tcSimplifyInfer, tcSimplifyInferCheck,
tcSimplifyCheck, tcSimplifyRestricted,
tcSimplifyToDicts, tcSimplifyIPs,
+ tcSimplifySuperClasses,
tcSimplifyTop, tcSimplifyInteractive,
tcSimplifyBracket,
#include "HsVersions.h"
-import {-# SOURCE #-} TcUnify( unifyTauTy )
-import TcEnv -- temp
-import HsSyn ( HsBind(..), LHsBinds, HsExpr(..), LHsExpr, pprLHsBinds )
-import TcHsSyn ( TcId, TcDictBinds, mkHsApp, mkHsTyApp, mkHsDictApp )
+import {-# SOURCE #-} TcUnify( unifyType )
+import TypeRep ( Type(..) )
+import HsSyn ( HsBind(..), HsExpr(..), LHsExpr, emptyLHsBinds )
+import TcHsSyn ( mkHsApp, mkHsTyApp, mkHsDictApp )
import TcRnMonad
import Inst ( lookupInst, LookupInstResult(..),
- tyVarsOfInst, fdPredsOfInsts, fdPredsOfInst, newDicts,
+ tyVarsOfInst, fdPredsOfInsts, newDicts,
isDict, isClassDict, isLinearInst, linearInstType,
- isStdClassTyVarDict, isMethodFor, isMethod,
+ isMethodFor, isMethod,
instToId, tyVarsOfInsts, cloneDict,
ipNamesOfInsts, ipNamesOfInst, dictPred,
- instBindingRequired,
- newDictsFromOld, tcInstClassOp,
- getDictClassTys, isTyVarDict,
- instLoc, zonkInst, tidyInsts, tidyMoreInsts,
- Inst, pprInsts, pprDictsInFull, pprInstInFull, tcGetInstEnvs,
- isIPDict, isInheritableInst, pprDFuns, pprDictsTheta
+ fdPredsOfInst,
+ newDictsAtLoc, tcInstClassOp,
+ getDictClassTys, isTyVarDict, instLoc,
+ zonkInst, tidyInsts, tidyMoreInsts,
+ pprInsts, pprDictsInFull, pprInstInFull, tcGetInstEnvs,
+ isInheritableInst, pprDictsTheta
)
-import TcEnv ( tcGetGlobalTyVars, tcLookupId, findGlobals )
-import InstEnv ( lookupInstEnv, classInstances )
-import TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, checkAmbiguity )
-import TcType ( TcTyVar, TcTyVarSet, ThetaType, TyVarDetails(VanillaTv),
- mkClassPred, isOverloadedTy, mkTyConApp,
+import TcEnv ( tcGetGlobalTyVars, tcLookupId, findGlobals, pprBinders,
+ lclEnvElts, tcMetaTy )
+import InstEnv ( lookupInstEnv, classInstances, pprInstances )
+import TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, zonkTcPredType,
+ checkAmbiguity, checkInstTermination )
+import TcType ( TcTyVar, TcTyVarSet, ThetaType, TcPredType, tidyPred,
+ mkClassPred, isOverloadedTy, mkTyConApp, isSkolemTyVar,
mkTyVarTy, tcGetTyVar, isTyVarClassPred, mkTyVarTys,
- tyVarsOfPred, tcEqType, pprPred )
+ tyVarsOfPred, tcEqType, pprPred, mkPredTy, tcIsTyVarTy )
+import TcIface ( checkWiredInTyCon )
import Id ( idType, mkUserLocal )
import Var ( TyVar )
-import Name ( getOccName, getSrcLoc )
+import TyCon ( TyCon )
+import Name ( Name, getOccName, getSrcLoc )
import NameSet ( NameSet, mkNameSet, elemNameSet )
import Class ( classBigSig, classKey )
-import FunDeps ( oclose, grow, improve, pprEquationDoc )
-import PrelInfo ( isNumericClass )
+import FunDeps ( oclose, grow, improve, pprEquation )
+import PrelInfo ( isNumericClass, isStandardClass )
import PrelNames ( splitName, fstName, sndName, integerTyConName,
showClassKey, eqClassKey, ordClassKey )
-import Subst ( mkTopTyVarSubst, substTheta, substTy )
-import TysWiredIn ( pairTyCon, doubleTy )
+import Type ( zipTopTvSubst, substTheta, substTy )
+import TysWiredIn ( pairTyCon, doubleTy, doubleTyCon )
import ErrUtils ( Message )
+import BasicTypes ( TopLevelFlag, isNotTopLevel )
import VarSet
import VarEnv ( TidyEnv )
import FiniteMap
import Util ( zipEqual, isSingleton )
import List ( partition )
import SrcLoc ( Located(..) )
-import CmdLineOpts
+import DynFlags ( DynFlag(..) )
+import StaticFlags
\end{code}
we follow the argument of Question 2 and generalise over ?y.
+Question 4: top level
+~~~~~~~~~~~~~~~~~~~~~
+At the top level, monomorhism makes no sense at all.
+
+ module Main where
+ main = let ?x = 5 in print foo
+
+ foo = woggle 3
+
+ woggle :: (?x :: Int) => Int -> Int
+ woggle y = ?x + y
+
+We definitely don't want (foo :: Int) with a top-level implicit parameter
+(?x::Int) becuase there is no way to bind it.
+
Possible choices
~~~~~~~~~~~~~~~~
try_me inst
| isFreeWhenInferring qtvs inst = Free
| isClassDict inst = DontReduceUnlessConstant -- Dicts
- | otherwise = ReduceMe -- Lits and Methods
+ | otherwise = ReduceMe NoSCs -- Lits and Methods
in
- traceTc (text "infloop" <+> vcat [ppr tau_tvs', ppr wanteds', ppr preds, ppr (grow preds tau_tvs'), ppr qtvs]) `thenM_`
+ traceTc (text "infloop" <+> vcat [ppr tau_tvs', ppr wanteds', ppr preds,
+ ppr (grow preds tau_tvs'), ppr qtvs]) `thenM_`
-- Step 2
reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
-- global type variables in the environment; so you don't
-- need to worry about setting them before calling tcSimplifyCheck
tcSimplifyCheck doc qtvs givens wanted_lie
- = tcSimplCheck doc get_qtvs
- givens wanted_lie `thenM` \ (qtvs', binds) ->
- returnM binds
+ = ASSERT( all isSkolemTyVar qtvs )
+ do { (qtvs', frees, binds) <- tcSimplCheck doc get_qtvs AddSCs givens wanted_lie
+ ; extendLIEs frees
+ ; return binds }
where
- get_qtvs = zonkTcTyVarsAndFV qtvs
+-- get_qtvs = zonkTcTyVarsAndFV qtvs
+ get_qtvs = return (mkVarSet qtvs) -- All skolems
-- tcSimplifyInferCheck is used when we know the constraints we are to simplify
TcDictBinds) -- Bindings
tcSimplifyInferCheck doc tau_tvs givens wanted_lie
- = tcSimplCheck doc get_qtvs givens wanted_lie
+ = do { (qtvs', frees, binds) <- tcSimplCheck doc get_qtvs AddSCs givens wanted_lie
+ ; extendLIEs frees
+ ; return (qtvs', binds) }
where
-- Figure out which type variables to quantify over
-- You might think it should just be the signature tyvars,
Here is the workhorse function for all three wrappers.
\begin{code}
-tcSimplCheck doc get_qtvs givens wanted_lie
- = check_loop givens wanted_lie `thenM` \ (qtvs, frees, binds, irreds) ->
-
- -- Complain about any irreducible ones
- mappM zonkInst given_dicts_and_ips `thenM` \ givens' ->
- groupErrs (addNoInstanceErrs (Just doc) givens') irreds `thenM_`
+tcSimplCheck doc get_qtvs want_scs givens wanted_lie
+ = do { (qtvs, frees, binds, irreds) <- check_loop givens wanted_lie
- -- Done
- extendLIEs frees `thenM_`
- returnM (qtvs, binds)
+ -- Complain about any irreducible ones
+ ; if not (null irreds)
+ then do { givens' <- mappM zonkInst given_dicts_and_ips
+ ; groupErrs (addNoInstanceErrs (Just doc) givens') irreds }
+ else return ()
+ ; returnM (qtvs, frees, binds) }
where
given_dicts_and_ips = filter (not . isMethod) givens
-- For error reporting, filter out methods, which are
-- When checking against a given signature we always reduce
-- until we find a match against something given, or can't reduce
try_me inst | isFreeWhenChecking qtvs' ip_set inst = Free
- | otherwise = ReduceMe
+ | otherwise = ReduceMe want_scs
in
reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
%************************************************************************
%* *
+ tcSimplifySuperClasses
+%* *
+%************************************************************************
+
+Note [SUPERCLASS-LOOP 1]
+~~~~~~~~~~~~~~~~~~~~~~~~
+We have to be very, very careful when generating superclasses, lest we
+accidentally build a loop. Here's an example:
+
+ class S a
+
+ class S a => C a where { opc :: a -> a }
+ class S b => D b where { opd :: b -> b }
+
+ instance C Int where
+ opc = opd
+
+ instance D Int where
+ opd = opc
+
+From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
+Simplifying, we may well get:
+ $dfCInt = :C ds1 (opd dd)
+ dd = $dfDInt
+ ds1 = $p1 dd
+Notice that we spot that we can extract ds1 from dd.
+
+Alas! Alack! We can do the same for (instance D Int):
+
+ $dfDInt = :D ds2 (opc dc)
+ dc = $dfCInt
+ ds2 = $p1 dc
+
+And now we've defined the superclass in terms of itself.
+
+Solution: never generate a superclass selectors at all when
+satisfying the superclass context of an instance declaration.
+
+Two more nasty cases are in
+ tcrun021
+ tcrun033
+
+\begin{code}
+tcSimplifySuperClasses qtvs givens sc_wanteds
+ = ASSERT( all isSkolemTyVar qtvs )
+ do { (_, frees, binds1) <- tcSimplCheck doc get_qtvs NoSCs givens sc_wanteds
+ ; binds2 <- tc_simplify_top doc False NoSCs frees
+ ; return (binds1 `unionBags` binds2) }
+ where
+ get_qtvs = return (mkVarSet qtvs)
+ doc = ptext SLIT("instance declaration superclass context")
+\end{code}
+
+
+%************************************************************************
+%* *
\subsection{tcSimplifyRestricted}
%* *
%************************************************************************
tcSimplifyRestricted -- Used for restricted binding groups
-- i.e. ones subject to the monomorphism restriction
:: SDoc
+ -> TopLevelFlag
+ -> [Name] -- Things bound in this group
-> TcTyVarSet -- Free in the type of the RHSs
-> [Inst] -- Free in the RHSs
-> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
-- quantify over; by definition there are none.
-- They are all thrown back in the LIE
-tcSimplifyRestricted doc tau_tvs wanteds
+tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
-- Zonk everything in sight
= mappM zonkInst wanteds `thenM` \ wanteds' ->
zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenM` \ tau_tvs' ->
constrained_tvs = tyVarsOfInsts constrained_dicts
qtvs = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs')
`minusVarSet` constrained_tvs
- try_me inst | isFreeWrtTyVars qtvs inst = Free
- | otherwise = ReduceMe
in
traceTc (text "tcSimplifyRestricted" <+> vcat [
pprInsts wanteds, pprInsts _frees, pprInsts constrained_dicts,
-- Remember that we may need to do *some* simplification, to
-- (for example) squash {Monad (ST s)} into {}. It's not enough
-- just to float all constraints
+ --
+ -- At top level, we *do* squash methods becuase we want to
+ -- expose implicit parameters to the test that follows
+ let
+ is_nested_group = isNotTopLevel top_lvl
+ try_me inst | isFreeWrtTyVars qtvs inst,
+ (is_nested_group || isDict inst) = Free
+ | otherwise = ReduceMe AddSCs
+ in
reduceContextWithoutImprovement
doc try_me wanteds' `thenM` \ (frees, binds, irreds) ->
ASSERT( null irreds )
- extendLIEs frees `thenM_`
- returnM (varSetElems qtvs, binds)
+
+ -- See "Notes on implicit parameters, Question 4: top level"
+ if is_nested_group then
+ extendLIEs frees `thenM_`
+ returnM (varSetElems qtvs, binds)
+ else
+ let
+ (non_ips, bad_ips) = partition isClassDict frees
+ in
+ addTopIPErrs bndrs bad_ips `thenM_`
+ extendLIEs non_ips `thenM_`
+ returnM (varSetElems qtvs, binds)
\end{code}
forall dIntegralInt.
fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
-because the scsel will mess up matching. Instead we want
+because the scsel will mess up RULE matching. Instead we want
forall dIntegralInt, dNumInt.
fromIntegral Int Int dIntegralInt dNumInt = id Int
-- Reduce methods and lits only; stop as soon as we get a dictionary
try_me inst | isDict inst = KeepDictWithoutSCs -- See notes above re "WithoutSCs"
- | otherwise = ReduceMe
+ | otherwise = ReduceMe NoSCs
\end{code}
-- Simplify any methods that mention the implicit parameter
try_me inst | isFreeWrtIPs ip_set inst = Free
- | otherwise = ReduceMe
+ | otherwise = ReduceMe NoSCs
simpl_loop givens wanteds
= mappM zonkInst givens `thenM` \ givens' ->
@LIE@), as well as the @HsBinds@ generated.
\begin{code}
-bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM (LHsBinds TcId)
+bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcDictBinds
+-- Simlifies only MethodInsts, and generate only bindings of form
+-- fm = f tys dicts
+-- We're careful not to even generate bindings of the form
+-- d1 = d2
+-- You'd think that'd be fine, but it interacts with what is
+-- arguably a bug in Match.tidyEqnInfo (see notes there)
bindInstsOfLocalFuns wanteds local_ids
| null overloaded_ids
-- Common case
= extendLIEs wanteds `thenM_`
- returnM emptyBag
+ returnM emptyLHsBinds
| otherwise
- = simpleReduceLoop doc try_me wanteds `thenM` \ (frees, binds, irreds) ->
+ = simpleReduceLoop doc try_me for_me `thenM` \ (frees, binds, irreds) ->
ASSERT( null irreds )
+ extendLIEs not_for_me `thenM_`
extendLIEs frees `thenM_`
returnM binds
where
doc = text "bindInsts" <+> ppr local_ids
overloaded_ids = filter is_overloaded local_ids
is_overloaded id = isOverloadedTy (idType id)
+ (for_me, not_for_me) = partition (isMethodFor overloaded_set) wanteds
overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
-- so it's worth building a set, so that
-- lookup (in isMethodFor) is faster
-
- try_me inst | isMethodFor overloaded_set inst = ReduceMe
- | otherwise = Free
+ try_me inst | isMethod inst = ReduceMe NoSCs
+ | otherwise = Free
\end{code}
\begin{code}
data WhatToDo
- = ReduceMe -- Try to reduce this
+ = ReduceMe WantSCs -- Try to reduce this
-- If there's no instance, behave exactly like
- -- DontReduce: add the inst to
- -- the irreductible ones, but don't
- -- produce an error message of any kind.
+ -- DontReduce: add the inst to the irreductible ones,
+ -- but don't produce an error message of any kind.
-- It might be quite legitimate such as (Eq a)!
| KeepDictWithoutSCs -- Return as irreducible; don't add its superclasses
| Free -- Return as free
reduceMe :: Inst -> WhatToDo
-reduceMe inst = ReduceMe
+reduceMe inst = ReduceMe AddSCs
data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
-- of a predicate when adding it to the avails
+ -- The reason for this flag is entirely the super-class loop problem
+ -- Note [SUPER-CLASS LOOP 1]
\end{code}
-- e.g. those "given" in a signature
Bool -- True <=> actually consumed (splittable IPs only)
- | NoRhs -- Used for Insts like (CCallable f)
- -- where no witness is required.
- -- ToDo: remove?
-
| Rhs -- Used when there is a RHS
(LHsExpr TcId) -- The RHS
[Inst] -- Insts free in the RHS; we need these too
instance Outputable Avail where
ppr = pprAvail
-pprAvail NoRhs = text "<no rhs>"
pprAvail IsFree = text "Free"
pprAvail Irred = text "Irred"
pprAvail (Given x b) = text "Given" <+> ppr x <+>
Nothing -> pprTrace "Urk: extractResults" (ppr w) $
go avails binds irreds frees ws
- Just NoRhs -> go avails binds irreds frees ws
Just IsFree -> go (add_free avails w) binds irreds (w:frees) ws
Just Irred -> go (add_given avails w) binds (w:irreds) frees ws
get_root irreds frees IsFree w = cloneDict w `thenM` \ w' ->
returnM (irreds, w':frees, instToId w')
- add_given avails w
- | instBindingRequired w = addToFM avails w (Given (instToId w) True)
- | otherwise = addToFM avails w NoRhs
- -- NB: make sure that CCallable/CReturnable use NoRhs rather
- -- than Given, else we end up with bogus bindings.
+ add_given avails w = addToFM avails w (Given (instToId w) True)
add_free avails w | isMethod w = avails
| otherwise = add_given avails w
tcImprove :: Avails -> TcM Bool -- False <=> no change
-- Perform improvement using all the predicates in Avails
tcImprove avails
- = tcGetInstEnvs `thenM` \ (home_ie, pkg_ie) ->
+ = tcGetInstEnvs `thenM` \ inst_envs ->
let
preds = [ (pred, pp_loc)
- | inst <- keysFM avails,
- let pp_loc = pprInstLoc (instLoc inst),
- pred <- fdPredsOfInst inst
+ | (inst, avail) <- fmToList avails,
+ pred <- get_preds inst avail,
+ let pp_loc = pprInstLoc (instLoc inst)
]
-- Avails has all the superclasses etc (good)
-- It also has all the intermediates of the deduction (good)
-- It does not have duplicates (good)
-- NB that (?x::t1) and (?x::t2) will be held separately in avails
-- so that improve will see them separate
+
+ -- For free Methods, we want to take predicates from their context,
+ -- but for Methods that have been squished their context will already
+ -- be in Avails, and we don't want duplicates. Hence this rather
+ -- horrid get_preds function
+ get_preds inst IsFree = fdPredsOfInst inst
+ get_preds inst other | isDict inst = [dictPred inst]
+ | otherwise = []
+
eqns = improve get_insts preds
- get_insts clas = classInstances home_ie clas ++ classInstances pkg_ie clas
+ get_insts clas = classInstances inst_envs clas
in
if null eqns then
returnM True
mappM_ unify eqns `thenM_`
returnM False
where
- unify ((qtvs, t1, t2), doc)
- = addErrCtxt doc $
- tcInstTyVars VanillaTv (varSetElems qtvs) `thenM` \ (_, _, tenv) ->
- unifyTauTy (substTy tenv t1) (substTy tenv t2)
+ unify ((qtvs, pairs), what1, what2)
+ = addErrCtxtM (mkEqnMsg what1 what2) $
+ tcInstTyVars (varSetElems qtvs) `thenM` \ (_, _, tenv) ->
+ mapM_ (unif_pr tenv) pairs
+ unif_pr tenv (ty1,ty2) = unifyType (substTy tenv ty1) (substTy tenv ty2)
+
+pprEquationDoc (eqn, (p1,w1), (p2,w2)) = vcat [pprEquation eqn, nest 2 (ppr p1), nest 2 (ppr p2)]
+
+mkEqnMsg (pred1,from1) (pred2,from2) tidy_env
+ = do { pred1' <- zonkTcPredType pred1; pred2' <- zonkTcPredType pred2
+ ; let { pred1'' = tidyPred tidy_env pred1'; pred2'' = tidyPred tidy_env pred2' }
+ ; let msg = vcat [ptext SLIT("When using functional dependencies to combine"),
+ nest 2 (sep [ppr pred1'' <> comma, nest 2 from1]),
+ nest 2 (sep [ppr pred2'' <> comma, nest 2 from2])]
+ ; return (tidy_env, msg) }
\end{code}
The main context-reduction function is @reduce@. Here's its game plan.
#ifdef DEBUG
(if n > 8 then
pprTrace "Interesting! Context reduction stack deeper than 8:"
- (nest 2 (pprStack stack))
+ (int n $$ ifPprDebug (nest 2 (pprStack stack)))
else (\x->x))
#endif
go wanteds state
-- First, see if the inst can be reduced to a constant in one step
try_simple addFree
- ; ReduceMe -> -- It should be reduced
+ ; ReduceMe want_scs -> -- It should be reduced
lookupInst wanted `thenM` \ lookup_result ->
case lookup_result of
GenInst wanteds' rhs -> addIrred NoSCs avails wanted `thenM` \ avails1 ->
reduceList stack try_me wanteds' avails1 `thenM` \ avails2 ->
- addWanted avails2 wanted rhs wanteds'
+ addWanted want_scs avails2 wanted rhs wanteds'
-- Experiment with temporarily doing addIrred *before* the reduceList,
-- which has the effect of adding the thing we are trying
-- to prove to the database before trying to prove the things it
-- the examples in [SUPERCLASS-LOOP]
-- So we do an addIrred before, and then overwrite it afterwards with addWanted
- SimpleInst rhs -> addWanted avails wanted rhs []
+ SimpleInst rhs -> addWanted want_scs avails wanted rhs []
NoInstance -> -- No such instance!
-- Add it and its superclasses
- addIrred AddSCs avails wanted
+ addIrred want_scs avails wanted
}
where
try_simple do_this_otherwise
= lookupInst wanted `thenM` \ lookup_result ->
case lookup_result of
- SimpleInst rhs -> addWanted avails wanted rhs []
+ SimpleInst rhs -> addWanted AddSCs avails wanted rhs []
other -> do_this_otherwise avails wanted
\end{code}
isAvailable :: Avails -> Inst -> Maybe Avail
isAvailable avails wanted = lookupFM avails wanted
-- NB 1: the Ord instance of Inst compares by the class/type info
- -- *not* by unique. So
+ -- *not* by unique. So
-- d1::C Int == d2::C Int
addLinearAvailable :: Avails -> Avail -> Inst -> TcM (Avails, [Inst])
--
addFree avails free = returnM (addToFM avails free IsFree)
-addWanted :: Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
-addWanted avails wanted rhs_expr wanteds
- = addAvailAndSCs avails wanted avail
+addWanted :: WantSCs -> Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
+addWanted want_scs avails wanted rhs_expr wanteds
+ = addAvailAndSCs want_scs avails wanted avail
where
- avail | instBindingRequired wanted = