%
\section[TcSimplify]{TcSimplify}
-Notes:
-
-Inference (local definitions)
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-If the inst constrains a local type variable, then
- [ReduceMe] if it's a literal or method inst, reduce it
-
- [DontReduce] otherwise see whether the inst is just a constant
- if succeed, use it
- if not, add original to context
- This check gets rid of constant dictionaries without
- losing sharing.
-
-If the inst does not constrain a local type variable then
- [Free] then throw it out as free.
-
-Inference (top level definitions)
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-If the inst does not constrain a local type variable, then
- [FreeIfTautological] try for tautology;
- if so, throw it out as free
- (discarding result of tautology check)
- if not, make original inst part of the context
- (eliminating superclasses as usual)
-
-If the inst constrains a local type variable, then
- as for inference (local defns)
-
-
-Checking (local defns)
-~~~~~~~~
-If the inst constrains a local type variable then
- [ReduceMe] reduce (signal error on failure)
-
-If the inst does not constrain a local type variable then
- [Free] throw it out as free.
-
-Checking (top level)
-~~~~~~~~~~~~~~~~~~~~
-If the inst constrains a local type variable then
- as for checking (local defns)
-
-If the inst does not constrain a local type variable then
- as for checking (local defns)
-
-
-
-Checking once per module
-~~~~~~~~~~~~~~~~~~~~~~~~~
-For dicts of the form (C a), where C is a std class
- and "a" is a type variable,
- [DontReduce] add to context
-
-otherwise [ReduceMe] always reduce
-
-[NB: we may generate one Tree [Int] dict per module, so
- sharing is not complete.]
-
-Sort out ambiguity at the end.
-
-Principal types
-~~~~~~~~~~~~~~~
-class C a where
- op :: a -> a
-
-f x = let g y = op (y::Int) in True
-
-Here the principal type of f is (forall a. a->a)
-but we'll produce the non-principal type
- f :: forall a. C Int => a -> a
-
-
-Ambiguity
-~~~~~~~~~
-Consider this:
-
- instance C (T a) Int where ...
- instance C (T a) Bool where ...
-
-and suppose we infer a context
-
- C (T x) y
-
-from some expression, where x and y are type varibles,
-and x is ambiguous, and y is being quantified over.
-Should we complain, or should we generate the type
-
- forall x y. C (T x) y => <type not involving x>
-
-The idea is that at the call of the function we might
-know that y is Int (say), so the "x" isn't really ambiguous.
-Notice that we have to add "x" to the type variables over
-which we generalise.
-
-Something similar can happen even if C constrains only ambiguous
-variables. Suppose we infer the context
-
- C [x]
-
-where x is ambiguous. Then we could infer the type
-
- forall x. C [x] => <type not involving x>
-
-in the hope that at the call site there was an instance
-decl such as
-
- instance Num a => C [a] where ...
-
-and hence the default mechanism would resolve the "a".
\begin{code}
module TcSimplify (
- tcSimplify, tcSimplifyAndCheck, tcSimplifyToDicts,
- tcSimplifyTop, tcSimplifyThetas, tcSimplifyCheckThetas,
- bindInstsOfLocalFuns, partitionPredsOfLIE
+ tcSimplifyInfer, tcSimplifyInferCheck, tcSimplifyCheck,
+ tcSimplifyToDicts, tcSimplifyIPs, tcSimplifyTop,
+ tcSimplifyThetas, tcSimplifyCheckThetas,
+ bindInstsOfLocalFuns
) where
#include "HsVersions.h"
import TcMonad
import Inst ( lookupInst, lookupSimpleInst, LookupInstResult(..),
- tyVarsOfInst,
- isDict, isClassDict, isMethod, notFunDep,
+ tyVarsOfInst, predsOfInsts,
+ isDict, isClassDict,
isStdClassTyVarDict, isMethodFor,
- instToId, instBindingRequired, instCanBeGeneralised,
- newDictFromOld, newFunDepFromDict,
+ instToId, tyVarsOfInsts,
+ instBindingRequired, instCanBeGeneralised,
+ newDictsFromOld, instMentionsIPs,
getDictClassTys, getIPs, isTyVarDict,
- getDictPred_maybe, getMethodTheta_maybe,
instLoc, pprInst, zonkInst, tidyInst, tidyInsts,
Inst, LIE, pprInsts, pprInstsInFull,
- mkLIE, emptyLIE, unitLIE, consLIE, plusLIE,
+ mkLIE,
lieToList
)
import TcEnv ( tcGetGlobalTyVars, tcGetInstEnv )
-import InstEnv ( lookupInstEnv, InstLookupResult(..) )
+import InstEnv ( lookupInstEnv, classInstEnv, InstLookupResult(..) )
-import TcType ( TcTyVarSet )
+import TcType ( zonkTcTyVarsAndFV )
import TcUnify ( unifyTauTy )
import Id ( idType )
+import Name ( Name )
+import NameSet ( mkNameSet )
import Class ( Class, classBigSig )
+import FunDeps ( oclose, grow, improve )
import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass )
import Type ( Type, ClassContext,
- mkTyVarTy, getTyVar,
+ mkTyVarTy, getTyVar,
isTyVarTy, splitSigmaTy, tyVarsOfTypes
)
import Subst ( mkTopTyVarSubst, substClasses )
-import PprType ( pprConstraint )
+import PprType ( pprClassPred )
import TysWiredIn ( unitTy )
import VarSet
import FiniteMap
import ListSetOps ( equivClasses )
import Util ( zipEqual, mapAccumL )
import List ( partition )
-import Maybe ( fromJust )
-import Maybes ( maybeToBool )
import CmdLineOpts
\end{code}
%************************************************************************
%* *
-\subsection[tcSimplify-main]{Main entry function}
+\subsection{NOTES}
%* *
%************************************************************************
-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.
+ --------------------------------------
+ Notes on quantification
+ --------------------------------------
+
+Suppose we are about to do a generalisation step.
+We have in our hand
+
+ G the environment
+ T the type of the RHS
+ C the constraints fromm that RHS
+
+The game is to figure out
+
+ Q the set of type variables over which to quantify
+ Ct the constraints we will *not* quantify over
+ Cq the constraints we will quantify over
+
+So we're going to infer the type
+
+ forall Q. Cq => T
+
+and float the constraints Ct further outwards.
+
+Here are the things that *must* be true:
+
+ (A) Q intersect fv(G) = EMPTY limits how big Q can be
+ (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
+
+(A) says we can't quantify over a variable that's free in the
+environment. (B) says we must quantify over all the truly free
+variables in T, else we won't get a sufficiently general type. We do
+not *need* to quantify over any variable that is fixed by the free
+vars of the environment G.
+
+ BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
+
+Example: class H x y | x->y where ...
