TcSimplify
\begin{code}
+{-# OPTIONS -w #-}
+-- The above warning supression flag is a temporary kludge.
+-- While working on this module you are encouraged to remove it and fix
+-- any warnings in the module. See
+-- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
+-- for details
+
module TcSimplify (
tcSimplifyInfer, tcSimplifyInferCheck,
tcSimplifyCheck, tcSimplifyRestricted,
tcSimplifyBracket, tcSimplifyCheckPat,
tcSimplifyDeriv, tcSimplifyDefault,
- bindInstsOfLocalFuns
+ bindInstsOfLocalFuns, bindIrreds,
+
+ misMatchMsg
) where
#include "HsVersions.h"
import TcEnv
import InstEnv
import TcGadt
-import TcMType
import TcType
+import TcMType
import TcIface
+import TcTyFuns
+import TypeRep
import Var
-import TyCon
import Name
import NameSet
import Class
import BasicTypes
import VarSet
import VarEnv
+import Module
import FiniteMap
import Bag
import Outputable
+import Maybes
import ListSetOps
import Util
+import UniqSet
import SrcLoc
import DynFlags
The Right Thing is to improve whenever the constraint set changes at
all. Not hard in principle, but it'll take a bit of fiddling to do.
-
-
- --------------------------------------
- Notes on quantification
- --------------------------------------
-
-Suppose we are about to do a generalisation step.
-We have in our hand
+Note [Choosing which variables to quantify]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose we are about to do a generalisation step. We have in our hand
G the environment
T the type of the RHS
(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.
+ (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!
So Q can be {c,d}, {b,c,d}
+In particular, it's perfectly OK to quantify over more type variables
+than strictly necessary; there is no need to quantify over 'b', since
+it is determined by 'a' which is free in the envt, but it's perfectly
+OK to do so. However we must not quantify over 'a' itself.
+
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:
+constraints can get into Ct instead of Cq. Here's a good way to
+choose Q:
Q = grow( fv(T), C ) \ oclose( fv(G), C )
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.
-
+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
Notes on implicit parameters
--------------------------------------
-Question 1: can we "inherit" implicit parameters
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Note [Inheriting implicit parameters]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this:
f x = (x::Int) + ?y
over implicit parameters. See the predicate isFreeWhenInferring.
+Note [Implicit parameters and ambiguity]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+What type should we infer for this?
+ f x = (show ?y, x::Int)
+Since we must quantify over the ?y, the most plausible type is
+ f :: (Show a, ?y::a) => Int -> (String, Int)
+But notice that the type of the RHS is (String,Int), with no type
+varibables mentioned at all! The type of f looks ambiguous. But
+it isn't, because at a call site we might have
+ let ?y = 5::Int in f 7
+and all is well. In effect, implicit parameters are, well, parameters,
+so we can take their type variables into account as part of the
+"tau-tvs" stuff. This is done in the function 'FunDeps.grow'.
+
+
Question 2: type signatures
~~~~~~~~~~~~~~~~~~~~~~~~~~~
BUT WATCH OUT: When you supply a type signature, we can't force you
:: SDoc
-> TcTyVarSet -- fv(T); type vars
-> [Inst] -- Wanted
- -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
- TcDictBinds, -- Bindings
- [TcId]) -- Dict Ids that must be bound here (zonked)
+ -> TcM ([TcTyVar], -- Tyvars to quantify (zonked and quantified)
+ [Inst], -- Dict Ids that must be bound here (zonked)
+ TcDictBinds) -- Bindings
-- Any free (escaping) Insts are tossed into the environment
\end{code}
\begin{code}
-tcSimplifyInfer doc tau_tvs wanted_lie
- = inferLoop doc (varSetElems tau_tvs)
- wanted_lie `thenM` \ (qtvs, frees, binds, irreds) ->
-
- extendLIEs frees `thenM_`
- returnM (qtvs, binds, map instToId irreds)
-
-inferLoop doc tau_tvs wanteds
- = -- Step 1
- zonkTcTyVarsAndFV tau_tvs `thenM` \ tau_tvs' ->
- mappM zonkInst wanteds `thenM` \ wanteds' ->
- tcGetGlobalTyVars `thenM` \ gbl_tvs ->
- let
- preds = fdPredsOfInsts wanteds'
- qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
-
- try_me inst
- | isFreeWhenInferring qtvs inst = Free
- | isClassDict inst = Irred -- Dicts
- | otherwise = ReduceMe NoSCs -- Lits and Methods
- env = mkRedEnv doc try_me []
- in
- traceTc (text "infloop" <+> vcat [ppr tau_tvs', ppr wanteds', ppr preds,
- ppr (grow preds tau_tvs'), ppr qtvs]) `thenM_`
- -- Step 2
- reduceContext env wanteds' `thenM` \ (improved, frees, binds, irreds) ->
-
- -- Step 3
- if not improved then
- returnM (varSetElems qtvs, frees, binds, irreds)
- else
- -- If improvement did some unification, we go round again. There
- -- are two subtleties:
- -- a) We start again with irreds, not wanteds
- -- Using an instance decl might have introduced a fresh type variable
- -- which might have been unified, so we'd get an infinite loop
- -- if we started again with wanteds! See example [LOOP]
- --
- -- b) It's also essential to re-process frees, because unification
- -- might mean that a type variable that looked free isn't now.
- --
- -- Hence the (irreds ++ frees)
-
- -- However, NOTICE that when we are done, we might have some bindings, but
- -- the final qtvs might be empty. See [NO TYVARS] below.
-
- inferLoop doc tau_tvs (irreds ++ frees) `thenM` \ (qtvs1, frees1, binds1, irreds1) ->
- returnM (qtvs1, frees1, binds `unionBags` binds1, irreds1)
+tcSimplifyInfer doc tau_tvs wanted
+ = do { tau_tvs1 <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
+ ; wanted' <- mappM zonkInst wanted -- Zonk before deciding quantified tyvars
+ ; gbl_tvs <- tcGetGlobalTyVars
+ ; let preds1 = fdPredsOfInsts wanted'
+ gbl_tvs1 = oclose preds1 gbl_tvs
+ qtvs = grow preds1 tau_tvs1 `minusVarSet` gbl_tvs1
+ -- See Note [Choosing which variables to quantify]
+
+ -- To maximise sharing, remove from consideration any
+ -- constraints that don't mention qtvs at all
+ ; let (free, bound) = partition (isFreeWhenInferring qtvs) wanted'
+ ; extendLIEs free
+
+ -- To make types simple, reduce as much as possible
+ ; traceTc (text "infer" <+> (ppr preds1 $$ ppr (grow preds1 tau_tvs1) $$ ppr gbl_tvs $$
+ ppr gbl_tvs1 $$ ppr free $$ ppr bound))
+ ; (irreds1, binds1) <- tryHardCheckLoop doc bound
+
+ -- Note [Inference and implication constraints]
+ ; let want_dict d = tyVarsOfInst d `intersectsVarSet` qtvs
+ ; (irreds2, binds2) <- approximateImplications doc want_dict irreds1
+
+ -- Now work out all over again which type variables to quantify,
+ -- exactly in the same way as before, but starting from irreds2. Why?
+ -- a) By now improvment may have taken place, and we must *not*
+ -- quantify over any variable free in the environment
+ -- tc137 (function h inside g) is an example
+ --
+ -- b) Do not quantify over constraints that *now* do not
+ -- mention quantified type variables, because they are
+ -- simply ambiguous (or might be bound further out). Example:
+ -- f :: Eq b => a -> (a, b)
+ -- g x = fst (f x)
+ -- From the RHS of g we get the MethodInst f77 :: alpha -> (alpha, beta)
+ -- We decide to quantify over 'alpha' alone, but free1 does not include f77
+ -- because f77 mentions 'alpha'. Then reducing leaves only the (ambiguous)
+ -- constraint (Eq beta), which we dump back into the free set
+ -- See test tcfail181
+ --
+ -- c) irreds may contain type variables not previously mentioned,
+ -- e.g. instance D a x => Foo [a]
+ -- wanteds = Foo [a]
+ -- Then after simplifying we'll get (D a x), and x is fresh
+ -- We must quantify over x else it'll be totally unbound
+ ; tau_tvs2 <- zonkTcTyVarsAndFV (varSetElems tau_tvs1)
+ ; gbl_tvs2 <- zonkTcTyVarsAndFV (varSetElems gbl_tvs1)
+ -- Note that we start from gbl_tvs1
+ -- We use tcGetGlobalTyVars, then oclose wrt preds2, because
+ -- we've already put some of the original preds1 into frees
+ -- E.g. wanteds = C a b (where a->b)
+ -- gbl_tvs = {a}
+ -- tau_tvs = {b}
+ -- Then b is fixed by gbl_tvs, so (C a b) will be in free, and
+ -- irreds2 will be empty. But we don't want to generalise over b!
+ ; let preds2 = fdPredsOfInsts irreds2 -- irreds2 is zonked
+ qtvs = grow preds2 tau_tvs2 `minusVarSet` oclose preds2 gbl_tvs2
+ ; let (free, irreds3) = partition (isFreeWhenInferring qtvs) irreds2
+ ; extendLIEs free
+
+ -- Turn the quantified meta-type variables into real type variables
+ ; qtvs2 <- zonkQuantifiedTyVars (varSetElems qtvs)
+
+ -- We can't abstract over any remaining unsolved
+ -- implications so instead just float them outwards. Ugh.
+ ; let (q_dicts0, implics) = partition isAbstractableInst irreds3
+ ; loc <- getInstLoc (ImplicOrigin doc)
+ ; implic_bind <- bindIrreds loc qtvs2 q_dicts0 implics
+
+ -- Prepare equality instances for quantification
+ ; let (q_eqs0,q_dicts) = partition isEqInst q_dicts0
+ ; q_eqs <- mappM finalizeEqInst q_eqs0
+
+ ; return (qtvs2, q_eqs ++ q_dicts, binds1 `unionBags` binds2 `unionBags` implic_bind) }
+ -- NB: when we are done, we might have some bindings, but
+ -- the final qtvs might be empty. See Note [NO TYVARS] below.
