClassPredTys_maybe, isClassPred, isIPPred,
+ inheritablePred, predHasFDs )
+import Id ( idType )
+import NameSet ( mkNameSet )
+import Class ( classBigSig )
+import FunDeps ( oclose, grow, improve )
+import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass )
+
+import Subst ( mkTopTyVarSubst, substTheta, substTy )
+import TysWiredIn ( unitTy )
+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 <=> Don't simplify const insts
- -> [TyVar] -- ``Global'' type variables
- -> [TyVar] -- ``Local'' type variables
- -> [Inst] -- Given; these constrain only local tyvars
- -> [Inst] -- Wanted
- -> TcM ([Inst], -- Free
- [(Inst,TypecheckedExpr)],-- Bindings
- [Inst]) -- Remaining wanteds; no dups
-
-tcSimpl dont_squash_consts global_tvs local_tvs givens wanteds
- =
- -- Make sure the insts and type variables are fixed points of the substitution
- applyTcSubstAndExpectTyVars global_tvs `thenNF_Tc` \ global_tvs ->
- applyTcSubstAndExpectTyVars local_tvs `thenNF_Tc` \ local_tvs ->
- applyTcSubstToInsts givens `thenNF_Tc` \ givens ->
- applyTcSubstToInsts wanteds `thenNF_Tc` \ wanteds ->
- let
- is_elem1 = isIn "tcSimpl1"
- is_elem2 = isIn "tcSimpl2"
- in
- -- Deal with duplicates and type constructors
- elimTyCons
- dont_squash_consts (\tv -> tv `is_elem1` global_tvs)
- givens wanteds `thenTc` \ (globals, tycon_binds, locals_and_ambigs) ->
+ forall Q. Cq => T
- -- Now disambiguate if necessary
- let
- (ambigs, unambigs) = partition (is_ambiguous local_tvs) locals_and_ambigs
- (locals, cant_generalise) = partition instCanBeGeneralised unambigs
- in
- checkTc (not (null cant_generalise)) (genCantGenErr cant_generalise) `thenTc_`
+and float the constraints Ct further outwards.
- (if (null ambigs) then
+Here are the things that *must* be true:
- -- No ambiguous dictionaries. Just bash on with the results
- -- of the elimTyCons
- returnTc (globals, tycon_binds, locals_and_ambigs)
+ (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
- else
+(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.
- -- 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.
+ BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
- -- 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.
+Example: class H x y | x->y where ...
- disambiguateDicts ambigs `thenTc_`
- applyTcSubstToInsts givens `thenNF_Tc` \ givens ->
- applyTcSubstToInsts wanteds `thenNF_Tc` \ wanteds ->
- elimTyCons
- dont_squash_consts (\tv -> tv `is_elem2` global_tvs)
- givens wanteds
+ fv(G) = {a} C = {H a b, H c d}
+ T = c -> b
- ) {- End of the "if" -} `thenTc` \ (globals, tycon_binds, locals) ->
+ (A) Q intersect {a} is empty
+ (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
- -- Deal with superclass relationships
- elimSCs givens locals `thenNF_Tc` \ (sc_binds, locals2) ->
+ So Q can be {c,d}, {b,c,d}
- -- Finished
- returnTc (globals, sc_binds ++ tycon_binds, locals2)
- where
- is_ambiguous local_tvs (Dict _ _ ty _)
- = getTyVar "is_ambiguous" ty `not_elem` local_tvs
- where
- not_elem = isn'tIn "is_ambiguous"
-\end{code}
+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.
-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.
-\begin{code}
-tcSimplify
- :: Bool -- True <=> top level
- -> [TyVar] -- ``Global'' type variables
- -> [TyVar] -- ``Local'' type variables
- -> [Inst] -- Wanted
- -> TcM ([Inst], -- Free
- [(Inst, TypecheckedExpr)],-- Bindings
- [Inst]) -- Remaining wanteds; no dups
-
-tcSimplify top_level global_tvs local_tvs wanteds
- = tcSimpl (not top_level) global_tvs local_tvs [] wanteds
-\end{code}
+-----------------------------------------
+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: you *must* quantify over implicit parameters. See
+isFreeAndInheritable.
+
+BUT WATCH OUT: for *expressions*, this isn't right. Consider:
+
+ (?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).
+
+
+Question 2: type signatures
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+OK, so is it legal to give an 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.
+
+Conclusion: the above type signature is illegal. You'll get a message
+of the form "could not deduce (?y::Int) from ()".
-@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.
+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.
+
\begin{code}
-tcSimplifyAndCheck
- :: Bool -- True <=> top level
- -> [TyVar] -- ``Global'' type variables
- -> [TyVar] -- ``Local'' type variables
- -> [Inst] -- Given
- -> [Inst] -- Wanted
- -> UnifyErrContext -- Context info for error
- -> TcM ([Inst], -- Free
- [(Inst, TypecheckedExpr)]) -- Bindings
-
-tcSimplifyAndCheck top_level global_tvs local_tvs givens wanteds err_ctxt
- = tcSimpl (not top_level) global_tvs local_tvs givens wanteds
- `thenTc` \ (free_insts, binds, wanteds') ->
- checkTc (not (null wanteds')) (reduceErr wanteds' err_ctxt)
- `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 :: [TyVar] -- ``Local'' type variables; guaranteed fixpoint of subst
- -> [Inst] -- Given
- -> UnifyErrContext
- -> TcM ([Inst], -- Free
- [(Inst, TypecheckedExpr)]) -- Bindings
-
-tcSimplifyRank2 local_tvs givens err_ctxt
- = applyTcSubstToInsts givens `thenNF_Tc` \ givens' ->
- elimTyCons False
- (\tv -> not (tv `is_elem` local_tvs))
- -- This predicate claims that all
- -- any non-local tyvars are global,
- -- thereby postponing dealing with
- -- ambiguity until the enclosing Gen
- [] givens' `thenTc` \ (free, dict_binds, wanteds) ->
-
- checkTc (not (null wanteds)) (reduceErr wanteds err_ctxt) `thenTc_`
-
- returnTc (free, dict_binds)
- where
- is_elem = isIn "tcSimplifyRank2"
+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_`
+
+ returnTc (qtvs, mkLIE frees, binds, map instToId irreds)
+
+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
+ | isFreeAndInheritable 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) ->
+
+ -- 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)
+
+ 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)!
\begin{code}
-tcSimplifyTop :: [Inst] -> TcM [(Inst, TypecheckedExpr)]
-tcSimplifyTop dicts
- = tcSimpl False [] [] [] dicts `thenTc` \ (_, binds, _) ->
- returnTc binds
+isFreeAndInheritable qtvs inst
+ = isFree qtvs inst -- Constrains no quantified vars
+ && all inheritablePred (predsOfInst inst) -- And no implicit parameter involved
+ -- (see "Notes on implicit parameters")
+
+isFree qtvs inst
+ = not (tyVarsOfInst inst `intersectsVarSet` qtvs)
\end{code}
-@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.
+
+%************************************************************************
+%* *
+\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}
-tcSimplifyThetas :: (Class -> TauType -> InstOrigin) -- Creates an origin for the dummy dicts
- -> [(Class, TauType)] -- Simplify this
- -> TcM [(Class, TauType)] -- Result
+tcSimplifyCheck
+ :: SDoc
+ -> [TcTyVar] -- Quantify over these
+ -> [Inst] -- Given
+ -> LIE -- Wanted
+ -> TcM (LIE, -- Free
+ TcDictBinds) -- Bindings
+
+-- tcSimplifyCheck is used when checking exprssion 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 isFree 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 isFreeAndInheritable 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}
-tcSimplifyThetas mk_inst_origin theta
- = let
- dicts = map mk_dummy_dict theta
- in
- -- Do the business (this is just the heart of "tcSimpl")
- elimTyCons False (\tv -> False) [] dicts `thenTc` \ (_, _, dicts2) ->
+Here is the workhorse function for all three wrappers.
- -- Deal with superclass relationships
- elimSCs [] dicts2 `thenNF_Tc` \ (_, dicts3) ->
+\begin{code}
+tcSimplCheck doc is_free get_qtvs givens wanted_lie
+ = check_loop givens (lieToList wanted_lie) `thenTc` \ (qtvs, frees, binds, irreds) ->
- returnTc (map unmk_dummy_dict dicts3)
- where
- mk_dummy_dict (clas, ty)
- = Dict uniq clas ty (mk_inst_origin clas ty)
+ -- Complain about any irreducible ones
+ complainCheck doc givens irreds `thenNF_Tc_`
- uniq = panic "tcSimplifyThetas:uniq"
+ -- Done
+ returnTc (qtvs, mkLIE frees, binds)
- unmk_dummy_dict (Dict _ clas ty _) = (clas, ty)
+ where
+ 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 | is_free qtvs' 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}
-@tcSimplifyCheckThetas@ just checks class-type constraints, essentially;
-used with \tr{default} declarations. We are only interested in
-whether it worked or not.
+
+%************************************************************************
+%* *
+\subsection{tcSimplifyRestricted}
+%* *
+%************************************************************************
\begin{code}
-tcSimplifyCheckThetas :: InstOrigin -- context; for error msg
- -> [(Class, TauType)] -- Simplify this
- -> TcM ()
+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) ->
-tcSimplifyCheckThetas origin theta
- = let
- dicts = map mk_dummy_dict theta
+ -- Next, figure out the tyvars we will quantify over
+ zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenNF_Tc` \ tau_tvs' ->
+ tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
+ let
+ constrained_tvs = tyVarsOfInsts constrained_dicts
+ qtvs = (tau_tvs' `minusVarSet` oclose (predsOfInsts constrained_dicts) gbl_tvs)
+ `minusVarSet` constrained_tvs
in
- -- Do the business (this is just the heart of "tcSimpl")
- elimTyCons False (\tv -> False) [] dicts `thenTc` \ _ ->
- returnTc ()
- where
- mk_dummy_dict (clas, ty)
- = Dict uniq clas ty origin
+ -- 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 isFreeAndInheritable) in which we quantify over
+ -- all *non-inheritable* constraints too. This implements choice
+ -- (B) under "implicit parameter and monomorphism" above.
+ mapNF_Tc zonkInst (lieToList wanted_lie) `thenNF_Tc` \ wanteds' ->
+ let
+ try_me inst | isFree qtvs inst = Free
+ | otherwise = ReduceMe
+ in
+ 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
- uniq = panic "tcSimplifyCheckThetas:uniq"
+ returnTc (varSetElems qtvs, mkLIE frees, binds)
\end{code}
%************************************************************************
%* *
-\subsection[elimTyCons]{@elimTyCons@}
+\subsection{tcSimplifyToDicts}
%* *
%************************************************************************
-\begin{code}
-elimTyCons :: Bool -- True <=> Don't simplify const insts
- -> (TyVar -> Bool) -- Free tyvar predicate
- -> [Inst] -- Given
- -> [Inst] -- Wanted
- -> TcM ([Inst], -- Free
- [(Inst, TypecheckedExpr)], -- Bindings
- [Inst] -- Remaining wanteds; no dups;
- -- dicts only (no Methods)
- )
-\end{code}
+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
-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.
+ f_spec = _inline_me_ (f Int dNumInt)
-When we do @eTC givens (wanted:wanteds)@ [some details omitted], we...
+But that means that we must simplify the Method for f to (f Int dNumInt)!
+So tcSimplifyToDicts squeezes out all Methods.
- (1) first look up @wanted@; this gives us one binding to heave in:
- wanted = rhs
+IMPORTANT NOTE: we *don't* want to do superclass commoning up. Consider
- (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!
+ fromIntegral :: (Integral a, Num b) => a -> b
+ {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
- (3) we simplify the remaining @wanteds@ (recursive call), giving
- us yet more bindings.
+Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
+want to get
-The final arrangement of the {\em non-recursive} bindings is
+ forall dIntegralInt.
+ fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
- let <simpler-wanted-binds> in
- let wanted = rhs in
- let <yet-more-bindings> ...
+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}
-elimTyCons dont_squash_consts is_free_tv givens wanteds
- = eTC givens wanteds
+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)
+
where
- eTC :: [Inst] -> [Inst]
- -> TcM ([Inst], [(Inst, TypecheckedExpr)], [Inst])
-
- eTC _ [] = returnTc ([], [], [])
-
- eTC givens (wanted:wanteds) = try givens wanted wanteds
- (extractTyVarsFromInst wanted)
- (find_equiv givens wanted)
- -- find_equiv looks in "givens" for an inst equivalent to "wanted"
- -- This is used only in Case 2 below; it's like a guard which also
- -- returns a result.
-
- try :: [Inst] -> Inst -> [Inst] -> [TyVar] -> (Maybe Inst)
- -> TcM ([Inst], [(Inst, TypecheckedExpr)], [Inst])
-
- -- Case 0: same as existing dict, so build a simple binding
- try givens wanted wanteds tvs_of_wanted (Just this)
- = eTC givens wanteds `thenTc` \ (frees, binds, wanteds') ->
- let
- -- Create a new binding iff it's needed
- new_binds | instBindingRequired wanted = (wanted, Var (mkInstId this)):binds
- | otherwise = binds
- in
- returnTc (frees, new_binds, wanteds')
-
- -- 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
- try givens wanted wanteds tvs_of_wanted _ | null tvs_of_wanted
- = simplify_it dont_squash_consts {- If dont_squash_consts is true,
- simplify only if trival -}
- givens wanted wanteds
-
- -- Case 2: constrains free vars only, so fling it out the top in free_ids
- try givens wanted wanteds tvs_of_wanted _
- | all is_free_tv tvs_of_wanted
- = eTC (wanted:givens) wanteds `thenTc` \ (frees, binds, wanteds') ->
- returnTc (wanted:frees, binds, wanteds')
-
- -- Case 3: is a dict constraining only a tyvar,
- -- so return it as part of the "wanteds" result
- try givens wanted wanteds tvs_of_wanted _
- | isTyVarDict wanted
- = eTC (wanted:givens) wanteds `thenTc` \ (frees, binds, wanteds') ->
- returnTc (frees, binds, wanted:wanteds')
-
- -- Case 4: is not a simple dict, so look up in instance environment
- try givens wanted wanteds tvs_of_wanted _
- = simplify_it False {- Simplify even if not trivial -}
- givens wanted wanteds
-
- simplify_it only_if_trivial givens wanted wanteds
- = if not (instBindingRequired wanted) then
- -- No binding required for this chap, so squash right away
- lookupNoBindInst_Tc wanted `thenTc` \ simpler_wanteds ->
-
- eTC givens simpler_wanteds `thenTc` \ (frees1, binds1, wanteds1) ->
- let
- new_givens = [new_given | (new_given,rhs) <- binds1]
- -- Typically binds1 is empty
- in
- eTC givens wanteds `thenTc` \ (frees2, binds2, wanteds2) ->
-
- returnTc (frees1 ++ frees2,
- binds1 ++ binds2,
- wanteds1 ++ wanteds2)
-
- else -- An binding is required for this inst
- lookupInst_Tc wanted `thenTc` \ (rhs, simpler_wanteds) ->
-
- if (only_if_trivial && not_var rhs) then
- -- Ho ho! It isn't trivial to simplify "wanted",
- -- because the rhs isn't a simple variable. The flag
- -- dont_squash_consts tells us to give up now and
- -- just fling it out the top.
- eTC (wanted:givens) wanteds `thenTc` \ (frees, binds, wanteds') ->
- returnTc (wanted:frees, binds, wanteds')
- else
- -- Aha! Either it's easy, or dont_squash_consts is
- -- False, so we must do it right here.
-
- eTC givens simpler_wanteds `thenTc` \ (frees1, binds1, wanteds1) ->
- let
- new_givens = [new_given | (new_given,rhs) <- binds1]
- in
- eTC (new_givens ++ [wanted] ++ wanteds1 ++ givens) wanteds
- `thenTc` \ (frees2, binds2, wanteds2) ->
- returnTc (frees1 ++ frees2,
- binds1 ++ [(wanted, rhs)] ++ binds2,
- wanteds1 ++ wanteds2)
- where
- not_var :: TypecheckedExpr -> Bool
- not_var (Var _) = False
- not_var other = True
-
- find_equiv :: [Inst] -> Inst -> Maybe Inst
- -- Look through the argument list for an inst which is
- -- equivalent to the second arg.
-
- find_equiv [] wanted = Nothing
- find_equiv (given:givens) wanted
- | wanted `matchesInst` given = Just given
- | otherwise = find_equiv givens wanted
+ 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[elimSCs]{@elimSCs@}
+\subsection{Filtering at a dynamic binding}
%* *
%************************************************************************
-\begin{code}
-elimSCs :: [Inst] -- Given; no dups
- -> [Inst] -- Wanted; no dups; all dictionaries, all
- -- constraining just a type variable
- -> NF_TcM ([(Inst,TypecheckedExpr)], -- Bindings
- [Inst]) -- Minimal wanted set
-
-elimSCs givens wanteds
- = -- Sort the wanteds so that subclasses occur before superclasses
- elimSCs_help
- [dict | dict@(Dict _ _ _ _) <- givens] -- Filter out non-dictionaries
- (sortSC wanteds)
-
-elimSCs_help :: [Inst] -- Given; no dups
- -> [Inst] -- Wanted; no dups;
- -> NF_TcM ([(Inst,TypecheckedExpr)],-- Bindings
- [Inst]) -- Minimal wanted set
-
-elimSCs_help given [] = returnNF_Tc ([], [])
-
-elimSCs_help givens (wanted@(Dict _ wanted_class wanted_ty wanted_orig):wanteds)
- = case (trySC givens wanted_class wanted_ty) of
-
- Nothing -> -- No superclass relnship found
- elimSCs_help (wanted:givens) wanteds `thenNF_Tc` \ (binds, wanteds') ->
- returnNF_Tc (binds, wanted:wanteds')
-
- Just (given, classes) -> -- Aha! There's a superclass relnship
-
- -- Build intermediate dictionaries
- let
- theta = [ (clas, wanted_ty) | clas <- classes ]
- in
- newDicts wanted_orig theta `thenNF_Tc` \ intermediates ->
-
- -- Deal with the recursive call
- elimSCs_help (wanted : (intermediates ++ givens)) wanteds
- `thenNF_Tc` \ (binds, wanteds') ->
-
- -- 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
- let
- new_binds = mk_binds wanted wanted_class (intermediates ++ [given]) []
- in
- returnNF_Tc (new_binds ++ binds, wanteds')
- where
- mk_binds :: Inst -- Define this
- -> Class -- ...whose class is this
- -> [Inst] -- In terms of this sub-class chain
- -> [(Inst, TypecheckedExpr)] -- Push the binding on front of these
- -> [(Inst, TypecheckedExpr)]
-
- mk_binds dict clas [] binds_so_far = binds_so_far
- mk_binds dict clas (dict_sub@(Dict _ dict_sub_class ty _):dicts_sub) binds_so_far
- = mk_binds dict_sub dict_sub_class dicts_sub (new_bind:binds_so_far)
- where
- new_bind = (dict, DictApp (TyApp (Var (getSuperDictSelId dict_sub_class clas))
- [ty])
- [mkInstId dict_sub])
-
-
-trySC :: [Inst] -- Givens
- -> Class -> UniType -- Wanted
- -> Maybe (Inst, [Class]) -- Nothing if no link; Just (given, classes)
- -- if wanted can be given in terms of given, with
- -- intermediate classes specified
-trySC givens wanted_class wanted_ty
- = case subclass_relns of
- [] -> Nothing
- ((given, classes, _): _) -> Just (given, classes)
- where
- subclass_relns :: [(Inst, [Class], Int)] -- Subclass of wanted,
- -- intervening classes,
- -- and number of intervening classes
- -- Sorted with shortest link first
- subclass_relns = sortLt reln_lt (catMaybes (map find_subclass_reln givens))
+When we have
+ let ?x = R in B
- reln_lt :: (Inst, [Class], Int) -> (Inst, [Class], Int) -> Bool
- (_,_,n1) `reln_lt` (_,_,n2) = n1 < n2
+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.
- find_subclass_reln given@(Dict _ given_class given_ty _)
- | wanted_ty == given_ty
- = case (wanted_class `isSuperClassOf` given_class) of
+Actually, the constraints from B might improve the types in ?x. For example
- Just classes -> Just (given,
- classes,
- length classes)
+ f :: (?x::Int) => Char -> Char
+ let ?x = 3 in f 'c'
- Nothing -> Nothing
+then the constraint (?x::Int) arising from the call to f will
+force the binding for ?x to be of type Int.
- | otherwise = Nothing
+\begin{code}
+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 ip_names
+ wanteds = lieToList wanted_lie
+ ip_names = map instName given_ips
+ ip_set = mkNameSet ip_names
+ -- Simplify any methods that mention the implicit parameter
+ try_me inst | inst `instMentionsIPs` ip_set = ReduceMe
+ | otherwise = Free
-sortSC :: [Inst] -- Expected to be all dicts (no MethodIds), all of
- -- which constrain type variables
- -> [Inst] -- Sorted with subclasses before superclasses
+ simpl_loop givens wanteds
+ = mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' ->
+ mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
-sortSC dicts = sortLt lt dicts
- where
- (Dict _ c1 ty1 _) `lt` (Dict _ c2 ty2 _)
- = tv1 `ltTyVar` tv2 ||
- (tv1 `eqTyVar` tv2 && maybeToBool (c2 `isSuperClassOf` c1))
- where
- tv1 = getTyVar "sortSC" ty1
- tv2 = getTyVar "sortSC" ty2
+ 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}
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 @Binds@ generated.
+@LIE@), as well as the @HsBinds@ generated.
\begin{code}
-bindInstsOfLocalFuns :: LIE -> [Id] -> NF_TcM (LIE, TypecheckedMonoBinds)
+bindInstsOfLocalFuns :: LIE -> [TcId] -> TcM (LIE, TcMonoBinds)
bindInstsOfLocalFuns init_lie local_ids
- = let
- insts = unMkLIE 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
+
+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 RedState = (Avails, -- What's available
+ [Inst]) -- Insts for which try_me returned Free
+
+type Avails = FiniteMap Inst Avail
+
+data Avail
+ = Irred -- Used for irreducible dictionaries,
+ -- which are going to be lambda bound
+
+ | BoundTo TcId -- Used for dictionaries for which we have a binding
+ -- e.g. those "given" in a signature
+
+ | 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
+
+pprAvails avails = vcat [ppr inst <+> equals <+> pprAvail avail
+ | (inst,avail) <- fmToList avails ]
+
+instance Outputable Avail where
+ ppr = pprAvail
+
+pprAvail NoRhs = text "<no rhs>"
+pprAvail Irred = text "Irred"
+pprAvail (BoundTo x) = text "Bound to" <+> ppr x
+pprAvail (Rhs rhs bs) = ppr rhs <+> braces (ppr bs)
+\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}
+bindsAndIrreds :: Avails
+ -> [Inst] -- Wanted
+ -> (TcDictBinds, -- Bindings
+ [Inst]) -- Irreducible ones
+
+bindsAndIrreds avails wanteds
+ = go avails EmptyMonoBinds [] wanteds
+ where
+ go avails binds irreds [] = (binds, irreds)
+
+ go avails binds irreds (w:ws)
+ = case lookupFM avails w of
+ Nothing -> -- Free guys come out here
+ -- (If we didn't do addFree we could use this as the
+ -- criterion for free-ness, and pick up the free ones here too)
+ go avails binds irreds ws
+
+ Just NoRhs -> go avails binds irreds ws
+
+ Just Irred -> go (addToFM avails w (BoundTo (instToId w))) binds (w:irreds) ws
+
+ Just (BoundTo id) -> go avails new_binds irreds ws
+ where
+ -- For implicit parameters, all occurrences share the same
+ -- Id, so there is no need for synonym bindings
+ new_binds | new_id == id = binds
+ | otherwise = addBind binds new_id (HsVar id)
+ new_id = instToId w
+
+ Just (Rhs rhs ws') -> go avails' (addBind binds id rhs) irreds (ws' ++ ws)
+ where
+ id = instToId w
+ avails' = addToFM avails w (BoundTo id)
+
+addBind binds id rhs = binds `AndMonoBinds` VarMonoBind id 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` \ state@(avails, frees) ->
+
+ -- Do improvement, using everything in avails
+ -- In particular, avails includes all superclasses of everything
+ tcImprove avails `thenTc` \ no_improvement ->
+
+ 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_`
+ let
+ (binds, irreds) = bindsAndIrreds avails wanteds
+ in
+ 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 ppr_eqn 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)
+ ppr_eqn ((qtvs, t1, t2), doc)
+ = vcat [ptext SLIT("forall") <+> braces (pprWithCommas ppr (varSetElems qtvs))
+ <+> ppr t1 <+> ptext SLIT(":=:") <+> ppr t2,
+ nest 2 doc]
+\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]
+ -> RedState
+ -> TcM RedState
+\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 RedState
+ that contains the state of the algorithm
+
+ It returns a RedState.
+
+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
+ | isAvailable state wanted
+ = returnTc state
+
+ | 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 :: RedState -> Inst -> Bool
+isAvailable (avails, _) wanted = wanted `elemFM` avails
+ -- NB: the Ord instance of Inst compares by the class/type info
+ -- *not* by unique. So
+ -- d1::C Int == d2::C Int
+
+-------------------------
+addFree :: RedState -> Inst -> NF_TcM RedState
+ -- 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.
+ --
+ -- 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 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!
+ -- Solution: never put methods in avail till they are captured
+ -- in which case addFree isn't used
+ --
+ -- NB3: make sure that CCallable/CReturnable use NoRhs rather
+ -- than BoundTo, else we end up with bogus bindings.
+ -- c.f. instBindingRequired in addWanted
+addFree (avails, frees) free
+ | isDict free = returnNF_Tc (addToFM avails free avail, free:frees)
+ | otherwise = returnNF_Tc (avails, free:frees)
+ where
+ avail | instBindingRequired free = BoundTo (instToId free)
+ | otherwise = NoRhs
+
+addWanted :: RedState -> Inst -> TcExpr -> [Inst] -> NF_TcM RedState
+addWanted state@(avails, frees) 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 (isAvailable state wanted) )
+ returnNF_Tc (addToFM avails wanted avail, frees)
+ where
+ avail | instBindingRequired wanted = Rhs rhs_expr wanteds
+ | otherwise = ASSERT( null wanteds ) NoRhs
+
+addGiven :: RedState -> Inst -> NF_TcM RedState
+addGiven state given = addAvailAndSCs state given (BoundTo (instToId given))
+
+addIrred :: WantSCs -> RedState -> Inst -> NF_TcM RedState
+addIrred NoSCs (avails,frees) irred = returnNF_Tc (addToFM avails irred Irred, frees)
+addIrred AddSCs state irred = addAvailAndSCs state irred Irred
+
+addAvailAndSCs :: RedState -> Inst -> Avail -> NF_TcM RedState
+addAvailAndSCs (avails, frees) wanted avail
+ = add_avail_and_scs avails wanted avail `thenNF_Tc` \ avails' ->
+ returnNF_Tc (avails', frees)
+
+---------------------
+add_avail_and_scs :: Avails -> Inst -> Avail -> NF_TcM Avails
+add_avail_and_scs avails wanted avail
+ = add_scs (addToFM avails wanted avail) wanted
+
+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 (BoundTo _) -> returnNF_Tc avails -- See Note [SUPER] below
+ other -> add_avail_and_scs 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 BoundTo. Crudely, BoundTo is cheaper
+than a selection.
+
+
+%************************************************************************
+%* *
+\section{tcSimplifyTop: defaulting}
+%* *
+%************************************************************************
+
+
+If a dictionary constrains a type variable which is
+ * 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
+ * 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.
+
+So here is the function which does the work. It takes the ambiguous
+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.
+
+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") try_me 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
- bind_insts insts [] EmptyMonoBinds
+ -- 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
- bind_insts :: [Inst] -- Insts to mangle
- -> [Inst] -- accum. Insts to return
- -> TypecheckedMonoBinds -- accum. Binds to return
- -> NF_TcM (LIE, TypecheckedMonoBinds)
-
- bind_insts [] acc_insts acc_binds
- = returnNF_Tc (mkLIE acc_insts, acc_binds)
-
- bind_insts (inst@(Method uniq id tys orig):insts) acc_insts acc_binds
- | id `is_elem` local_ids
- = noFailTc (lookupInst_Tc inst) `thenNF_Tc` \ (expr, dict_insts) ->
- let
- bind = VarMonoBind (mkInstId inst) expr
- in
- bind_insts insts (dict_insts ++ acc_insts) (bind `AndMonoBinds` acc_binds)
-
- bind_insts (some_other_inst:insts) acc_insts acc_binds
- -- Either not a method, or a method instance for an id not in local_ids
- = bind_insts insts (some_other_inst:acc_insts) acc_binds
-
- is_elem = isIn "bindInstsOfLocalFuns"
+ wanteds = lieToList wanted_lie
+ try_me inst = ReduceMe
+
+ 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}
+
+@disambigOne@ assumes that its arguments dictionaries constrain all
+the same type variable.
+
+ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
+@()@ instead of @Int@. I reckon this is the Right Thing to do since
+the most common use of defaulting is code like:
+\begin{verbatim}
+ _ccall_ foo `seqPrimIO` bar
+\end{verbatim}
+Since we're not using the result of @foo@, the result if (presumably)
+@void@.
+
+\begin{code}
+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
+
+ -- Failure here is caused by there being no type in the
+ -- 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 ->
+ let
+ try_default [] -- No defaults work, so fail
+ = failTc
+
+ try_default (default_ty : default_tys)
+ = tryTc_ (try_default default_tys) $ -- If default_ty fails, we try
+ -- default_tys instead
+ tcSimplifyCheckThetas [] theta `thenTc` \ _ ->
+ returnTc default_ty
+ where
+ theta = [mkClassPred clas [default_ty] | clas <- classes]
+ in
+ -- See if any default works, and if so bind the type variable to it
+ -- 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)
+ try_me 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
+ = addAmbigErrs dicts `thenNF_Tc_`
+ returnTc EmptyMonoBinds
+
+ where
+ try_me inst = ReduceMe -- This reduce should not fail
+ 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.)
+
+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}
+tcSimplifyThetas :: ThetaType -- Wanted
+ -> TcM ThetaType -- Needed
+
+tcSimplifyThetas wanteds
+ = doptsTc Opt_GlasgowExts `thenNF_Tc` \ glaExts ->
+ reduceSimple [] 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 | glaExts = [pred | pred <- irreds,
+ isEmptyVarSet (tyVarsOfPred pred)]
+ | otherwise = [pred | pred <- irreds,
+ not (isTyVarClassPred pred)]
+ in
+ if null bad_guys then
+ returnTc irreds
+ else
+ mapNF_Tc addNoInstErr bad_guys `thenNF_Tc_`
+ failTc
+\end{code}
+
+@tcSimplifyCheckThetas@ just checks class-type constraints, essentially;
+used with \tr{default} declarations. We are only interested in
+whether it worked or not.
+
+\begin{code}
+tcSimplifyCheckThetas :: ThetaType -- Given
+ -> ThetaType -- Wanted
+ -> TcM ()
+
+tcSimplifyCheckThetas givens wanteds
+ = reduceSimple givens wanteds `thenNF_Tc` \ irreds ->
+ if null irreds then
+ returnTc ()
+ else
+ mapNF_Tc addNoInstErr irreds `thenNF_Tc_`
+ failTc
+\end{code}
+
+
+\begin{code}
+type AvailsSimple = FiniteMap PredType Bool
+ -- True => irreducible
+ -- False => given, or can be derived from a given or from an irreducible
+
+reduceSimple :: ThetaType -- Given
+ -> ThetaType -- Wanted
+ -> NF_TcM ThetaType -- Irreducible
+
+reduceSimple givens wanteds
+ = reduce_simple (0,[]) givens_fm wanteds `thenNF_Tc` \ givens_fm' ->
+ returnNF_Tc [pred | (pred,True) <- fmToList givens_fm']
+ where
+ givens_fm = foldl addNonIrred emptyFM givens
+
+reduce_simple :: (Int,ThetaType) -- Stack
+ -> AvailsSimple
+ -> ThetaType
+ -> NF_TcM AvailsSimple
+
+reduce_simple (n,stack) avails wanteds
+ = go avails wanteds
+ where
+ go avails [] = returnNF_Tc avails
+ go avails (w:ws) = reduce_simple_help (n+1,w:stack) avails w `thenNF_Tc` \ avails' ->
+ go avails' ws
+
+reduce_simple_help stack givens wanted
+ | wanted `elemFM` givens
+ = returnNF_Tc givens
+
+ | Just (clas, tys) <- getClassPredTys_maybe wanted
+ = lookupSimpleInst clas tys `thenNF_Tc` \ maybe_theta ->
+ case maybe_theta of
+ Nothing -> returnNF_Tc (addSimpleIrred givens wanted)
+ Just theta -> reduce_simple stack (addNonIrred givens wanted) theta
+
+ | otherwise
+ = returnNF_Tc (addSimpleIrred givens wanted)
+
+addSimpleIrred :: AvailsSimple -> PredType -> AvailsSimple
+addSimpleIrred givens pred
+ = addSCs (addToFM givens pred True) pred
+
+addNonIrred :: AvailsSimple -> PredType -> AvailsSimple
+addNonIrred givens pred
+ = addSCs (addToFM givens pred False) pred
+
+addSCs givens pred
+ | not (isClassPred pred) = givens
+ | otherwise = foldl add givens sc_theta
+ where
+ Just (clas,tys) = getClassPredTys_maybe pred
+ (tyvars, sc_theta_tmpl, _, _) = classBigSig clas
+ sc_theta = substTheta (mkTopTyVarSubst tyvars tys) sc_theta_tmpl
+
+ add givens ct
+ = case lookupFM givens ct of
+ Nothing -> -- Add it and its superclasses
+ addSCs (addToFM givens ct False) ct
+
+ Just True -> -- Set its flag to False; superclasses already done
+ addToFM givens ct False
+
+ Just False -> -- Already done
+ givens
+
+\end{code}
+
+
+%************************************************************************
+%* *
+\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}
+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 `thenNF_Tc` \ givens' ->
+ mapNF_Tc (addNoInstanceErrs doc givens') (groupInsts irreds) `thenNF_Tc_`
+ returnNF_Tc ()
+ where
+ given_dicts = filter isDict givens
+ -- Filter out methods, which are only added to
+ -- the given set as an optimisation
+
+addNoInstanceErrs what_doc givens dicts
+ = 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 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
+addNoInstErr pred
+ = addErrTc (ptext SLIT("No instance for") <+> quotes (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}
projects / ghc-hetmet.git / blobdiff