+
+ fv(G) = {a} C = {H a b, H c d}
+ T = c -> b
+
+ (A) Q intersect {a} is empty
+ (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
+
+ So Q can be {c,d}, {b,c,d}
+
+Other things being equal, however, we'd like to quantify over as few
+variables as possible: smaller types, fewer type applications, more
+constraints can get into Ct instead of Cq.
+
+
+-----------------------------------------
+We will make use of
+
+ fv(T) the free type vars of T
+
+ oclose(vs,C) The result of extending the set of tyvars vs
+ using the functional dependencies from C
+
+ grow(vs,C) The result of extend the set of tyvars vs
+ using all conceivable links from C.
+
+ E.g. vs = {a}, C = {H [a] b, K (b,Int) c, Eq e}
+ Then grow(vs,C) = {a,b,c}
+
+ Note that grow(vs,C) `superset` grow(vs,simplify(C))
+ That is, simplfication can only shrink the result of grow.
+
+Notice that
+ oclose is conservative one way: v `elem` oclose(vs,C) => v is definitely fixed by vs
+ grow is conservative the other way: if v might be fixed by vs => v `elem` grow(vs,C)
+
+
+-----------------------------------------
+
+Choosing Q
+~~~~~~~~~~
+Here's a good way to choose Q:
+
+ Q = grow( fv(T), C ) \ oclose( fv(G), C )
+
+That is, quantify over all variable that that MIGHT be fixed by the
+call site (which influences T), but which aren't DEFINITELY fixed by
+G. This choice definitely quantifies over enough type variables,
+albeit perhaps too many.
+
+Why grow( fv(T), C ) rather than fv(T)? Consider
+
+ class H x y | x->y where ...
+
+ T = c->c
+ C = (H c d)
+
+ If we used fv(T) = {c} we'd get the type
+
+ forall c. H c d => c -> b
+
+ And then if the fn was called at several different c's, each of
+ which fixed d differently, we'd get a unification error, because
+ d isn't quantified. Solution: quantify d. So we must quantify
+ everything that might be influenced by c.
+
+Why not oclose( fv(T), C )? Because we might not be able to see
+all the functional dependencies yet:
+
+ class H x y | x->y where ...
+ instance H x y => Eq (T x y) where ...
+
+ T = c->c
+ C = (Eq (T c d))
+
+ Now oclose(fv(T),C) = {c}, because the functional dependency isn't
+ apparent yet, and that's wrong. We must really quantify over d too.
+
+
+There really isn't any point in quantifying over any more than
+grow( fv(T), C ), because the call sites can't possibly influence
+any other type variables.
+
+
+
+ --------------------------------------
+ Notes on ambiguity
+ --------------------------------------
+
+It's very hard to be certain when a type is ambiguous. Consider
+
+ class K x
+ class H x y | x -> y
+ instance H x y => K (x,y)
+
+Is this type ambiguous?
+ forall a b. (K (a,b), Eq b) => a -> a
+
+Looks like it! But if we simplify (K (a,b)) we get (H a b) and
+now we see that a fixes b. So we can't tell about ambiguity for sure
+without doing a full simplification. And even that isn't possible if
+the context has some free vars that may get unified. Urgle!
+
+Here's another example: is this ambiguous?
+ forall a b. Eq (T b) => a -> a
+Not if there's an insance decl (with no context)
+ instance Eq (T b) where ...
+
+You may say of this example that we should use the instance decl right
+away, but you can't always do that:
+
+ class J a b where ...
+ instance J Int b where ...
+
+ f :: forall a b. J a b => a -> a
+
+(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 heres 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-ambigous can then float out, and get smashed at top level
+(which squashes out the constants, like Eq (T a) above)
+
+
+ --------------------------------------
+ Notes on implicit parameters
+ --------------------------------------
+
+Consider
+
+ f x = ...?y...
+
+Then we get an LIE like (?y::Int). Doesn't constrain a type variable,
+but we must nevertheless infer a type like
+
+ f :: (?y::Int) => Int -> Int
+
+so that f is passed the value of y at the call site. Is this legal?
+
+ f :: Int -> Int
+ f x = x + ?y
+
+Should f be overloaded on "?y" ? Or does the type signature say that it
+shouldn't be? Our position is that it should be illegal. Otherwise
+you can change the *dynamic* semantics by adding a type signature:
+
+ (let f x = x + ?y -- f :: (?y::Int) => Int -> Int
+ 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)
+
+URK! Let's not do this. So this is illegal:
+
+ f :: Int -> Int
+ f x = x + ?y
+
+BOTTOM LINE: you *must* quantify over implicit parameters.
+
+
+ --------------------------------------
+ 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
+
+
+%************************************************************************
+%* *
+\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}
-tcSimplify
+tcSimplifyInfer
:: SDoc
- -> TcTyVarSet -- ``Local'' type variables
- -- ASSERT: this tyvar set is already zonked
- -> LIE -- Wanted
- -> TcM (LIE, -- Free
- TcDictBinds, -- Bindings
- LIE) -- Remaining wanteds; no dups
-
-tcSimplify str local_tvs wanted_lie
-{- this is just an optimization, and interferes with implicit params,
- disable it for now. same goes for tcSimplifyAndCheck
- | isEmptyVarSet local_tvs
- = returnTc (wanted_lie, EmptyMonoBinds, emptyLIE)
+ -> [TcTyVar] -- 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}
- | otherwise
--}
- = reduceContext str try_me [] wanteds `thenTc` \ (binds, frees, irreds) ->
+
+\begin{code}
+tcSimplifyInfer doc tau_tvs wanted_lie
+ = inferLoop doc 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
- cant_generalise = filter (not . instCanBeGeneralised) irreds
- in
- checkTc (null cant_generalise)
- (genCantGenErr cant_generalise) `thenTc_`
-
- -- Check for ambiguous insts.
- -- You might think these can't happen (I did) because an ambiguous
- -- inst like (Eq a) will get tossed out with "frees", and eventually
- -- dealt with by tcSimplifyTop.
- -- But we can get stuck with
- -- C a b
- -- where "a" is one of the local_tvs, but "b" is unconstrained.
- -- Then we must yell about the ambiguous b
- -- But we must only do so if "b" really is unconstrained; so
- -- we must grab the global tyvars to answer that question
- tcGetGlobalTyVars `thenNF_Tc` \ global_tvs ->
- let
- avail_tvs = local_tvs `unionVarSet` global_tvs
- (irreds', bad_guys) = partition (isEmptyVarSet . ambig_tv_fn) irreds
- ambig_tv_fn dict = tyVarsOfInst dict `minusVarSet` avail_tvs
+ preds = predsOfInsts wanteds'
+ qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
+
+ try_me inst
+ | isFree qtvs inst = Free
+ | isClassDict inst = DontReduceUnlessConstant -- Dicts
+ | otherwise = ReduceMe AddToIrreds -- Lits and Methods
in
- addAmbigErrs ambig_tv_fn bad_guys `thenNF_Tc_`
+ -- 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
+ inferLoop doc tau_tvs wanteds
+\end{code}
+\begin{code}
+isFree qtvs inst
+ = not (tyVarsOfInst inst `intersectsVarSet` qtvs) -- Constrains no quantified vars
+ && null (getIPs inst) -- And no implicit parameter involved
+ -- (see "Notes on implicit parameters")
+\end{code}
- -- Finished
- returnTc (mkLIE frees, binds, mkLIE irreds')
- where
- wanteds = lieToList wanted_lie
- try_me inst
- -- Does not constrain a local tyvar
- | isEmptyVarSet (tyVarsOfInst inst `intersectVarSet` local_tvs)
- && null (getIPs inst)
- = -- if is_top_level then
- -- FreeIfTautological -- Special case for inference on
- -- -- top-level defns
- -- else
- Free
-
- -- We're infering (not checking) the type, and
- -- the inst constrains a local type variable
- | isClassDict inst = DontReduceUnlessConstant -- Dicts
- | otherwise = ReduceMe AddToIrreds -- Lits and Methods
-\end{code}
+%************************************************************************
+%* *
+\subsection{tcSimplifyCheck}
+%* *
+%************************************************************************
-@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.
+@tcSimplifyCheck@ is used when we know exactly the set of variables
+we are going to quantify over.
\begin{code}
-tcSimplifyAndCheck
+tcSimplifyCheck
:: SDoc
- -> TcTyVarSet -- ``Local'' type variables
- -- ASSERT: this tyvar set is already zonked
- -> LIE -- Given; constrain only local tyvars
+ -> [TcTyVar] -- Quantify over these
+ -> [Inst] -- Given
-> LIE -- Wanted
-> TcM (LIE, -- Free
- TcDictBinds) -- Bindings
+ TcDictBinds) -- Bindings
-tcSimplifyAndCheck str local_tvs given_lie wanted_lie
-{-
- | isEmptyVarSet local_tvs
- -- This can happen quite legitimately; for example in
- -- instance Num Int where ...
- = returnTc (wanted_lie, EmptyMonoBinds)
-
- | otherwise
--}
- = reduceContext str try_me givens wanteds `thenTc` \ (binds, frees, irreds) ->
+tcSimplifyCheck doc qtvs givens wanted_lie
+ = checkLoop doc qtvs givens (lieToList wanted_lie) `thenTc` \ (frees, binds, irreds) ->
-- Complain about any irreducible ones
- mapNF_Tc complain irreds `thenNF_Tc_`
+ complainCheck doc givens irreds `thenNF_Tc_`
-- Done
returnTc (mkLIE frees, binds)
+
+checkLoop doc qtvs givens wanteds
+ = -- Step 1
+ zonkTcTyVarsAndFV qtvs `thenNF_Tc` \ qtvs' ->
+ mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' ->
+ mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
+ 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 | isFree qtvs' inst = Free
+ | otherwise = ReduceMe AddToIrreds
+ in
+ -- Step 2
+ reduceContext doc try_me givens' wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
+
+ -- Step 3
+ if no_improvement then
+ returnTc (frees, binds, irreds)
+ else
+ checkLoop doc qtvs givens wanteds
+
+complainCheck doc givens irreds
+ = mapNF_Tc zonkInst given_dicts `thenNF_Tc` \ givens' ->
+ mapNF_Tc (addNoInstanceErr doc given_dicts) irreds `thenNF_Tc_`
+ returnTc ()
where
- givens = lieToList given_lie
- wanteds = lieToList wanted_lie
- given_dicts = filter isClassDict givens
+ given_dicts = filter isDict givens
+ -- Filter out methods, which are only added to
+ -- the given set as an optimisation
+\end{code}
+
+
+
+%************************************************************************
+%* *
+\subsection{tcSimplifyAndCheck}
+%* *
+%************************************************************************
+
+@tcSimplifyInferCheck@ is used when we know the consraints we are to simplify
+against, but we don't know the type variables over which we are going to quantify.
+
+\begin{code}
+tcSimplifyInferCheck
+ :: SDoc
+ -> [TcTyVar] -- fv(T)
+ -> [Inst] -- Given
+ -> LIE -- Wanted
+ -> TcM ([TcTyVar], -- Variables over which to quantify
+ LIE, -- Free
+ TcDictBinds) -- Bindings
+
+tcSimplifyInferCheck doc tau_tvs givens wanted
+ = inferCheckLoop doc tau_tvs givens (lieToList wanted) `thenTc` \ (qtvs, frees, binds, irreds) ->
- try_me inst
- -- Does not constrain a local tyvar
- | isEmptyVarSet (tyVarsOfInst inst `intersectVarSet` local_tvs)
- && (not (isMethod inst) || null (getIPs inst))
- = Free
+ -- Complain about any irreducible ones
+ complainCheck doc givens irreds `thenNF_Tc_`
+
+ -- Done
+ returnTc (qtvs, mkLIE frees, binds)
- -- When checking against a given signature we always reduce
- -- until we find a match against something given, or can't reduce
- | otherwise
- = ReduceMe AddToIrreds
+inferCheckLoop doc tau_tvs givens wanteds
+ = -- Step 1
+ zonkTcTyVarsAndFV tau_tvs `thenNF_Tc` \ tau_tvs' ->
+ mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' ->
+ mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
+ tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
- complain dict = mapNF_Tc zonkInst givens `thenNF_Tc` \ givens ->
- addNoInstanceErr str given_dicts dict
+ let
+ -- Figure out what we are going to generalise 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.
+ qtvs = (tau_tvs' `unionVarSet` tyVarsOfInsts givens') `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
+
+ -- When checking against a given signature we always reduce
+ -- until we find a match against something given, or can't reduce
+ try_me inst | isFree qtvs inst = Free
+ | otherwise = ReduceMe AddToIrreds
+ in
+ -- Step 2
+ 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
+ inferCheckLoop doc tau_tvs givens wanteds
\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.
So tcSimplifyToDicts squeezes out all Methods.
\begin{code}
-tcSimplifyToDicts :: LIE -> TcM (LIE, TcDictBinds)
+tcSimplifyToDicts :: LIE -> TcM ([Inst], TcDictBinds)
tcSimplifyToDicts wanted_lie
- = reduceContext (text "tcSimplifyToDicts") try_me [] wanteds `thenTc` \ (binds, frees, irreds) ->
+ = 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 (mkLIE irreds, binds)
+ returnTc (irreds, binds)
+
where
+ doc = text "tcSimplifyToDicts"
wanteds = lieToList wanted_lie
-- Reduce methods and lits only; stop as soon as we get a dictionary
| otherwise = ReduceMe AddToIrreds
\end{code}
-The following function partitions a LIE by a predicate defined
-over `Pred'icates (an unfortunate overloading of terminology!).
-This means it sometimes has to split up `Methods', in which case
-a binding is generated.
-It is used in `with' bindings to extract from the LIE the implicit
-parameters being bound.
+%************************************************************************
+%* *
+\subsection{Filtering at a dynamic binding}
+%* *
+%************************************************************************
+
+When we have
+ let ?x = R in B
+
+we must discharge all the ?x constraints from B. We also do an improvement
+step; if we have ?x::t1 and ?x::t2 we must unify t1, t2. No need to iterate, though.
\begin{code}
-partitionPredsOfLIE pred lie
- = foldlTc (partPreds pred) (emptyLIE, emptyLIE, EmptyMonoBinds) insts
- where insts = lieToList lie
-
--- warning: the term `pred' is overloaded here!
-partPreds pred (lie1, lie2, binds) inst
- | maybeToBool maybe_pred
- = if pred p then
- returnTc (consLIE inst lie1, lie2, binds)
- else
- returnTc (lie1, consLIE inst lie2, binds)
- where maybe_pred = getDictPred_maybe inst
- Just p = maybe_pred
-
--- the assumption is that those satisfying `pred' are being extracted,
--- so we leave the method untouched when nothing satisfies `pred'
-partPreds pred (lie1, lie2, binds1) inst
- | maybeToBool maybe_theta
- = if any pred theta then
- zonkInst inst `thenTc` \ inst' ->
- tcSimplifyToDicts (unitLIE inst') `thenTc` \ (lie3, binds2) ->
- partitionPredsOfLIE pred lie3 `thenTc` \ (lie1', lie2', EmptyMonoBinds) ->
- returnTc (lie1 `plusLIE` lie1',
- lie2 `plusLIE` lie2',
- binds1 `AndMonoBinds` binds2)
- else
- returnTc (lie1, consLIE inst lie2, binds1)
- where maybe_theta = getMethodTheta_maybe inst
- Just theta = maybe_theta
+tcSimplifyIPs :: [Name] -- The implicit parameters bound here
+ -> LIE
+ -> TcM (LIE, TcDictBinds)
+tcSimplifyIPs ip_names wanted_lie
+ = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
+ -- The irreducible ones should be a subset of the implicit
+ -- parameters we provided
+ ASSERT( all here_ip irreds )
+ returnTc (mkLIE frees, binds)
+
+ where
+ doc = text "tcSimplifyIPs" <+> ppr ip_names
+ wanteds = lieToList wanted_lie
+ ip_set = mkNameSet ip_names
+ here_ip ip = isDict ip && ip `instMentionsIPs` ip_set
-partPreds pred (lie1, lie2, binds) inst
- = returnTc (lie1, consLIE inst lie2, binds)
+ -- Simplify any methods that mention the implicit parameter
+ try_me inst | inst `instMentionsIPs` ip_set = ReduceMe AddToIrreds
+ | otherwise = Free
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
+%* *
+%************************************************************************
+
+When doing a binding group, we may have @Insts@ of local functions.
+For example, we might have...
+\begin{verbatim}
+let f x = x + 1 -- orig local function (overloaded)
+ f.1 = f Int -- two instances of f
+ f.2 = f Float
+ in
+ (f.1 5, f.2 6.7)
+\end{verbatim}
+The point is: we must drop the bindings for @f.1@ and @f.2@ here,
+where @f@ is in scope; those @Insts@ must certainly not be passed
+upwards towards the top-level. If the @Insts@ were binding-ified up
+there, they would have unresolvable references to @f@.
+
+We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
+For each method @Inst@ in the @init_lie@ that mentions one of the
+@Ids@, we create a binding. We return the remaining @Insts@ (in an
+@LIE@), as well as the @HsBinds@ generated.
+
+\begin{code}
+bindInstsOfLocalFuns :: LIE -> [TcId] -> TcM (LIE, TcMonoBinds)
+
+bindInstsOfLocalFuns init_lie local_ids
+ | 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 = case splitSigmaTy (idType id) of
+ (_, theta, _) -> not (null theta)
+
+ 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 AddToIrreds
+ | otherwise = Free
\end{code}
| Free -- Return as free
- | FreeIfTautological -- Return as free iff it's tautological;
- -- if not, return as irreducible
- -- The FreeIfTautological case is to allow the possibility
- -- of generating functions with types like
- -- f :: C Int => Int -> Int
- -- Here, the C Int isn't a tautology presumably because Int
- -- isn't an instance of C in this module; but perhaps it will
- -- be at f's call site(s). Haskell doesn't allow this at
- -- present.
-
data NoInstanceAction
= Stop -- Fail; no error message
-- (Only used when tautology checking.)
\begin{code}
-type RedState s
- = (Avails s, -- What's available
- [Inst], -- Insts for which try_me returned Free
- [Inst] -- Insts for which try_me returned DontReduce
- )
+type RedState = (Avails, -- What's available
+ [Inst]) -- Insts for which try_me returned Free
-type Avails s = FiniteMap Inst Avail
+type Avails = FiniteMap Inst Avail
data Avail
- = Avail
- TcId -- The "main Id"; that is, the Id for the Inst that
- -- caused this avail to be put into the finite map in the first place
- -- It is this Id that is bound to the RHS.
-
- RHS -- The RHS: an expression whose value is that Inst.
- -- The main Id should be bound to this RHS
-
- [TcId] -- Extra Ids that must all be bound to the main Id.
- -- At the end we generate a list of bindings
- -- { i1 = main_id; i2 = main_id; i3 = main_id; ... }
-
-data RHS
- = NoRhs -- Used for irreducible dictionaries,
- -- which are going to be lambda bound, or for those that are
- -- suppplied as "given" when checking againgst a signature.
- --
- -- NoRhs is also used for Insts like (CCallable f)
+ = 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
- Bool -- True => the RHS simply selects a superclass dictionary
- -- from a subclass dictionary.
- -- False => not so.
- -- This is useful info, because superclass selection
- -- is cheaper than building the dictionary using its dfun,
- -- and we can sometimes replace the latter with the former
-
- | PassiveScSel -- Used for as-yet-unactivated RHSs. For example suppose we have
- -- an (Ord t) dictionary; then we put an (Eq t) entry in
- -- the finite map, with an PassiveScSel. Then if the
- -- the (Eq t) binding is ever *needed* we make it an Rhs
- TcExpr
- [Inst] -- List of Insts that are free in the RHS.
- -- If the main Id is subsequently needed, we toss this list into
- -- the needed-inst pool so that we make sure their bindings
- -- will actually be produced.
- --
- -- Invariant: these Insts are already in the finite mapping
-
+ TcExpr -- The RHS
+ [Inst] -- Insts free in the RHS; we need these too
pprAvails avails = vcat (map pprAvail (eltsFM avails))
-pprAvail (Avail main_id rhs ids)
- = ppr main_id <> colon <+> brackets (ppr ids) <+> pprRhs rhs
-
instance Outputable Avail where
ppr = pprAvail
-pprRhs NoRhs = text "<no rhs>"
-pprRhs (Rhs rhs b) = ppr rhs
-pprRhs (PassiveScSel rhs is) = text "passive" <+> ppr rhs
+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 = binds `AndMonoBinds` new_bind
+ new_bind = VarMonoBind new_id (HsVar id)
+ new_id = instToId w
+
+ Just (Rhs rhs ws') -> go avails' (binds `AndMonoBinds` new_bind) irreds (ws' ++ ws)
+ where
+ id = instToId w
+ avails' = addToFM avails w (BoundTo id)
+ new_bind = VarMonoBind id rhs
\end{code}
%* *
%************************************************************************
-The main entry point for context reduction is @reduceContext@:
+When the "what to do" predicate doesn't depend on the quantified type variables,
+matters are easier. We don't need to do any zonking, unless the improvement step
+does something, in which case we zonk before iterating.
+
+The "given" set is always empty.
\begin{code}
-reduceContext :: SDoc -> (Inst -> WhatToDo)
- -> [Inst] -- Given
- -> [Inst] -- Wanted
- -> TcM (TcDictBinds,
- [Inst], -- Free
- [Inst]) -- Irreducible
-
-reduceContext str try_me givens wanteds
- = -- Zonking first
- mapNF_Tc zonkInst givens `thenNF_Tc` \ givens ->
- mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds ->
- -- JRL - process fundeps last. We eliminate fundeps by seeing
- -- what available classes generate them, so we need to process the
- -- classes first. (would it be useful to make LIEs ordered in the first place?)
- let (wantedOther, wantedFds) = partition notFunDep wanteds
- wanteds' = wantedOther ++ wantedFds in
+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 wanteds
+\end{code}
-{-
- pprTrace "reduceContext" (vcat [
+
+
+\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 "----------------------",
- str,
+ 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` \ avails ->
+ foldlNF_Tc addGiven (emptyFM, []) givens `thenNF_Tc` \ init_state ->
-- Do the real work
- reduceList (0,[]) try_me wanteds' (avails, [], []) `thenNF_Tc` \ (avails, frees, irreds) ->
+ 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 ->
- -- Extract the bindings from avails
- let
- binds = foldFM add_bind EmptyMonoBinds avails
-
- add_bind _ (Avail main_id rhs ids) binds
- = foldr add_synonym (add_rhs_bind rhs binds) ids
- where
- add_rhs_bind (Rhs rhs _) binds = binds `AndMonoBinds` VarMonoBind main_id rhs
- add_rhs_bind other binds = binds
-
- -- Add the trivial {x = y} bindings
- -- The main Id can end up in the list when it's first added passively
- -- and then activated, so we have to filter it out. A bit of a hack.
- add_synonym id binds
- | id /= main_id = binds `AndMonoBinds` VarMonoBind id (HsVar main_id)
- | otherwise = binds
- in
{-
- pprTrace ("reduceContext end") (vcat [
+ traceTc (text "reduceContext end" <+> (vcat [
text "----------------------",
- str,
+ doc,
text "given" <+> ppr givens,
text "wanted" <+> ppr wanteds,
text "----",
text "avails" <+> pprAvails avails,
text "frees" <+> ppr frees,
- text "irreds" <+> ppr irreds,
+ text "no_improvement =" <+> ppr no_improvement,
text "----------------------"
- ]) $
+ ])) `thenNF_Tc_`
-}
- returnNF_Tc (binds, frees, irreds)
+ let
+ (binds, irreds) = bindsAndIrreds avails wanteds
+ in
+ returnTc (no_improvement, frees, binds, irreds)
+
+tcImprove avails
+ = tcGetInstEnv `thenTc` \ inst_env ->
+ let
+ preds = predsOfInsts (keysFM avails)
+ -- 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
+ mapTc_ (\ (t1,t2) -> unifyTauTy t1 t2) eqns `thenTc_`
+ returnTc False
\end{code}
The main context-reduction function is @reduce@. Here's its game plan.
-- along with its depth
-> (Inst -> WhatToDo)
-> [Inst]
- -> RedState s
- -> TcM (RedState s)
+ -> RedState
+ -> TcM RedState
\end{code}
@reduce@ is passed
go ws state'
-- Base case: we're done!
-reduce stack try_me wanted state@(avails, frees, irreds)
+reduce stack try_me wanted state
-- It's the same as an existing inst, or a superclass thereof
- | wanted `elemFM` avails
- = returnTc (activate avails wanted, frees, irreds)
+ | isAvailable state wanted
+ = returnTc state
| otherwise
= case try_me wanted of {
- ReduceMe no_instance_action -> -- It should be reduced
- lookupInst wanted `thenNF_Tc` \ lookup_result ->
- case lookup_result of
- GenInst wanteds' rhs -> use_instance wanteds' rhs
- SimpleInst rhs -> use_instance [] rhs
-
- NoInstance -> -- No such instance!
- case no_instance_action of
- Stop -> failTc
- AddToIrreds -> add_to_irreds
- ;
- Free -> -- It's free and this isn't a top-level binding, so just chuck it upstairs
- -- First, see if the inst can be reduced to a constant in one step
- lookupInst wanted `thenNF_Tc` \ lookup_result ->
- case lookup_result of
- SimpleInst rhs -> use_instance [] rhs
- other -> add_to_frees
-
-
-
- ;
- FreeIfTautological -> -- It's free and this is a top level binding, so
- -- check whether it's a tautology or not
- tryTc_
- add_to_irreds -- If tautology trial fails, add to irreds
-
- -- If tautology succeeds, just add to frees
- (reduce stack try_me_taut wanted (avails, [], []) `thenTc_`
- returnTc (avails, wanted:frees, irreds))
-
-
- ;
-
- DontReduce -> add_to_irreds
- ;
-
- DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced)
- -- See if the inst can be reduced to a constant in one step
- lookupInst wanted `thenNF_Tc` \ lookup_result ->
- case lookup_result of
- SimpleInst rhs -> use_instance [] rhs
- other -> add_to_irreds
- }
- where
- -- The three main actions
- add_to_frees = let
- avails' = addFree avails wanted
- -- Add the thing to the avails set so any identical Insts
- -- will be commoned up with it right here
- in
- returnTc (avails', wanted:frees, irreds)
-
- add_to_irreds = addGiven avails wanted `thenNF_Tc` \ avails' ->
- returnTc (avails', frees, wanted:irreds)
-
- use_instance wanteds' rhs = addWanted avails wanted rhs `thenNF_Tc` \ avails' ->
- reduceList stack try_me wanteds' (avails', frees, irreds)
-
-
- -- The try-me to use when trying to identify tautologies
- -- It blunders on reducing as much as possible
- try_me_taut inst = ReduceMe Stop -- No error recovery
-\end{code}
-
-
-\begin{code}
-activate :: Avails s -> Inst -> Avails s
- -- Activate the binding for Inst, ensuring that a binding for the
- -- wanted Inst will be generated.
- -- (Activate its parent if necessary, recursively).
- -- Precondition: the Inst is in Avails already
-
-activate avails wanted
- | not (instBindingRequired wanted)
- = avails
-
- | otherwise
- = case lookupFM avails wanted of
-
- Just (Avail main_id (PassiveScSel rhs insts) ids) ->
- foldl activate avails' insts -- Activate anything it needs
- where
- avails' = addToFM avails wanted avail'
- avail' = Avail main_id (Rhs rhs True) (wanted_id : ids) -- Activate it
-
- Just (Avail main_id other_rhs ids) -> -- Just add to the synonyms list
- addToFM avails wanted (Avail main_id other_rhs (wanted_id : ids))
-
- Nothing -> panic "activate"
- where
- wanted_id = instToId wanted
-
-addWanted avails wanted rhs_expr
- = ASSERT( not (wanted `elemFM` avails) )
- addFunDeps (addToFM avails wanted avail) wanted
- -- NB: we don't add the thing's superclasses too!
- -- Why not? Because addWanted is used when we've successfully used an
- -- instance decl to reduce something; e.g.
- -- d:Ord [a] = dfunOrd (d1:Eq [a]) (d2:Ord a)
- -- Note that we pass the superclasses to the dfun, so they will be "wanted".
- -- If we put the superclasses of "d" in avails, then we might end up
- -- expressing "d1" in terms of "d", which would be a disaster.
- where
- avail = Avail (instToId wanted) rhs []
-
- rhs | instBindingRequired wanted = Rhs rhs_expr False -- Not superclass selection
- | otherwise = NoRhs
-
-addFree :: Avails s -> Inst -> (Avails s)
- -- 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 secon occurrence of truncate, we get
- -- t3 = truncate at f
- -- t4 = t3 at i
- -- When simplifying with i,f free, we might still notice that
- -- t1=t3; but alas, the binding for t2 (which mentions t1)
- -- will continue to float out!
- -- Solution: never put methods in avail till they are captured
- -- in which case addFree isn't used
-addFree avails free
- | isDict free = addToFM avails free (Avail (instToId free) NoRhs [])
- | otherwise = avails
-
-addGiven :: Avails s -> Inst -> NF_TcM (Avails s)
-addGiven avails given
- = -- ASSERT( not (given `elemFM` avails) )
- -- This assertion isn't necessarily true. It's permitted
- -- to given a redundant context in a type signature (eg (Ord a, Eq a) => ...)
- -- and when typechecking instance decls we generate redundant "givens" too.
- addAvail avails given avail
- where
- avail = Avail (instToId given) NoRhs []
-
-addAvail avails wanted avail
- = addSuperClasses (addToFM avails wanted avail) wanted
-
-addSuperClasses :: Avails s -> Inst -> NF_TcM (Avails s)
- -- Add all the superclasses of the Inst to Avails
- -- Invariant: the Inst is already in Avails.
-
-addSuperClasses avails dict
- | not (isClassDict dict)
- = returnNF_Tc avails
-
- | otherwise -- It is a dictionary
- = foldlNF_Tc add_sc avails (zipEqual "addSuperClasses" sc_theta' sc_sels) `thenNF_Tc` \ avails' ->
- addFunDeps avails' dict
- where
- (clas, tys) = getDictClassTys dict
- (tyvars, sc_theta, sc_sels, _) = classBigSig clas
- sc_theta' = substClasses (mkTopTyVarSubst tyvars tys) sc_theta
-
- add_sc avails ((super_clas, super_tys), sc_sel)
- = newDictFromOld dict super_clas super_tys `thenNF_Tc` \ super_dict ->
- let
- sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys)
- [instToId dict]
- in
- case lookupFM avails super_dict of
-
- Just (Avail main_id (Rhs rhs False {- not sc selection -}) ids) ->
- -- Already there, but not as a superclass selector
- -- No need to look at its superclasses; since it's there
- -- already they must be already in avails
- -- However, we must remember to activate the dictionary
- -- from which it is (now) generated
- returnNF_Tc (activate avails' dict)
- where
- avails' = addToFM avails super_dict avail
- avail = Avail main_id (Rhs sc_sel_rhs True) ids -- Superclass selection
-
- Just (Avail _ _ _) -> returnNF_Tc avails
- -- Already there; no need to do anything
-
- Nothing ->
- -- Not there at all, so add it, and its superclasses
- addAvail avails super_dict avail
- where
- avail = Avail (instToId super_dict)
- (PassiveScSel sc_sel_rhs [dict])
- []
-
-addFunDeps :: Avails s -> Inst -> NF_TcM (Avails s)
- -- Add in the functional dependencies generated by the inst
-addFunDeps avails inst
- = newFunDepFromDict inst `thenNF_Tc` \ fdInst_maybe ->
- case fdInst_maybe of
- Nothing -> returnNF_Tc avails
- Just fdInst ->
- let fdAvail = Avail (instToId (fromJust fdInst_maybe)) NoRhs [] in
- addAvail avails fdInst fdAvail
-\end{code}
-
-%************************************************************************
-%* *
-\subsection[simple]{@Simple@ versions}
-%* *
-%************************************************************************
-
-Much simpler versions when there are no bindings to make!
-
-@tcSimplifyThetas@ simplifies class-type constraints formed by
-@deriving@ declarations and when specialising instances. We are
-only interested in the simplified bunch of class/type constraints.
-
-It simplifies to constraints of the form (C a b c) where
-a,b,c are type variables. This is required for the context of
-instance declarations.
-
-\begin{code}
-tcSimplifyThetas :: ClassContext -- Wanted
- -> TcM ClassContext -- 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 = [ct | ct@(clas,tys) <- irreds,
- isEmptyVarSet (tyVarsOfTypes tys)]
- | otherwise = [ct | ct@(clas,tys) <- irreds,
- not (all isTyVarTy tys)]
- in
- if null bad_guys then
- returnTc irreds
- else
- mapNF_Tc addNoInstErr bad_guys `thenNF_Tc_`
- failTc
-\end{code}
-
-@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 :: ClassContext -- Given
- -> ClassContext -- 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 (Class,[Type]) Bool
- -- True => irreducible
- -- False => given, or can be derived from a given or from an irreducible
-
-reduceSimple :: ClassContext -- Given
- -> ClassContext -- Wanted
- -> NF_TcM ClassContext -- Irreducible
-
-reduceSimple givens wanteds
- = reduce_simple (0,[]) givens_fm wanteds `thenNF_Tc` \ givens_fm' ->
- returnNF_Tc [ct | (ct,True) <- fmToList givens_fm']
- where
- givens_fm = foldl addNonIrred emptyFM givens
-
-reduce_simple :: (Int,ClassContext) -- Stack
- -> AvailsSimple
- -> ClassContext
- -> 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@(clas,tys)
- | wanted `elemFM` givens
- = returnNF_Tc givens
-
- | otherwise
- = lookupSimpleInst clas tys `thenNF_Tc` \ maybe_theta ->
-
- case maybe_theta of
- Nothing -> returnNF_Tc (addIrred givens wanted)
- Just theta -> reduce_simple stack (addNonIrred givens wanted) theta
-
-addIrred :: AvailsSimple -> (Class,[Type]) -> AvailsSimple
-addIrred givens ct@(clas,tys)
- = addSCs (addToFM givens ct True) ct
-
-addNonIrred :: AvailsSimple -> (Class,[Type]) -> AvailsSimple
-addNonIrred givens ct@(clas,tys)
- = addSCs (addToFM givens ct False) ct
-
-addSCs givens ct@(clas,tys)
- = foldl add givens sc_theta
- where
- (tyvars, sc_theta_tmpl, _, _) = classBigSig clas
- sc_theta = substClasses (mkTopTyVarSubst tyvars tys) sc_theta_tmpl
+ DontReduce -> addIrred state wanted
- add givens ct@(clas, tys)
- = case lookupFM givens ct of
- Nothing -> -- Add it and its superclasses
- addSCs (addToFM givens ct False) ct
+ ; 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
- Just True -> -- Set its flag to False; superclasses already done
- addToFM givens ct False
+ ; 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
- Just False -> -- Already done
- givens
-
-\end{code}
+ ; ReduceMe no_instance_action -> -- 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 []
-%************************************************************************
-%* *
-\subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
-%* *
-%************************************************************************
+ NoInstance -> -- No such instance!
+ case no_instance_action of
+ Stop -> failTc
+ AddToIrreds -> addIrred state wanted
-When doing a binding group, we may have @Insts@ of local functions.
-For example, we might have...
-\begin{verbatim}
-let f x = x + 1 -- orig local function (overloaded)
- f.1 = f Int -- two instances of f
- f.2 = f Float
- in
- (f.1 5, f.2 6.7)
-\end{verbatim}
-The point is: we must drop the bindings for @f.1@ and @f.2@ here,
-where @f@ is in scope; those @Insts@ must certainly not be passed
-upwards towards the top-level. If the @Insts@ were binding-ified up
-there, they would have unresolvable references to @f@.
+ }
+ 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}
-We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
-For each method @Inst@ in the @init_lie@ that mentions one of the
-@Ids@, we create a binding. We return the remaining @Insts@ (in an
-@LIE@), as well as the @HsBinds@ generated.
\begin{code}
-bindInstsOfLocalFuns :: LIE -> [TcId] -> TcM (LIE, TcMonoBinds)
+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
+
+addGiven :: RedState -> Inst -> NF_TcM RedState
+addGiven state given = add_avail state given (BoundTo (instToId given))
+
+addIrred :: RedState -> Inst -> NF_TcM RedState
+addIrred state irred = add_avail state irred Irred
+
+addWanted :: RedState -> Inst -> TcExpr -> [Inst] -> NF_TcM RedState
+addWanted state wanted rhs_expr wanteds
+ = ASSERT( not (isAvailable state wanted) )
+ add_avail state wanted avail
+ where
+ avail | instBindingRequired wanted = Rhs rhs_expr wanteds
+ | otherwise = ASSERT( null wanteds ) NoRhs
+
+add_avail :: RedState -> Inst -> Avail -> NF_TcM RedState
+add_avail (avails, frees) wanted avail
+ = addAvail avails wanted avail `thenNF_Tc` \ avails' ->
+ returnNF_Tc (avails', frees)
+
+---------------------
+addAvail :: Avails -> Inst -> Avail -> NF_TcM Avails
+addAvail avails wanted avail
+ = addSuperClasses (addToFM avails wanted avail) wanted
-bindInstsOfLocalFuns init_lie local_ids
- | null overloaded_ids || null lie_for_here
- -- Common case
- = returnTc (init_lie, EmptyMonoBinds)
+addSuperClasses :: Avails -> Inst -> NF_TcM Avails
+ -- Add all the superclasses of the Inst to Avails
+ -- Invariant: the Inst is already in Avails.
- | otherwise
- = reduceContext (text "bindInsts" <+> ppr local_ids)
- try_me [] lie_for_here `thenTc` \ (binds, frees, irreds) ->
- ASSERT( null irreds )
- returnTc (mkLIE frees `plusLIE` mkLIE lie_not_for_here, binds)
- where
- overloaded_ids = filter is_overloaded local_ids
- is_overloaded id = case splitSigmaTy (idType id) of
- (_, theta, _) -> not (null theta)
+addSuperClasses avails dict
+ | not (isClassDict dict)
+ = returnNF_Tc avails
- 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
+ | otherwise -- It is a dictionary
+ = newDictsFromOld dict sc_theta' `thenNF_Tc` \ sc_dicts ->
+ foldlNF_Tc add_sc avails (zipEqual "addSuperClasses" sc_dicts sc_sels)
+ where
+ (clas, tys) = getDictClassTys dict
+ (tyvars, sc_theta, sc_sels, _) = classBigSig clas
+ sc_theta' = substClasses (mkTopTyVarSubst tyvars tys) sc_theta
- -- No sense in repeatedly zonking lots of
- -- constant constraints so filter them out here
- (lie_for_here, lie_not_for_here) = partition (isMethodFor overloaded_set)
- (lieToList init_lie)
- try_me inst | isMethodFor overloaded_set inst = ReduceMe AddToIrreds
- | otherwise = Free
+ 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 -> addAvail 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[Disambig]{Disambiguation of overloading}
+\section{tcSimplifyTop: defaulting}
%* *
%************************************************************************
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.
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.
-@tcSimplifyTop@ is called once per module to simplify
-all the constant and ambiguous Insts.
\begin{code}
tcSimplifyTop :: LIE -> TcM TcDictBinds
tcSimplifyTop wanted_lie
- = reduceContext (text "tcSimplTop") try_me [] wanteds `thenTc` \ (binds1, frees, irreds) ->
+ = 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:_) = isEmptyVarSet (tyVarsOfInst d `intersectVarSet` non_std_tyvars)
+ 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
mapTc disambigGroup std_oks `thenTc` \ binds_ambig ->
-- And complain about the ones that don't
- mapNF_Tc complain bad_guys `thenNF_Tc_`
+ addTopAmbigErrs bad_guys `thenNF_Tc_`
- returnTc (binds1 `andMonoBinds` andMonoBindList binds_ambig)
+ returnTc (binds `andMonoBinds` andMonoBindList binds_ambig)
where
wanteds = lieToList wanted_lie
try_me inst = ReduceMe AddToIrreds
d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
- complain d | not (null (getIPs d)) = addTopIPErr d
- | isEmptyVarSet (tyVarsOfInst d) = addTopInstanceErr d
- | otherwise = addAmbigErr tyVarsOfInst d
-
get_tv d = case getDictClassTys d of
(clas, [ty]) -> getTyVar "tcSimplifyTop" ty
get_clas d = case getDictClassTys d of
in
-- See if any default works, and if so bind the type variable to it
-- If not, add an AmbigErr
- recoverTc (complain dicts `thenNF_Tc_` returnTc EmptyMonoBinds) $
+ 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_`
- reduceContext (text "disambig" <+> ppr dicts)
- try_me [] dicts `thenTc` \ (binds, frees, ambigs) ->
+ 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
returnTc EmptyMonoBinds
| otherwise -- No defaults
- = complain dicts `thenNF_Tc_`
+ = addAmbigErrs dicts `thenNF_Tc_`
returnTc EmptyMonoBinds
where
- complain = addAmbigErrs tyVarsOfInst
try_me inst = ReduceMe AddToIrreds -- This reduce should not fail
tyvar = get_tv (head dicts) -- Should be non-empty
classes = map get_clas dicts
]
-Errors and contexts
-~~~~~~~~~~~~~~~~~~~
+
+%************************************************************************
+%* *
+\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 :: ClassContext -- Wanted
+ -> TcM ClassContext -- 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 = [ct | ct@(clas,tys) <- irreds,
+ isEmptyVarSet (tyVarsOfTypes tys)]
+ | otherwise = [ct | ct@(clas,tys) <- irreds,
+ not (all isTyVarTy tys)]
+ in
+ if null bad_guys then
+ returnTc irreds
+ else
+ mapNF_Tc addNoInstErr bad_guys `thenNF_Tc_`
+ failTc
+\end{code}
+
+@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 :: ClassContext -- Given
+ -> ClassContext -- 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 (Class,[Type]) Bool
+ -- True => irreducible
+ -- False => given, or can be derived from a given or from an irreducible
+
+reduceSimple :: ClassContext -- Given
+ -> ClassContext -- Wanted
+ -> NF_TcM ClassContext -- Irreducible
+
+reduceSimple givens wanteds
+ = reduce_simple (0,[]) givens_fm wanteds `thenNF_Tc` \ givens_fm' ->
+ returnNF_Tc [ct | (ct,True) <- fmToList givens_fm']
+ where
+ givens_fm = foldl addNonIrred emptyFM givens
+
+reduce_simple :: (Int,ClassContext) -- Stack
+ -> AvailsSimple
+ -> ClassContext
+ -> 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@(clas,tys)
+ | wanted `elemFM` givens
+ = returnNF_Tc givens
+
+ | otherwise
+ = 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
+
+addSimpleIrred :: AvailsSimple -> (Class,[Type]) -> AvailsSimple
+addSimpleIrred givens ct@(clas,tys)
+ = addSCs (addToFM givens ct True) ct
+
+addNonIrred :: AvailsSimple -> (Class,[Type]) -> AvailsSimple
+addNonIrred givens ct@(clas,tys)
+ = addSCs (addToFM givens ct False) ct
+
+addSCs givens ct@(clas,tys)
+ = foldl add givens sc_theta
+ where
+ (tyvars, sc_theta_tmpl, _, _) = classBigSig clas
+ sc_theta = substClasses (mkTopTyVarSubst tyvars tys) sc_theta_tmpl
+
+ add givens ct@(clas, tys)
+ = 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}
-genCantGenErr insts -- Can't generalise these Insts
- = sep [ptext SLIT("Cannot generalise these overloadings (in a _ccall_):"),
- nest 4 (pprInstsInFull insts)
- ]
+addTopAmbigErrs dicts
+ = mapNF_Tc complain tidy_dicts
+ where
+ fixed_tvs = oclose (predsOfInsts tidy_dicts) emptyVarSet
+ (tidy_env, tidy_dicts) = tidyInsts emptyTidyEnv dicts
+ complain d | not (null (getIPs d)) = addTopIPErr tidy_env d
+ | tyVarsOfInst d `subVarSet` fixed_tvs = addTopInstanceErr tidy_env d
+ | otherwise = addAmbigErr tidy_env d
+
+addTopIPErr tidy_env tidy_dict
+ = addInstErrTcM (instLoc tidy_dict)
+ (tidy_env,
+ ptext SLIT("Unbound implicit parameter") <+> quotes (pprInst tidy_dict))
+
+-- Used for top-level irreducibles
+addTopInstanceErr tidy_env tidy_dict
+ = addInstErrTcM (instLoc tidy_dict)
+ (tidy_env,
+ ptext SLIT("No instance for") <+> quotes (pprInst tidy_dict))
-addAmbigErrs ambig_tv_fn dicts = mapNF_Tc (addAmbigErr ambig_tv_fn) dicts
+addAmbigErrs dicts
+ = mapNF_Tc (addAmbigErr tidy_env) tidy_dicts
+ where
+ (tidy_env, tidy_dicts) = tidyInsts emptyTidyEnv dicts
-addAmbigErr ambig_tv_fn dict
- = addInstErrTcM (instLoc dict)
+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 (ambig_tv_fn tidy_dict)
- (tidy_env, tidy_dict) = tidyInst emptyTidyEnv dict
+ ambig_tvs = varSetElems (tyVarsOfInst tidy_dict)
warnDefault dicts default_ty
= doptsTc Opt_WarnTypeDefaults `thenTc` \ warn_flag ->
warnTc True (vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+> quotes (ppr default_ty),
pprInstsInFull dicts])
-addTopIPErr dict
- = addInstErrTcM (instLoc dict)
- (tidy_env,
- ptext SLIT("Unbound implicit parameter") <+> quotes (pprInst tidy_dict))
- where
- (tidy_env, tidy_dict) = tidyInst emptyTidyEnv dict
-
--- Used for top-level irreducibles
-addTopInstanceErr dict
- = addInstErrTcM (instLoc dict)
- (tidy_env,
- ptext SLIT("No instance for") <+> quotes (pprInst tidy_dict))
- where
- (tidy_env, tidy_dict) = tidyInst emptyTidyEnv dict
-
-- 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.
-addNoInstanceErr str givens dict
+addNoInstanceErr what_doc givens dict
= tcGetInstEnv `thenNF_Tc` \ inst_env ->
let
doc = vcat [sep [herald <+> quotes (pprInst tidy_dict),
quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst tidy_dict))))]
fix1 = sep [ptext SLIT("Add") <+> quotes (pprInst tidy_dict),
- ptext SLIT("to the") <+> str]
+ ptext SLIT("to the") <+> what_doc]
fix2 | isTyVarDict dict || ambig_overlap
= empty
-- Used for the ...Thetas variants; all top level
addNoInstErr (c,ts)
- = addErrTc (ptext SLIT("No instance for") <+> quotes (pprConstraint c ts))
+ = addErrTc (ptext SLIT("No instance for") <+> quotes (pprClassPred c ts))
reduceDepthErr n stack
= vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
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