+
+approximateImplications :: SDoc -> (Inst -> Bool) -> [Inst] -> TcM ([Inst], TcDictBinds)
+-- Note [Inference and implication constraints]
+-- Given a bunch of Dict and ImplicInsts, try to approximate the implications by
+-- - fetching any dicts inside them that are free
+-- - using those dicts as cruder constraints, to solve the implications
+-- - returning the extra ones too
+
+approximateImplications doc want_dict irreds
+ | null extra_dicts
+ = return (irreds, emptyBag)
+ | otherwise
+ = do { extra_dicts' <- mapM cloneDict extra_dicts
+ ; tryHardCheckLoop doc (extra_dicts' ++ irreds) }
+ -- By adding extra_dicts', we make them
+ -- available to solve the implication constraints
+ where
+ extra_dicts = get_dicts (filter isImplicInst irreds)
+
+ get_dicts :: [Inst] -> [Inst] -- Returns only Dicts
+ -- Find the wanted constraints in implication constraints that satisfy
+ -- want_dict, and are not bound by forall's in the constraint itself
+ get_dicts ds = concatMap get_dict ds
+
+ get_dict d@(Dict {}) | want_dict d = [d]
+ | otherwise = []
+ get_dict (ImplicInst {tci_tyvars = tvs, tci_wanted = wanteds})
+ = [ d | let tv_set = mkVarSet tvs
+ , d <- get_dicts wanteds
+ , not (tyVarsOfInst d `intersectsVarSet` tv_set)]
+ get_dict i@(EqInst {}) | want_dict i = [i]
+ | otherwise = []
+ get_dict other = pprPanic "approximateImplications" (ppr other)
\end{code}
-Example [LOOP]
+Note [Inference and implication constraints]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose we have a wanted implication constraint (perhaps arising from
+a nested pattern match) like
+ C a => D [a]
+and we are now trying to quantify over 'a' when inferring the type for
+a function. In principle it's possible that there might be an instance
+ instance (C a, E a) => D [a]
+so the context (E a) would suffice. The Right Thing is to abstract over
+the implication constraint, but we don't do that (a) because it'll be
+surprising to programmers and (b) because we don't have the machinery to deal
+with 'given' implications.
+
+So our best approximation is to make (D [a]) part of the inferred
+context, so we can use that to discharge the implication. Hence
+the strange function get_dictsin approximateImplications.
+
+The common cases are more clear-cut, when we have things like
+ forall a. C a => C b
+Here, abstracting over (C b) is not an approximation at all -- but see
+Note [Freeness and implications].
+
+See Trac #1430 and test tc228.
- 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
+\begin{code}
+-----------------------------------------------------------
+-- 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
+ :: InstLoc
+ -> TcTyVarSet -- fv(T)
+ -> [Inst] -- Given
+ -> [Inst] -- Wanted
+ -> TcM ([TyVar], -- Fully zonked, and quantified
+ TcDictBinds) -- Bindings
-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)!
+tcSimplifyInferCheck loc tau_tvs givens wanteds
+ = do { traceTc (text "tcSimplifyInferCheck <-" <+> ppr wanteds)
+ ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
+
+ -- 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.
+ ; let all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
+ ; all_tvs <- zonkTcTyVarsAndFV all_tvs
+ ; gbl_tvs <- tcGetGlobalTyVars
+ ; let qtvs = varSetElems (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
+
+ ; qtvs' <- zonkQuantifiedTyVars qtvs
+
+ -- Now we are back to normal (c.f. tcSimplCheck)
+ ; implic_bind <- bindIrreds loc qtvs' givens irreds
-[NO TYVARS]
+ ; traceTc (text "tcSimplifyInferCheck ->" <+> ppr (implic_bind))
+ ; return (qtvs', binds `unionBags` implic_bind) }
+\end{code}
+Note [Squashing methods]
+~~~~~~~~~~~~~~~~~~~~~~~~~
+Be careful if you want to float methods more:
+ 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)
+may continue to float out!
+
+
+Note [NO TYVARS]
+~~~~~~~~~~~~~~~~~
class Y a b | a -> b where
y :: a -> X b
\begin{code}
isFreeWhenInferring :: TyVarSet -> Inst -> Bool
isFreeWhenInferring qtvs inst
- = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
- && isInheritableInst inst -- And no implicit parameter involved
- -- (see "Notes on implicit parameters")
+ = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
+ && isInheritableInst inst -- and no implicit parameter involved
+ -- see Note [Inheriting implicit parameters]
{- No longer used (with implication constraints)
isFreeWhenChecking :: TyVarSet -- Quantified tyvars
-> [Inst] -- Wanted
-> TcM TcDictBinds -- Bindings
tcSimplifyCheck loc qtvs givens wanteds
- = ASSERT( all isSkolemTyVar qtvs )
- do { (binds, irreds) <- innerCheckLoop loc AddSCs givens wanteds
- ; implic_bind <- makeImplicationBind loc [] emptyRefinement
- qtvs givens irreds
+ = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
+ do { traceTc (text "tcSimplifyCheck")
+ ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
+ ; implic_bind <- bindIrreds loc qtvs givens irreds
; return (binds `unionBags` implic_bind) }
-----------------------------------------------------------
-> [Inst] -- Wanted
-> TcM TcDictBinds -- Bindings
tcSimplifyCheckPat loc co_vars reft qtvs givens wanteds
- = ASSERT( all isSkolemTyVar qtvs )
- do { (binds, irreds) <- innerCheckLoop loc AddSCs givens wanteds
- ; implic_bind <- makeImplicationBind loc co_vars reft
- qtvs givens irreds
+ = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
+ do { traceTc (text "tcSimplifyCheckPat")
+ ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
+ ; implic_bind <- bindIrredsR loc qtvs co_vars reft
+ givens irreds
; return (binds `unionBags` implic_bind) }
-----------------------------------------------------------
-makeImplicationBind :: InstLoc -> [CoVar] -> Refinement
- -> [TcTyVar] -> [Inst] -> [Inst]
- -> TcM TcDictBinds
+bindIrreds :: InstLoc -> [TcTyVar]
+ -> [Inst] -> [Inst]
+ -> TcM TcDictBinds
+bindIrreds loc qtvs givens irreds
+ = bindIrredsR loc qtvs [] emptyRefinement givens irreds
+
+bindIrredsR :: InstLoc -> [TcTyVar] -> [CoVar]
+ -> Refinement -> [Inst] -> [Inst]
+ -> TcM TcDictBinds
-- Make a binding that binds 'irreds', by generating an implication
-- constraint for them, *and* throwing the constraint into the LIE
-makeImplicationBind loc co_vars reft qtvs givens irreds
+bindIrredsR loc qtvs co_vars reft givens irreds
+ | null irreds
+ = return emptyBag
+ | otherwise
= do { let givens' = filter isDict givens
-- The givens can include methods
+ -- See Note [Pruning the givens in an implication constraint]
- -- If there are no 'givens', then it's safe to
+ -- If there are no 'givens' *and* the refinement is empty
+ -- (the refinement is like more givens), then it's safe to
-- partition the 'wanteds' by their qtvs, thereby trimming irreds
-- See Note [Freeness and implications]
- ; irreds <- if null givens'
- then do
- { let qtv_set = mkVarSet qtvs
- (frees, real_irreds) = partition (isFreeWrtTyVars qtv_set) irreds
- ; extendLIEs frees
- ; return real_irreds }
- else
- return irreds
-
- -- If there are no irreds, we are done!
- ; if null irreds then
- return emptyBag
- else do
+ ; irreds' <- if null givens' && isEmptyRefinement reft
+ then do
+ { let qtv_set = mkVarSet qtvs
+ (frees, real_irreds) = partition (isFreeWrtTyVars qtv_set) irreds
+ ; extendLIEs frees
+ ; return real_irreds }
+ else return irreds
+
+ ; let all_tvs = qtvs ++ co_vars -- Abstract over all these
+ ; (implics, bind) <- makeImplicationBind loc all_tvs reft givens' irreds'
+ -- This call does the real work
+ -- If irreds' is empty, it does something sensible
+ ; extendLIEs implics
+ ; return bind }
- -- Otherwise we must generate a binding
- -- The binding looks like
- -- (ir1, .., irn) = f qtvs givens
- -- where f is (evidence for) the new implication constraint
- --
- -- This binding must line up the 'rhs' in reduceImplication
- { uniq <- newUnique
+makeImplicationBind :: InstLoc -> [TcTyVar] -> Refinement
+ -> [Inst] -> [Inst]
+ -> TcM ([Inst], TcDictBinds)
+-- Make a binding that binds 'irreds', by generating an implication
+-- constraint for them, *and* throwing the constraint into the LIE
+-- The binding looks like
+-- (ir1, .., irn) = f qtvs givens
+-- where f is (evidence for) the new implication constraint
+-- f :: forall qtvs. {reft} givens => (ir1, .., irn)
+-- qtvs includes coercion variables
+--
+-- This binding must line up the 'rhs' in reduceImplication
+makeImplicationBind loc all_tvs reft
+ givens -- Guaranteed all Dicts (TOMDO: true?)
+ irreds
+ | null irreds -- If there are no irreds, we are done
+ = return ([], emptyBag)
+ | otherwise -- Otherwise we must generate a binding
+ = do { uniq <- newUnique
; span <- getSrcSpanM
- ; let all_tvs = qtvs ++ co_vars -- Abstract over all these
- name = mkInternalName uniq (mkVarOcc "ic") (srcSpanStart span)
+ ; let (eq_givens, dict_givens) = partition isEqInst givens
+ eq_tyvar_cos = map TyVarTy $ uniqSetToList $ tyVarsOfTypes $ map eqInstType eq_givens
+ ; let name = mkInternalName uniq (mkVarOcc "ic") span
implic_inst = ImplicInst { tci_name = name, tci_reft = reft,
tci_tyvars = all_tvs,
- tci_given = givens',
+ tci_given = (eq_givens ++ dict_givens),
tci_wanted = irreds, tci_loc = loc }
-
- ; let n_irreds = length irreds
- irred_ids = map instToId irreds
- tup_ty = mkTupleTy Boxed n_irreds (map idType irred_ids)
- pat = TuplePat (map nlVarPat irred_ids) Boxed tup_ty
+ ; let -- only create binder for dict_irreds
+ (eq_irreds, dict_irreds) = partition isEqInst irreds
+ n_dict_irreds = length dict_irreds
+ dict_irred_ids = map instToId dict_irreds
+ tup_ty = mkTupleTy Boxed n_dict_irreds (map idType dict_irred_ids)
+ pat = TuplePat (map nlVarPat dict_irred_ids) Boxed tup_ty
rhs = L span (mkHsWrap co (HsVar (instToId implic_inst)))
- co = mkWpApps (map instToId givens') <.> mkWpTyApps (mkTyVarTys all_tvs)
- bind | n_irreds==1 = VarBind (head irred_ids) rhs
- | otherwise = PatBind { pat_lhs = L span pat,
- pat_rhs = unguardedGRHSs rhs,
- pat_rhs_ty = tup_ty,
- bind_fvs = placeHolderNames }
- ; -- pprTrace "Make implic inst" (ppr implic_inst) $
- extendLIE implic_inst
- ; return (unitBag (L span bind)) }}
-
+ co = mkWpApps (map instToId dict_givens) <.> mkWpTyApps eq_tyvar_cos <.> mkWpTyApps (mkTyVarTys all_tvs)
+ bind | [dict_irred_id] <- dict_irred_ids = VarBind dict_irred_id rhs
+ | otherwise = PatBind { pat_lhs = L span pat,
+ pat_rhs = unguardedGRHSs rhs,
+ pat_rhs_ty = tup_ty,
+ bind_fvs = placeHolderNames }
+ ; -- pprTrace "Make implic inst" (ppr (implic_inst,irreds,dict_irreds,tup_ty)) $
+ return ([implic_inst], unitBag (L span bind)) }
-----------------------------------------------------------
-topCheckLoop :: SDoc
+tryHardCheckLoop :: SDoc
-> [Inst] -- Wanted
- -> TcM (TcDictBinds,
- [Inst]) -- Irreducible
+ -> TcM ([Inst], TcDictBinds)
-topCheckLoop doc wanteds
- = checkLoop (mkRedEnv doc try_me []) wanteds
+tryHardCheckLoop doc wanteds
+ = do { (irreds,binds,_) <- checkLoop (mkRedEnv doc try_me []) wanteds
+ ; return (irreds,binds)
+ }
where
try_me inst = ReduceMe AddSCs
+ -- Here's the try-hard bit
-----------------------------------------------------------
-innerCheckLoop :: InstLoc -> WantSCs
+gentleCheckLoop :: InstLoc
-> [Inst] -- Given
-> [Inst] -- Wanted
- -> TcM (TcDictBinds,
- [Inst]) -- Irreducible
+ -> TcM ([Inst], TcDictBinds)
-innerCheckLoop inst_loc want_scs givens wanteds
- = checkLoop env wanteds
+gentleCheckLoop inst_loc givens wanteds
+ = do { (irreds,binds,_) <- checkLoop env wanteds
+ ; return (irreds,binds)
+ }
where
env = mkRedEnv (pprInstLoc inst_loc) try_me givens
- try_me inst | isMethodOrLit inst = ReduceMe want_scs
- | otherwise = Irred
+ try_me inst | isMethodOrLit inst = ReduceMe AddSCs
+ | otherwise = Stop
-- When checking against a given signature
-- we MUST be very gentle: Note [Check gently]
\end{code}
Hence we have a dictionary for Show [a] available; and indeed we
need it. We are going to build an implication contraint
forall a. (b~[a]) => Show [a]
-Later, we will solve this constraint using the knowledge (Show b)
+Later, we will solve this constraint using the knowledg e(Show b)
But we MUST NOT reduce (Show [a]) to (Show a), else the whole
thing becomes insoluble. So we simplify gently (get rid of literals
and methods only, plus common up equal things), deferring the real
work until top level, when we solve the implication constraint
-with topCheckLooop.
+with tryHardCheckLooop.
\begin{code}
-----------------------------------------------------------
checkLoop :: RedEnv
-> [Inst] -- Wanted
- -> TcM (TcDictBinds,
- [Inst]) -- Irreducible
--- Precondition: the try_me never returns Free
--- givens are completely rigid
+ -> TcM ([Inst], TcDictBinds,
+ [Inst]) -- needed givens
+-- Precondition: givens are completely rigid
+-- Postcondition: returned Insts are zonked
checkLoop env wanteds
- = do { -- Givens are skolems, so no need to zonk them
- wanteds' <- mappM zonkInst wanteds
-
- ; (improved, _frees, binds, irreds) <- reduceContext env wanteds'
-
- ; ASSERT( null _frees )
-
- if not improved then
- return (binds, irreds)
- else do
+ = go env wanteds []
+ where go env wanteds needed_givens
+ = do { -- Givens are skolems, so no need to zonk them
+ wanteds' <- zonkInsts wanteds
+
+ ; (improved, binds, irreds, more_needed_givens) <- reduceContext env wanteds'
- { (binds1, irreds1) <- checkLoop env irreds
- ; return (binds `unionBags` binds1, irreds1) } }
+ ; let all_needed_givens = needed_givens ++ more_needed_givens
+
+ ; if not improved then
+ return (irreds, binds, all_needed_givens)
+ else do
+
+ -- If improvement did some unification, we go round again.
+ -- 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 Note [LOOP]
+ { (irreds1, binds1, all_needed_givens1) <- go env irreds all_needed_givens
+ ; return (irreds1, binds `unionBags` binds1, all_needed_givens1) } }
\end{code}
+Note [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
-\begin{code}
------------------------------------------------------------
--- 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
- :: InstLoc
- -> TcTyVarSet -- fv(T)
- -> [Inst] -- Given
- -> [Inst] -- Wanted
- -> TcM ([TcTyVar], -- Variables over which to quantify
- TcDictBinds) -- Bindings
-
-tcSimplifyInferCheck loc tau_tvs givens wanteds
- = do { (binds, irreds) <- innerCheckLoop loc AddSCs givens wanteds
+Wanted: Max Z (S x) y
- -- 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.
- ; let all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
- ; all_tvs <- zonkTcTyVarsAndFV all_tvs
- ; gbl_tvs <- tcGetGlobalTyVars
- ; let qtvs = varSetElems (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
+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)!
- -- Now we are back to normal (c.f. tcSimplCheck)
- ; implic_bind <- makeImplicationBind loc [] emptyRefinement
- qtvs givens irreds
- ; return (qtvs, binds `unionBags` implic_bind) }
-\end{code}
%************************************************************************
-> [Inst] -- Wanted
-> TcM TcDictBinds
tcSimplifySuperClasses loc givens sc_wanteds
- = do { (binds1, irreds) <- checkLoop env sc_wanteds
+ = do { traceTc (text "tcSimplifySuperClasses")
+ ; (irreds,binds1,_) <- checkLoop env sc_wanteds
; let (tidy_env, tidy_irreds) = tidyInsts irreds
; reportNoInstances tidy_env (Just (loc, givens)) tidy_irreds
; return binds1 }
where
env = mkRedEnv (pprInstLoc loc) try_me givens
try_me inst = ReduceMe NoSCs
- -- Like topCheckLoop, but with NoSCs
+ -- Like tryHardCheckLoop, but with NoSCs
\end{code}
-> [Name] -- Things bound in this group
-> TcTyVarSet -- Free in the type of the RHSs
-> [Inst] -- Free in the RHSs
- -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
+ -> TcM ([TyVar], -- Tyvars to quantify (zonked and quantified)
TcDictBinds) -- Bindings
-- tcSimpifyRestricted returns no constraints to
-- quantify over; by definition there are none.
tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
-- Zonk everything in sight
- = mappM zonkInst wanteds `thenM` \ wanteds' ->
+ = do { traceTc (text "tcSimplifyRestricted")
+ ; wanteds' <- zonkInsts wanteds
- -- 'reduceMe': Reduce as far as we can. Don't stop at
+ -- '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
-- BUT do no improvement! See Plan D above
-- HOWEVER, some unification may take place, if we instantiate
-- a method Inst with an equality constraint
- let env = mkNoImproveRedEnv doc reduceMe
- in
- reduceContext env wanteds' `thenM` \ (_imp, _frees, _binds, constrained_dicts) ->
+ ; let env = mkNoImproveRedEnv doc (\i -> ReduceMe AddSCs)
+ ; (_imp, _binds, constrained_dicts, _) <- reduceContext env wanteds'
-- Next, figure out the tyvars we will quantify over
- zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenM` \ tau_tvs' ->
- tcGetGlobalTyVars `thenM` \ gbl_tvs' ->
- mappM zonkInst constrained_dicts `thenM` \ constrained_dicts' ->
- let
- constrained_tvs' = tyVarsOfInsts constrained_dicts'
- qtvs' = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs')
- `minusVarSet` constrained_tvs'
- in
- traceTc (text "tcSimplifyRestricted" <+> vcat [
- pprInsts wanteds, pprInsts _frees, pprInsts constrained_dicts',
+ ; tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
+ ; gbl_tvs' <- tcGetGlobalTyVars
+ ; constrained_dicts' <- zonkInsts constrained_dicts
+
+ ; let qtvs1 = tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs'
+ -- As in tcSimplifyInfer
+
+ -- Do not quantify over constrained type variables:
+ -- this is the monomorphism restriction
+ constrained_tvs' = tyVarsOfInsts constrained_dicts'
+ qtvs = qtvs1 `minusVarSet` constrained_tvs'
+ pp_bndrs = pprWithCommas (quotes . ppr) bndrs
+
+ -- Warn in the mono
+ ; warn_mono <- doptM Opt_WarnMonomorphism
+ ; warnTc (warn_mono && (constrained_tvs' `intersectsVarSet` qtvs1))
+ (vcat[ ptext SLIT("the Monomorphism Restriction applies to the binding")
+ <> plural bndrs <+> ptext SLIT("for") <+> pp_bndrs,
+ ptext SLIT("Consider giving a type signature for") <+> pp_bndrs])
+
+ ; traceTc (text "tcSimplifyRestricted" <+> vcat [
+ pprInsts wanteds, pprInsts constrained_dicts',
ppr _binds,
- ppr constrained_tvs', ppr tau_tvs', ppr qtvs' ]) `thenM_`
+ ppr constrained_tvs', ppr tau_tvs', ppr qtvs ])
-- The first step may have squashed more methods than
-- necessary, so try again, this time more gently, knowing the exact
-- set of type variables to quantify over.
--
- -- We quantify only over constraints that are captured by qtvs';
+ -- We quantify only over constraints that are captured by qtvs;
-- these will just be a subset of non-dicts. This in contrast
-- to normal inference (using isFreeWhenInferring) in which we quantify over
-- all *non-inheritable* constraints too. This implements choice
--
-- At top level, we *do* squash methods becuase we want to
-- expose implicit parameters to the test that follows
- let
- is_nested_group = isNotTopLevel top_lvl
- try_me inst | isFreeWrtTyVars qtvs' inst,
- (is_nested_group || isDict inst) = Free
- | otherwise = ReduceMe AddSCs
- env = mkNoImproveRedEnv doc try_me
- in
- reduceContext env wanteds' `thenM` \ (_imp, frees, binds, irreds) ->
- ASSERT( null irreds )
+ ; let is_nested_group = isNotTopLevel top_lvl
+ try_me inst | isFreeWrtTyVars qtvs inst,
+ (is_nested_group || isDict inst) = Stop
+ | otherwise = ReduceMe AddSCs
+ env = mkNoImproveRedEnv doc try_me
+ ; (_imp, binds, irreds, _) <- reduceContext env wanteds'
-- See "Notes on implicit parameters, Question 4: top level"
- if is_nested_group then
- extendLIEs frees `thenM_`
- returnM (varSetElems qtvs', binds)
- else
- let
- (non_ips, bad_ips) = partition isClassDict frees
- in
- addTopIPErrs bndrs bad_ips `thenM_`
- extendLIEs non_ips `thenM_`
- returnM (varSetElems qtvs', binds)
+ ; ASSERT( all (isFreeWrtTyVars qtvs) irreds ) -- None should be captured
+ if is_nested_group then
+ extendLIEs irreds
+ else do { let (bad_ips, non_ips) = partition isIPDict irreds
+ ; addTopIPErrs bndrs bad_ips
+ ; extendLIEs non_ips }
+
+ ; qtvs' <- zonkQuantifiedTyVars (varSetElems qtvs)
+ ; return (qtvs', binds) }
\end{code}
\begin{code}
tcSimplifyBracket :: [Inst] -> TcM ()
tcSimplifyBracket wanteds
- = do { topCheckLoop doc wanteds
+ = do { tryHardCheckLoop doc wanteds
; return () }
where
doc = text "tcSimplifyBracket"
-- makes them the same.
tcSimplifyIPs given_ips wanteds
- = do { wanteds' <- mappM zonkInst wanteds
- ; given_ips' <- mappM zonkInst given_ips
+ = do { wanteds' <- zonkInsts wanteds
+ ; given_ips' <- zonkInsts given_ips
-- Unusually for checking, we *must* zonk the given_ips
; let env = mkRedEnv doc try_me given_ips'
- ; (improved, _frees, binds, irreds) <- reduceContext env wanteds'
+ ; (improved, binds, irreds, _) <- reduceContext env wanteds'
; if not improved then
ASSERT( all is_free irreds )
is_free inst = isFreeWrtIPs ip_set inst
-- Simplify any methods that mention the implicit parameter
- try_me inst | is_free inst = Irred
+ try_me inst | is_free inst = Stop
| otherwise = ReduceMe NoSCs
\end{code}
returnM emptyLHsBinds
| otherwise
- = do { (binds, irreds) <- checkLoop env for_me
+ = do { (irreds, binds,_) <- checkLoop env for_me
; extendLIEs not_for_me
; extendLIEs irreds
; return binds }
-- so it's worth building a set, so that
-- lookup (in isMethodFor) is faster
try_me inst | isMethod inst = ReduceMe NoSCs
- | otherwise = Irred
+ | otherwise = Stop
\end{code}
-- message of any kind.
-- It might be quite legitimate such as (Eq a)!
- | Irred -- Return as irreducible unless it can
+ | Stop -- Return as irreducible unless it can
-- be reduced to a constant in one step
-
- | Free -- Return as free
-
-reduceMe :: Inst -> WhatToDo
-reduceMe inst = ReduceMe AddSCs
+ -- Do not add superclasses; see
data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
-- of a predicate when adding it to the avails
-- Note [SUPER-CLASS LOOP 1]
\end{code}
+
%************************************************************************
%* *
\subsection[reduce]{@reduce@}
%* *
%************************************************************************
+Note [Ancestor Equalities]
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+During context reduction, we add to the wanted equalities also those
+equalities that (transitively) occur in superclass contexts of wanted
+class constraints. Consider the following code
+
+ class a ~ Int => C a
+ instance C Int
+
+If (C a) is wanted, we want to add (a ~ Int), which will be discharged by
+substituting Int for a. Hence, we ultimately want (C Int), which we
+discharge with the explicit instance.
\begin{code}
reduceContext :: RedEnv
-> [Inst] -- Wanted
-> TcM (ImprovementDone,
- [Inst], -- Free
TcDictBinds, -- Dictionary bindings
- [Inst]) -- Irreducible
+ [Inst], -- Irreducible
+ [Inst]) -- Needed givens
reduceContext env wanteds
= do { traceTc (text "reduceContext" <+> (vcat [
text "----------------------"
]))
- -- Build the Avail mapping from "givens"
- ; init_state <- foldlM addGiven emptyAvails (red_givens env)
-
- -- Do the real work
- ; avails <- reduceList env wanteds init_state
-
- ; let improved = availsImproved avails
- ; (binds, irreds, frees) <- extractResults avails wanteds
+ ; let givens = red_givens env
+ (given_eqs0, given_dicts0) = partition isEqInst givens
+ (wanted_eqs0, wanted_dicts) = partition isEqInst wanteds
+
+ -- We want to add as wanted equalities those that (transitively)
+ -- occur in superclass contexts of wanted class constraints.
+ -- See Note [Ancestor Equalities]
+ ; ancestor_eqs <- ancestorEqualities wanted_dicts
+ ; let wanted_eqs = wanted_eqs0 ++ ancestor_eqs
+ ; traceTc $ text "reduceContext: ancestor eqs" <+> ppr ancestor_eqs
+
+ -- 1. Normalise the *given* *equality* constraints
+ ; (given_eqs, eliminate_skolems) <- normaliseGivens given_eqs0
+
+ -- 2. Normalise the *given* *dictionary* constraints
+ -- wrt. the toplevel and given equations
+ ; (given_dicts, given_binds) <- normaliseGivenDicts given_eqs
+ given_dicts0
+
+ -- 3. Solve the *wanted* *equation* constraints
+ ; eq_irreds0 <- solveWanteds given_eqs wanted_eqs
+
+ -- 4. Normalise the *wanted* equality constraints with respect to
+ -- each other
+ ; eq_irreds <- normaliseWanteds eq_irreds0
+
+ -- 5. Build the Avail mapping from "given_dicts"
+ ; init_state <- foldlM addGiven emptyAvails given_dicts
+
+ -- 6. Solve the *wanted* *dictionary* constraints
+ -- This may expose some further equational constraints...
+ ; wanted_dicts' <- zonkInsts wanted_dicts
+ ; avails <- reduceList env wanted_dicts' init_state
+ ; (binds, irreds0, needed_givens) <- extractResults avails wanted_dicts'
+ ; traceTc $ text "reduceContext extractresults" <+> vcat
+ [ppr avails,ppr wanted_dicts',ppr binds,ppr needed_givens]
+
+ -- 7. Normalise the *wanted* *dictionary* constraints
+ -- wrt. the toplevel and given equations
+ ; (irreds1,normalise_binds1) <- normaliseWantedDicts given_eqs irreds0
+
+ -- 8. Substitute the wanted *equations* in the wanted *dictionaries*
+ ; (irreds,normalise_binds2) <- substEqInDictInsts eq_irreds irreds1
+
+ -- 9. eliminate the artificial skolem constants introduced in 1.
+ ; eliminate_skolems
+
+ -- If there was some FD improvement,
+ -- or new wanted equations have been exposed,
+ -- we should have another go at solving.
+ ; let improved = availsImproved avails
+ || (not $ isEmptyBag normalise_binds1)
+ || (not $ isEmptyBag normalise_binds2)
+ || (any isEqInst irreds)
; traceTc (text "reduceContext end" <+> (vcat [
text "----------------------",
text "wanted" <+> ppr wanteds,
text "----",
text "avails" <+> pprAvails avails,
- text "frees" <+> ppr frees,
text "improved =" <+> ppr improved,
+ text "irreds = " <+> ppr irreds,
+ text "binds = " <+> ppr binds,
+ text "needed givens = " <+> ppr needed_givens,
text "----------------------"
]))
- ; return (improved, frees, binds, irreds) }
+ ; return (improved,
+ given_binds `unionBags` normalise_binds1
+ `unionBags` normalise_binds2
+ `unionBags` binds,
+ irreds ++ eq_irreds,
+ needed_givens)
+ }
tcImproveOne :: Avails -> Inst -> TcM ImprovementDone
tcImproveOne avails inst
go wanteds state }
where
go [] state = return state
- go (w:ws) state = do { state' <- reduce (env {red_stack = (n+1, w:stk)}) w state
+ go (w:ws) state = do { traceTc (text "reduceList " <+> ppr (w:ws) <+> ppr state)
+ ; state' <- reduce (env {red_stack = (n+1, w:stk)}) w state
; go ws state' }
-- Base case: we're done!
reduce env wanted avails
-- It's the same as an existing inst, or a superclass thereof
| Just avail <- findAvail avails wanted
- = returnM avails
+ = do { traceTc (text "reduce: found " <+> ppr wanted)
+ ; returnM avails
+ }
| otherwise
- = case red_try_me env wanted of {
- Free -> try_simple addFree -- It's free so just chuck it upstairs
- ; Irred -> try_simple (addIrred AddSCs) -- Assume want superclasses
-
- ; ReduceMe want_scs -> -- It should be reduced
- reduceInst env avails wanted `thenM` \ (avails, lookup_result) ->
- case lookup_result of
- NoInstance -> -- No such instance!
+ = do { traceTc (text "reduce" <+> ppr avails <+> ppr wanted)
+ ; case red_try_me env wanted of {
+ Stop -> try_simple (addIrred NoSCs);
+ -- See Note [No superclasses for Stop]
+
+ ReduceMe want_scs -> do -- It should be reduced
+ { (avails, lookup_result) <- reduceInst env avails wanted
+ ; case lookup_result of
+ NoInstance -> addIrred want_scs avails wanted
-- Add it and its superclasses
- addIrred want_scs avails wanted
-
- GenInst [] rhs -> addWanted want_scs avails wanted rhs []
+
+ GenInst [] rhs -> addWanted want_scs avails wanted rhs []
- GenInst wanteds' rhs -> do { avails1 <- addIrred NoSCs avails wanted
- ; avails2 <- reduceList env wanteds' avails1
- ; addWanted want_scs avails2 wanted rhs wanteds' }
+ GenInst wanteds' rhs
+ -> do { avails1 <- addIrred NoSCs avails wanted
+ ; avails2 <- reduceList env wanteds' avails1
+ ; addWanted want_scs avails2 wanted rhs wanteds' } }
-- Temporarily do addIrred *before* the reduceList,
-- which has the effect of adding the thing we are trying
-- to prove to the database before trying to prove the things it
-- needs. See note [RECURSIVE DICTIONARIES]
-- NB: we must not do an addWanted before, because that adds the
- -- superclasses too, and thaat can lead to a spurious loop; see
+ -- superclasses too, and that can lead to a spurious loop; see
-- the examples in [SUPERCLASS-LOOP]
-- So we do an addIrred before, and then overwrite it afterwards with addWanted
-
- }
+ } }
where
-- First, see if the inst can be reduced to a constant in one step
-- Works well for literals (1::Int) and constant dictionaries (d::Num Int)
in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
becuase it lost legitimate superclass sharing, and it still didn't do the job:
I found a very obscure program (now tcrun021) in which improvement meant the
-simplifier got two bites a the cherry... so something seemed to be an Irred
+simplifier got two bites a the cherry... so something seemed to be an Stop
first time, but reducible next time.
Now we implement the Right Solution, which is to check for loops directly
; return (avails, result) }
\end{code}
+Note [Equational Constraints in Implication Constraints]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+An equational constraint is of the form
+ Given => Wanted
+where Given and Wanted may contain both equational and dictionary
+constraints. The delay and reduction of these two kinds of constraints
+is distinct:
+
+-) In the generated code, wanted Dictionary constraints are wrapped up in an
+ implication constraint that is created at the code site where the wanted
+ dictionaries can be reduced via a let-binding. This let-bound implication
+ constraint is deconstructed at the use-site of the wanted dictionaries.
+
+-) While the reduction of equational constraints is also delayed, the delay
+ is not manifest in the generated code. The required evidence is generated
+ in the code directly at the use-site. There is no let-binding and deconstruction
+ necessary. The main disadvantage is that we cannot exploit sharing as the
+ same evidence may be generated at multiple use-sites. However, this disadvantage
+ is limited because it only concerns coercions which are erased.
+
+The different treatment is motivated by the different in representation. Dictionary
+constraints require manifest runtime dictionaries, while equations require coercions
+which are types.
+
\begin{code}
---------------------------------------------
reduceImplication :: RedEnv
\begin{code}
-- ToDo: should we instantiate tvs? I think it's not necessary
--
- -- ToDo: what about improvement? There may be some improvement
- -- exposed as a result of the simplifications done by reduceList
- -- which are discarded if we back off.
- -- This is almost certainly Wrong, but we'll fix it when dealing
- -- better with equality constraints
+ -- Note on coercion variables:
+ --
+ -- The extra given coercion variables are bound at two different sites:
+ -- -) in the creation context of the implication constraint
+ -- the solved equational constraints use these binders
+ --
+ -- -) at the solving site of the implication constraint
+ -- the solved dictionaries use these binders
+ -- these binders are generated by reduceImplication
+ --
reduceImplication env orig_avails reft tvs extra_givens wanteds inst_loc
= do { -- Add refined givens, and the extra givens
- (refined_red_givens, avails)
- <- if isEmptyRefinement reft then return (red_givens env, orig_avails)
- else foldlM (addRefinedGiven reft) ([], orig_avails) (red_givens env)
- ; avails <- foldlM addGiven avails extra_givens
+ -- Todo fix this
+ (refined_red_givens,refined_avails)
+ <- if isEmptyRefinement reft then return (red_givens env,orig_avails)
+ else foldlM (addRefinedGiven reft) ([],orig_avails) (red_givens env)
-- Solve the sub-problem
; let try_me inst = ReduceMe AddSCs -- Note [Freeness and implications]
- env' = env { red_givens = refined_red_givens ++ extra_givens
+ env' = env { red_givens = refined_red_givens ++ extra_givens ++ availsInsts orig_avails
, red_try_me = try_me }
; traceTc (text "reduceImplication" <+> vcat
- [ ppr (red_givens env), ppr extra_givens, ppr reft, ppr wanteds ])
- ; avails <- reduceList env' wanteds avails
+ [ ppr orig_avails,
+ ppr (red_givens env), ppr extra_givens,
+ ppr reft, ppr wanteds])
+ ; (irreds,binds,needed_givens0) <- checkLoop env' wanteds
+ ; let needed_givens1 = [ng | ng <- needed_givens0, notElem ng extra_givens]
+
+ -- Note [Reducing implication constraints]
+ -- Tom -- update note, put somewhere!
+
+ ; traceTc (text "reduceImplication result" <+> vcat
+ [ppr irreds, ppr binds, ppr needed_givens1])
+-- ; avails <- reduceList env' wanteds avails
+--
+-- -- Extract the binding
+-- ; (binds, irreds) <- extractResults avails wanteds
+ ; (refinement_binds,needed_givens) <- extractLocalResults refined_avails needed_givens1
+ ; traceTc (text "reduceImplication local results" <+> vcat
+ [ppr refinement_binds, ppr needed_givens])
+
+ ; -- extract superclass binds
+ -- (sc_binds,_) <- extractResults avails []
+-- ; traceTc (text "reduceImplication sc_binds" <+> vcat
+-- [ppr sc_binds, ppr avails])
+--
+
+ -- We always discard the extra avails we've generated;
+ -- but we remember if we have done any (global) improvement
+-- ; let ret_avails = avails
+ ; let ret_avails = orig_avails
+-- ; let ret_avails = updateImprovement orig_avails avails
- -- Extract the binding
- ; (binds, irreds, _frees) <- extractResults avails wanteds
- -- No frees, because try_me never says Free
-
- ; let dict_ids = map instToId extra_givens
- co = mkWpTyLams tvs <.> mkWpLams dict_ids <.> WpLet binds
+ ; traceTc (text "reduceImplication condition" <+> ppr ((isEmptyLHsBinds binds) || (null irreds)))
+
+-- Porgress is no longer measered by the number of bindings
+-- ; if isEmptyLHsBinds binds then -- No progress
+ ; if (isEmptyLHsBinds binds) && (not $ null irreds) then
+ return (ret_avails, NoInstance)
+ else do
+ {
+ ; (implic_insts, bind) <- makeImplicationBind inst_loc tvs reft extra_givens irreds
+ -- This binding is useless if the recursive simplification
+ -- made no progress; but currently we don't try to optimise that
+ -- case. After all, we only try hard to reduce at top level, or
+ -- when inferring types.
+
+ ; let dict_wanteds = filter (not . isEqInst) wanteds
+ (extra_eq_givens, extra_dict_givens) = partition isEqInst extra_givens
+ dict_ids = map instToId extra_dict_givens
+ -- TOMDO: given equational constraints bug!
+ -- we need a different evidence for given
+ -- equations depending on whether we solve
+ -- dictionary constraints or equational constraints
+ eq_tyvars = uniqSetToList $ tyVarsOfTypes $ map eqInstType extra_eq_givens
+ -- dict_ids = map instToId extra_givens
+ co = mkWpTyLams tvs <.> mkWpTyLams eq_tyvars <.> mkWpLams dict_ids <.> WpLet (binds `unionBags` refinement_binds `unionBags` bind)
rhs = mkHsWrap co payload
loc = instLocSpan inst_loc
- payload | isSingleton wanteds = HsVar (instToId (head wanteds))
- | otherwise = ExplicitTuple (map (L loc . HsVar . instToId) wanteds) Boxed
+ payload | [dict_wanted] <- dict_wanteds = HsVar (instToId dict_wanted)
+ | otherwise = ExplicitTuple (map (L loc . HsVar . instToId) dict_wanteds) Boxed
+
+ ; traceTc (text "reduceImplication ->" <+> vcat
+ [ ppr ret_avails,
+ ppr implic_insts])
-- If there are any irreds, we back off and return NoInstance
- -- Either way, we discard the extra avails we've generated;
- -- but we remember if we have done any (global) improvement
- ; let ret_avails = updateImprovement orig_avails avails
- ; case irreds of
- [] -> return (ret_avails, GenInst [] (L loc rhs))
- other -> return (ret_avails, NoInstance)
- }
+ ; return (ret_avails, GenInst (implic_insts ++ needed_givens) (L loc rhs))
+ }
+ }
\end{code}
+Note [Reducing implication constraints]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose we are trying to simplify
+ (Ord a, forall b. C a b => (W [a] b, D c b))
+where
+ instance (C a b, Ord a) => W [a] b
+When solving the implication constraint, we'll start with
+ Ord a -> Irred
+in the Avails. Then we add (C a b -> Given) and solve. Extracting
+the results gives us a binding for the (W [a] b), with an Irred of
+(Ord a, D c b). Now, the (Ord a) comes from "outside" the implication,
+but the (D d b) is from "inside". So we want to generate a Rhs binding
+like this
+
+ ic = /\b \dc:C a b). (df a b dc do, ic' b dc)
+ depending on
+ do :: Ord a
+ ic' :: forall b. C a b => D c b
+
+The 'depending on' part of the Rhs is important, because it drives
+the extractResults code.
+
+The "inside" and "outside" distinction is what's going on with 'inner' and
+'outer' in reduceImplication
+
+
Note [Freeness and implications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It's hard to say when an implication constraint can be floated out. Consider
to float the (C Int) out, even though it mentions no type variable in
the constraints!
+Note [Pruning the givens in an implication constraint]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose we are about to form the implication constraint
+ forall tvs. Eq a => Ord b
+The (Eq a) cannot contribute to the (Ord b), because it has no access to
+the type variable 'b'. So we could filter out the (Eq a) from the givens.
+
+Doing so would be a bit tidier, but all the implication constraints get
+simplified away by the optimiser, so it's no great win. So I don't take
+advantage of that at the moment.
+
+If you do, BE CAREFUL of wobbly type variables.
+
+
%************************************************************************
%* *
Avails and AvailHow: the pool of evidence
type AvailEnv = FiniteMap Inst AvailHow
data AvailHow
- = IsFree -- Used for free Insts
- | IsIrred -- Used for irreducible dictionaries,
+ = IsIrred -- Used for irreducible dictionaries,
-- which are going to be lambda bound
| Given TcId -- Used for dictionaries for which we have a binding
-------------------------
pprAvail :: AvailHow -> SDoc
-pprAvail IsFree = text "Free"
pprAvail IsIrred = text "Irred"
pprAvail (Given x) = text "Given" <+> ppr x
-pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs)
+pprAvail (Rhs rhs bs) = text "Rhs" <+> sep [ppr rhs, braces (ppr bs)]
-------------------------
extendAvailEnv :: AvailEnv -> Inst -> AvailHow -> AvailEnv
-> [Inst] -- Wanted
-> TcM ( TcDictBinds, -- Bindings
[Inst], -- Irreducible ones
- [Inst]) -- Free ones
+ [Inst]) -- Needed givens, i.e. ones used in the bindings
extractResults (Avails _ avails) wanteds
= go avails emptyBag [] [] wanteds
where
go :: AvailEnv -> TcDictBinds -> [Inst] -> [Inst] -> [Inst]
-> TcM (TcDictBinds, [Inst], [Inst])
- go avails binds irreds frees []
- = returnM (binds, irreds, frees)
+ go avails binds irreds givens []
+ = returnM (binds, irreds, givens)
- go avails binds irreds frees (w:ws)
+ go avails binds irreds givens (w:ws)
= case findAvailEnv avails w of
- Nothing -> pprTrace "Urk: extractResults" (ppr w) $
- go avails binds irreds frees ws
-
- Just IsFree -> go (add_free avails w) binds irreds (w:frees) ws
- Just IsIrred -> go (add_given avails w) binds (w:irreds) frees ws
+ Nothing -> pprTrace "Urk: extractResults" (ppr w) $
+ go avails binds irreds givens ws
- Just (Given id) -> go avails new_binds irreds frees ws
- where
- new_binds | id == instToId w = binds
- | otherwise = addBind binds w (L (instSpan w) (HsVar id))
+ Just (Given id)
+ | id == w_id -> go avails binds irreds (w:givens) ws
+ | otherwise -> go avails (addBind binds w (nlHsVar id)) irreds (update_id w id:givens) ws
-- The sought Id can be one of the givens, via a superclass chain
-- and then we definitely don't want to generate an x=x binding!
- Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds frees (ws' ++ ws)
- where
+ Just IsIrred -> go (add_given avails w) binds (w:irreds) givens ws
+ -- The add_given handles the case where we want (Ord a, Eq a), and we
+ -- don't want to emit *two* Irreds for Ord a, one via the superclass chain
+ -- This showed up in a dupliated Ord constraint in the error message for
+ -- test tcfail043
+
+ Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds givens (ws' ++ ws)
+ where
new_binds = addBind binds w rhs
+ where
+ w_id = instToId w
+ update_id m@(Method{}) id = m {tci_id = id}
+ update_id w id = w {tci_name = idName id}
add_given avails w = extendAvailEnv avails w (Given (instToId w))
- add_free avails w | isMethod w = avails
- | otherwise = add_given avails w
- -- NB: Hack alert!
- -- Do *not* replace Free by Given if it's a method.
- -- The following situation shows why this is bad:
- -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
- -- From an application (truncate f i) we get
- -- t1 = truncate at f
- -- t2 = t1 at i
- -- If we have also have a second occurrence of truncate, we get
- -- t3 = truncate at f
- -- t4 = t3 at i
- -- When simplifying with i,f free, we might still notice that
- -- t1=t3; but alas, the binding for t2 (which mentions t1)
- -- will continue to float out!
-
-addBind binds inst rhs = binds `unionBags` unitBag (L (instSpan inst)
- (VarBind (instToId inst) rhs))
-instSpan wanted = instLocSpan (instLoc wanted)
+extractLocalResults :: Avails
+ -> [Inst] -- Wanted
+ -> TcM ( TcDictBinds, -- Bindings
+ [Inst]) -- Needed givens, i.e. ones used in the bindings
+
+extractLocalResults (Avails _ avails) wanteds
+ = go avails emptyBag [] wanteds
+ where
+ go :: AvailEnv -> TcDictBinds -> [Inst] -> [Inst]
+ -> TcM (TcDictBinds, [Inst])
+ go avails binds givens []
+ = returnM (binds, givens)
+
+ go avails binds givens (w:ws)
+ = case findAvailEnv avails w of
+ Nothing -> -- pprTrace "Urk: extractLocalResults" (ppr w) $
+ go avails binds givens ws
+
+ Just IsIrred ->
+ go avails binds givens ws
+
+ Just (Given id)
+ | id == w_id -> go avails binds (w:givens) ws
+ | otherwise -> go avails binds (w{tci_name=idName id}:givens) ws
+ -- The sought Id can be one of the givens, via a superclass chain
+ -- and then we definitely don't want to generate an x=x binding!
+
+ Just (Rhs rhs ws') -> go (add_given avails w) new_binds givens (ws' ++ ws)
+ where
+ new_binds = addBind binds w rhs
+ where
+ w_id = instToId w
+
+ add_given avails w = extendAvailEnv avails w (Given (instToId w))
\end{code}
-\begin{code}
--------------------------
-addFree :: Avails -> Inst -> TcM Avails
- -- When an Inst is tossed upstairs as 'free' we nevertheless add it
- -- to avails, so that any other equal Insts will be commoned up right
- -- here rather than also being tossed upstairs. This is really just
- -- an optimisation, and perhaps it is more trouble that it is worth,
- -- as the following comments show!
- --
- -- NB: do *not* add superclasses. If we have
- -- df::Floating a
- -- dn::Num a
- -- but a is not bound here, then we *don't* want to derive
- -- dn from df here lest we lose sharing.
- --
-addFree avails free = extendAvails avails free IsFree
+Note [No superclasses for Stop]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When we decide not to reduce an Inst -- the 'WhatToDo' --- we still
+add it to avails, so that any other equal Insts will be commoned up
+right here. However, we 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.
+\begin{code}
addWanted :: WantSCs -> Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
addWanted want_scs avails wanted rhs_expr wanteds
= addAvailAndSCs want_scs avails wanted avail
addRefinedGiven reft (refined_givens, avails) given
| isDict given -- We sometimes have 'given' methods, but they
-- are always optional, so we can drop them
- , Just (co, pred) <- refinePred reft (dictPred given)
+ , let pred = dictPred given
+ , isRefineablePred pred -- See Note [ImplicInst rigidity]
+ , Just (co, pred) <- refinePred reft pred
= do { new_given <- newDictBndr (instLoc given) pred
; let rhs = L (instSpan given) $
HsWrap (WpCo co) (HsVar (instToId given))
-- and hopefully the optimiser will spot the duplicated work
| otherwise
= return (refined_givens, avails)
+\end{code}
+Note [ImplicInst rigidity]
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ C :: forall ab. (Eq a, Ord b) => b -> T a
+
+ ...(case x of C v -> <body>)...
+
+From the case (where x::T ty) we'll get an implication constraint
+ forall b. (Eq ty, Ord b) => <body-constraints>
+Now suppose <body-constraints> itself has an implication constraint
+of form
+ forall c. <reft> => <payload>
+Then, we can certainly apply the refinement <reft> to the Ord b, becuase it is
+existential, but we probably should not apply it to the (Eq ty) because it may
+be wobbly. Hence the isRigidInst
+
+@Insts@ are ordered by their class/type info, rather than by their
+unique. This allows the context-reduction mechanism to use standard finite
+maps to do their stuff. It's horrible that this code is here, rather
+than with the Avails handling stuff in TcSimplify
+
+\begin{code}
addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
addIrred want_scs avails irred = ASSERT2( not (irred `elemAvails` avails), ppr irred $$ ppr avails )
addAvailAndSCs want_scs avails irred IsIrred
where
is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys
-- Note: this compares by *type*, not by Unique
- deps = findAllDeps (unitVarSet (instToId inst)) avail
+ deps = findAllDeps (unitVarSet (instToVar inst)) avail
dep_tys = map idType (varSetElems deps)
findAllDeps :: IdSet -> AvailHow -> IdSet
; addSCs is_loop avails' sc_dict }
where
sc_sel_rhs = L (instSpan dict) (HsWrap co_fn (HsVar sc_sel))
- co_fn = WpApp (instToId dict) <.> mkWpTyApps tys
+ co_fn = WpApp (instToVar dict) <.> mkWpTyApps tys
is_given :: Inst -> Bool
is_given sc_dict = case findAvail avails sc_dict of
Just (Given _) -> True -- Given is cheaper than superclass selection
other -> False
+
+-- From the a set of insts obtain all equalities that (transitively) occur in
+-- superclass contexts of class constraints (aka the ancestor equalities).
+--
+ancestorEqualities :: [Inst] -> TcM [Inst]
+ancestorEqualities
+ = mapM mkWantedEqInst -- turn only equality predicates..
+ . filter isEqPred -- ..into wanted equality insts
+ . bagToList
+ . addAEsToBag emptyBag -- collect the superclass constraints..
+ . map dictPred -- ..of all predicates in a bag
+ . filter isClassDict
+ where
+ addAEsToBag :: Bag PredType -> [PredType] -> Bag PredType
+ addAEsToBag bag [] = bag
+ addAEsToBag bag (pred:preds)
+ | pred `elemBag` bag = addAEsToBag bag preds
+ | isEqPred pred = addAEsToBag bagWithPred preds
+ | isClassPred pred = addAEsToBag bagWithPred predsWithSCs
+ | otherwise = addAEsToBag bag preds
+ where
+ bagWithPred = bag `snocBag` pred
+ predsWithSCs = preds ++ substTheta (zipTopTvSubst tyvars tys) sc_theta
+ --
+ (tyvars, sc_theta, _, _) = classBigSig clas
+ (clas, tys) = getClassPredTys pred
\end{code}
+
%************************************************************************
%* *
\section{tcSimplifyTop: defaulting}
-- The TcLclEnv should be valid here, solely to improve
-- error message generation for the monomorphism restriction
tc_simplify_top doc interactive wanteds
- = do { wanteds <- mapM zonkInst wanteds
+ = do { dflags <- getDOpts
+ ; wanteds <- zonkInsts wanteds
; mapM_ zonkTopTyVar (varSetElems (tyVarsOfInsts wanteds))
- ; (binds1, irreds1) <- topCheckLoop doc wanteds
+ ; traceTc (text "tc_simplify_top 0: " <+> ppr wanteds)
+ ; (irreds1, binds1) <- tryHardCheckLoop doc1 wanteds
+ ; traceTc (text "tc_simplify_top 1: " <+> ppr irreds1)
+ ; (irreds2, binds2) <- approximateImplications doc2 (\d -> True) irreds1
+ ; traceTc (text "tc_simplify_top 2: " <+> ppr irreds2)
- ; if null irreds1 then
- return binds1
- else do
- -- OK, so there are some errors
- { -- Use the defaulting rules to do extra unification
- -- NB: irreds are already zonked
- ; extended_default <- if interactive then return True
- else doptM Opt_ExtendedDefaultRules
- ; disambiguate extended_default irreds1 -- Does unification
- ; (binds2, irreds2) <- topCheckLoop doc irreds1
-
- -- Deal with implicit parameter
- ; let (bad_ips, non_ips) = partition isIPDict irreds2
+ -- Use the defaulting rules to do extra unification
+ -- NB: irreds2 are already zonked
+ ; (irreds3, binds3) <- disambiguate doc3 interactive dflags irreds2
+
+ -- Deal with implicit parameters
+ ; let (bad_ips, non_ips) = partition isIPDict irreds3
(ambigs, others) = partition isTyVarDict non_ips
; topIPErrs bad_ips -- Can arise from f :: Int -> Int
; addNoInstanceErrs others
; addTopAmbigErrs ambigs
- ; return (binds1 `unionBags` binds2) }}
+ ; return (binds1 `unionBags` binds2 `unionBags` binds3) }
+ where
+ doc1 = doc <+> ptext SLIT("(first round)")
+ doc2 = doc <+> ptext SLIT("(approximate)")
+ doc3 = doc <+> ptext SLIT("(disambiguate)")
\end{code}
If a dictionary constrains a type variable which is
@void@.
\begin{code}
-disambiguate :: Bool -> [Inst] -> TcM ()
+disambiguate :: SDoc -> Bool -> DynFlags -> [Inst] -> TcM ([Inst], TcDictBinds)
-- Just does unification to fix the default types
-- The Insts are assumed to be pre-zonked
-disambiguate extended_defaulting insts
+disambiguate doc interactive dflags insts
+ | null insts
+ = return (insts, emptyBag)
+
| null defaultable_groups
- = return ()
+ = do { traceTc (text "disambigutate, no defaultable groups" <+> vcat [ppr unaries, ppr insts, ppr bad_tvs, ppr defaultable_groups])
+ ; return (insts, emptyBag) }
+
| otherwise
= do { -- Figure out what default types to use
- mb_defaults <- getDefaultTys
- ; default_tys <- case mb_defaults of
- Just tys -> return tys
- Nothing -> -- No use-supplied default;
- -- use [Integer, Double]
- do { integer_ty <- tcMetaTy integerTyConName
- ; checkWiredInTyCon doubleTyCon
- ; return [integer_ty, doubleTy] }
- ; mapM_ (disambigGroup default_tys) defaultable_groups }
+ default_tys <- getDefaultTys extended_defaulting ovl_strings
+
+ ; traceTc (text "disambiguate1" <+> vcat [ppr insts, ppr unaries, ppr bad_tvs, ppr defaultable_groups])
+ ; mapM_ (disambigGroup default_tys) defaultable_groups
+
+ -- disambigGroup does unification, hence try again
+ ; tryHardCheckLoop doc insts }
+
where
- unaries :: [(Inst,Class, TcTyVar)] -- (C tv) constraints
- bad_tvs :: TcTyVarSet -- Tyvars mentioned by *other* constraints
- (unaries, bad_tvs) = getDefaultableDicts insts
+ extended_defaulting = interactive || dopt Opt_ExtendedDefaultRules dflags
+ ovl_strings = dopt Opt_OverloadedStrings dflags
+
+ unaries :: [(Inst, Class, TcTyVar)] -- (C tv) constraints
+ bad_tvs :: TcTyVarSet -- Tyvars mentioned by *other* constraints
+ (unaries, bad_tvs_s) = partitionWith find_unary insts
+ bad_tvs = unionVarSets bad_tvs_s
+
+ -- Finds unary type-class constraints
+ find_unary d@(Dict {tci_pred = ClassP cls [ty]})
+ | Just tv <- tcGetTyVar_maybe ty = Left (d,cls,tv)
+ find_unary inst = Right (tyVarsOfInst inst)
-- Group by type variable
defaultable_groups :: [[(Inst,Class,TcTyVar)]]
defaultable_group :: [(Inst,Class,TcTyVar)] -> Bool
defaultable_group ds@((_,_,tv):_)
- = not (isSkolemTyVar tv) -- Note [Avoiding spurious errors]
+ = isTyConableTyVar tv -- Note [Avoiding spurious errors]
&& not (tv `elemVarSet` bad_tvs)
&& defaultable_classes [c | (_,c,_) <- ds]
defaultable_group [] = panic "defaultable_group"
defaultable_classes clss
| extended_defaulting = any isInteractiveClass clss
- | otherwise = all isStandardClass clss && any isNumericClass clss
+ | otherwise = all is_std_class clss && (any is_num_class clss)
-- In interactive mode, or with -fextended-default-rules,
-- we default Show a to Show () to avoid graututious errors on "show []"
isInteractiveClass cls
- = isNumericClass cls
- || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
+ = is_num_class cls || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
+
+ is_num_class cls = isNumericClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
+ -- is_num_class adds IsString to the standard numeric classes,
+ -- when -foverloaded-strings is enabled
+ is_std_class cls = isStandardClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
+ -- Similarly is_std_class
+-----------------------
disambigGroup :: [Type] -- The default types
-> [(Inst,Class,TcTyVar)] -- All standard classes of form (C a)
-> TcM () -- Just does unification, to fix the default types
disambigGroup default_tys dicts
= try_default default_tys
where
- (_,_,tyvar) = head dicts -- Should be non-empty
+ (_,_,tyvar) = ASSERT(not (null dicts)) head dicts -- Should be non-empty
classes = [c | (_,c,_) <- dicts]
try_default [] = return ()
-- After this we can't fail
; warnDefault dicts default_ty
- ; unifyType default_ty (mkTyVarTy tyvar) }
+ ; unifyType default_ty (mkTyVarTy tyvar)
+ ; return () -- TOMDO: do something with the coercion
+ }
+
+
+-----------------------
+getDefaultTys :: Bool -> Bool -> TcM [Type]
+getDefaultTys extended_deflts ovl_strings
+ = do { mb_defaults <- getDeclaredDefaultTys
+ ; case mb_defaults of {
+ Just tys -> return tys ; -- User-supplied defaults
+ Nothing -> do
+
+ -- No use-supplied default
+ -- Use [Integer, Double], plus modifications
+ { integer_ty <- tcMetaTy integerTyConName
+ ; checkWiredInTyCon doubleTyCon
+ ; string_ty <- tcMetaTy stringTyConName
+ ; return (opt_deflt extended_deflts unitTy
+ -- Note [Default unitTy]
+ ++
+ [integer_ty,doubleTy]
+ ++
+ opt_deflt ovl_strings string_ty) } } }
+ where
+ opt_deflt True ty = [ty]
+ opt_deflt False ty = []
\end{code}
+Note [Default unitTy]
+~~~~~~~~~~~~~~~~~~~~~
+In interative mode (or with -fextended-default-rules) we add () as the first type we
+try when defaulting. This has very little real impact, except in the following case.
+Consider:
+ Text.Printf.printf "hello"
+This has type (forall a. IO a); it prints "hello", and returns 'undefined'. We don't
+want the GHCi repl loop to try to print that 'undefined'. The neatest thing is to
+default the 'a' to (), rather than to Integer (which is what would otherwise happen;
+and then GHCi doesn't attempt to print the (). So in interactive mode, we add
+() to the list of defaulting types. See Trac #1200.
+
Note [Avoiding spurious errors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When doing the unification for defaulting, we check for skolem
\begin{code}
tcSimplifyDeriv :: InstOrigin
- -> TyCon
-> [TyVar]
-> ThetaType -- Wanted
-> TcM ThetaType -- Needed
+-- Given instance (wanted) => C inst_ty
+-- Simplify 'wanted' as much as possible
-tcSimplifyDeriv orig tc tyvars theta
- = tcInstTyVars tyvars `thenM` \ (tvs, _, tenv) ->
+tcSimplifyDeriv orig tyvars theta
+ = do { (tvs, _, tenv) <- tcInstTyVars tyvars
-- The main loop may do unification, and that may crash if
-- it doesn't see a TcTyVar, so we have to instantiate. Sigh
-- ToDo: what if two of them do get unified?
- newDictBndrsO orig (substTheta tenv theta) `thenM` \ wanteds ->
- topCheckLoop doc wanteds `thenM` \ (_, irreds) ->
-
- doptM Opt_GlasgowExts `thenM` \ gla_exts ->
- doptM Opt_AllowUndecidableInstances `thenM` \ undecidable_ok ->
- let
- inst_ty = mkTyConApp tc (mkTyVarTys tvs)
- (ok_insts, bad_insts) = partition is_ok_inst irreds
- is_ok_inst inst
- = isDict inst -- Exclude implication consraints
- && (isTyVarClassPred pred || (gla_exts && ok_gla_pred pred))
- where
- pred = dictPred inst
-
- ok_gla_pred pred = null (checkInstTermination [inst_ty] [pred])
- -- See Note [Deriving context]
-
- tv_set = mkVarSet tvs
- simpl_theta = map dictPred ok_insts
- weird_preds = [pred | pred <- simpl_theta
- , not (tyVarsOfPred pred `subVarSet` tv_set)]
-
- -- Check for a bizarre corner case, when the derived instance decl should
- -- have form instance C a b => D (T a) where ...
- -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
- -- of problems; in particular, it's hard to compare solutions for
- -- equality when finding the fixpoint. So I just rule it out for now.
-
- rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
- -- This reverse-mapping is a Royal Pain,
- -- but the result should mention TyVars not TcTyVars
- in
- -- In effect, the bad and wierd insts cover all of the cases that
- -- would make checkValidInstance fail; if it were called right after tcSimplifyDeriv
- -- * wierd_preds ensures unambiguous instances (checkAmbiguity in checkValidInstance)
- -- * ok_gla_pred ensures termination (checkInstTermination in checkValidInstance)
- addNoInstanceErrs bad_insts `thenM_`
- mapM_ (addErrTc . badDerivedPred) weird_preds `thenM_`
- returnM (substTheta rev_env simpl_theta)
+ ; wanteds <- newDictBndrsO orig (substTheta tenv theta)
+ ; (irreds, _) <- tryHardCheckLoop doc wanteds
+
+ ; let (tv_dicts, others) = partition ok irreds
+ ; addNoInstanceErrs others
+ -- See Note [Exotic derived instance contexts] in TcMType
+
+ ; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
+ simpl_theta = substTheta rev_env (map dictPred tv_dicts)
+ -- This reverse-mapping is a pain, but the result
+ -- should mention the original TyVars not TcTyVars
+
+ ; return simpl_theta }
where
doc = ptext SLIT("deriving classes for a data type")
-\end{code}
-Note [Deriving context]
-~~~~~~~~~~~~~~~~~~~~~~~
-With -fglasgow-exts, we allow things like (C Int a) in the simplified
-context for a derived instance declaration, because at a use of this
-instance, we might know that a=Bool, and have an instance for (C Int
-Bool)
-
-We nevertheless insist that each predicate meets the termination
-conditions. If not, the deriving mechanism generates larger and larger
-constraints. Example:
- data Succ a = S a
- data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
-
-Note the lack of a Show instance for Succ. First we'll generate
- instance (Show (Succ a), Show a) => Show (Seq a)
-and then
- instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
-and so on. Instead we want to complain of no instance for (Show (Succ a)).
-
+ ok dict | isDict dict = validDerivPred (dictPred dict)
+ | otherwise = False
+\end{code}
@tcSimplifyDefault@ just checks class-type constraints, essentially;
tcSimplifyDefault theta
= newDictBndrsO DefaultOrigin theta `thenM` \ wanteds ->
- topCheckLoop doc wanteds `thenM` \ (_, irreds) ->
+ tryHardCheckLoop doc wanteds `thenM` \ (irreds, _) ->
addNoInstanceErrs irreds `thenM_`
if null irreds then
returnM ()
addTopIPErrs bndrs []
= return ()
addTopIPErrs bndrs ips
- = addErrTcM (tidy_env, mk_msg tidy_ips)
+ = do { dflags <- getDOpts
+ ; addErrTcM (tidy_env, mk_msg dflags tidy_ips) }
where
(tidy_env, tidy_ips) = tidyInsts ips
- mk_msg ips = vcat [sep [ptext SLIT("Implicit parameters escape from"),
- nest 2 (ptext SLIT("the monomorphic top-level binding(s) of")
+ mk_msg dflags ips
+ = vcat [sep [ptext SLIT("Implicit parameters escape from"),
+ nest 2 (ptext SLIT("the monomorphic top-level binding")
+ <> plural bndrs <+> ptext SLIT("of")
<+> pprBinders bndrs <> colon)],
- nest 2 (vcat (map ppr_ip ips)),
- monomorphism_fix]
+ nest 2 (vcat (map ppr_ip ips)),
+ monomorphism_fix dflags]
ppr_ip ip = pprPred (dictPred ip) <+> pprInstArising ip
topIPErrs :: [Inst] -> TcM ()
= groupErrs (report_no_instances tidy_env mb_what) insts
report_no_instances tidy_env mb_what insts
- = do { inst_envs <- tcGetInstEnvs
- ; let (implics, insts1) = partition isImplicInst insts
- (insts2, overlaps) = partitionWith (check_overlap inst_envs) insts1
- ; traceTc (text "reportNoInstnces" <+> vcat
- [ppr implics, ppr insts1, ppr insts2])
- ; mapM_ complain_implic implics
- ; mapM_ (\doc -> addErrTcM (tidy_env, doc)) overlaps
- ; groupErrs complain_no_inst insts2 }
+ = do { inst_envs <- tcGetInstEnvs
+ ; let (implics, insts1) = partition isImplicInst insts
+ (insts2, overlaps) = partitionWith (check_overlap inst_envs) insts1
+ (eqInsts, insts3) = partition isEqInst insts2
+ ; traceTc (text "reportNoInstances" <+> vcat
+ [ppr implics, ppr insts1, ppr insts2])
+ ; mapM_ complain_implic implics
+ ; mapM_ (\doc -> addErrTcM (tidy_env, doc)) overlaps
+ ; groupErrs complain_no_inst insts3
+ ; mapM_ complain_eq eqInsts
+ }
where
complain_no_inst insts = addErrTcM (tidy_env, mk_no_inst_err insts)
(Just (tci_loc inst, tci_given inst))
(tci_wanted inst)
+ complain_eq EqInst {tci_left = lty, tci_right = rty,
+ tci_loc = InstLoc _ _ ctxt}
+ = do { (env, msg) <- misMatchMsg lty rty
+ ; setErrCtxt ctxt $
+ failWithTcM (env, msg)
+ }
+
check_overlap :: (InstEnv,InstEnv) -> Inst -> Either Inst SDoc
-- Right msg => overlap message
-- Left inst => no instance
| not (isClassDict wanted) = Left wanted
| otherwise
= case lookupInstEnv inst_envs clas tys of
- -- The case of exactly one match and no unifiers means
- -- a successful lookup. That can't happen here, becuase
- -- dicts only end up here if they didn't match in Inst.lookupInst
+ -- The case of exactly one match and no unifiers means a
+ -- successful lookup. That can't happen here, because dicts
+ -- only end up here if they didn't match in Inst.lookupInst
#ifdef DEBUG
([m],[]) -> pprPanic "reportNoInstance" (ppr wanted)
#endif
(clas,tys) = getDictClassTys wanted
mk_overlap_msg dict (matches, unifiers)
- = vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for")
+ = ASSERT( not (null matches) )
+ vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for")
<+> pprPred (dictPred dict))),
sep [ptext SLIT("Matching instances") <> colon,
nest 2 (vcat [pprInstances ispecs, pprInstances unifiers])],
- ASSERT( not (null matches) )
if not (isSingleton matches)
then -- Two or more matches
empty
ASSERT( not (null unifiers) )
parens (vcat [ptext SLIT("The choice depends on the instantiation of") <+>
quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
- ptext SLIT("Use -fallow-incoherent-instances to use the first choice above")])]
+ ptext SLIT("To pick the first instance above, use -fallow-incoherent-instances"),
+ ptext SLIT("when compiling the other instance declarations")])]
where
- ispecs = [ispec | (_, ispec) <- matches]
+ ispecs = [ispec | (ispec, _) <- matches]
mk_no_inst_err insts
| null insts = empty
-- Try to identify the offending variable
-- ASSUMPTION: the Insts are fully zonked
mkMonomorphismMsg tidy_env inst_tvs
- = findGlobals (mkVarSet inst_tvs) tidy_env `thenM` \ (tidy_env, docs) ->
- returnM (tidy_env, mk_msg docs)
+ = do { dflags <- getDOpts
+ ; (tidy_env, docs) <- findGlobals (mkVarSet inst_tvs) tidy_env
+ ; return (tidy_env, mk_msg dflags docs) }
where
- mk_msg [] = ptext SLIT("Probable fix: add a type signature that fixes these type variable(s)")
+ mk_msg _ _ | any isRuntimeUnk inst_tvs
+ = vcat [ptext SLIT("Cannot resolve unknown runtime types:") <+>
+ (pprWithCommas ppr inst_tvs),
+ ptext SLIT("Use :print or :force to determine these types")]
+ mk_msg _ [] = ptext SLIT("Probable fix: add a type signature that fixes these type variable(s)")
-- This happens in things like
-- f x = show (read "foo")
-- where monomorphism doesn't play any role
- mk_msg docs = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
- nest 2 (vcat docs),
- monomorphism_fix
- ]
-monomorphism_fix :: SDoc
-monomorphism_fix = ptext SLIT("Probable fix:") <+>
- (ptext SLIT("give these definition(s) an explicit type signature")
- $$ ptext SLIT("or use -fno-monomorphism-restriction"))
+ mk_msg dflags docs
+ = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
+ nest 2 (vcat docs),
+ monomorphism_fix dflags]
+
+isRuntimeUnk :: TcTyVar -> Bool
+isRuntimeUnk x | SkolemTv RuntimeUnkSkol <- tcTyVarDetails x = True
+ | otherwise = False
+
+monomorphism_fix :: DynFlags -> SDoc
+monomorphism_fix dflags
+ = ptext SLIT("Probable fix:") <+> vcat
+ [ptext SLIT("give these definition(s) an explicit type signature"),
+ if dopt Opt_MonomorphismRestriction dflags
+ then ptext SLIT("or use -fno-monomorphism-restriction")
+ else empty] -- Only suggest adding "-fno-monomorphism-restriction"
+ -- if it is not already set!
warnDefault ups default_ty
= doptM Opt_WarnTypeDefaults `thenM` \ warn_flag ->
quotes (ppr default_ty),
pprDictsInFull tidy_dicts]
--- Used for the ...Thetas variants; all top level
-badDerivedPred pred
- = vcat [ptext SLIT("Can't derive instances where the instance context mentions"),
- ptext SLIT("type variables that are not data type parameters"),
- nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)]
-
reduceDepthErr n stack
= vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
ptext SLIT("Use -fcontext-stack=N to increase stack size to N"),
nest 4 (pprStack stack)]
pprStack stack = vcat (map pprInstInFull stack)
+
+-----------------------
+misMatchMsg :: TcType -> TcType -> TcM (TidyEnv, SDoc)
+-- Generate the message when two types fail to match,
+-- going to some trouble to make it helpful.
+-- The argument order is: actual type, expected type
+misMatchMsg ty_act ty_exp
+ = do { env0 <- tcInitTidyEnv
+ ; ty_exp <- zonkTcType ty_exp
+ ; ty_act <- zonkTcType ty_act
+ ; (env1, pp_exp, extra_exp) <- ppr_ty env0 ty_exp
+ ; (env2, pp_act, extra_act) <- ppr_ty env1 ty_act
+ ; return (env2,
+ sep [sep [ptext SLIT("Couldn't match expected type") <+> pp_exp,
+ nest 7 $
+ ptext SLIT("against inferred type") <+> pp_act],
+ nest 2 (extra_exp $$ extra_act)]) }
+
+ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
+ppr_ty env ty
+ = do { let (env1, tidy_ty) = tidyOpenType env ty
+ ; (env2, extra) <- ppr_extra env1 tidy_ty
+ ; return (env2, quotes (ppr tidy_ty), extra) }
+
+-- (ppr_extra env ty) shows extra info about 'ty'
+ppr_extra env (TyVarTy tv)
+ | isSkolemTyVar tv || isSigTyVar tv
+ = return (env1, pprSkolTvBinding tv1)
+ where
+ (env1, tv1) = tidySkolemTyVar env tv
+
+ppr_extra env ty = return (env, empty) -- Normal case
\end{code}
projects / ghc-hetmet.git / blobdiff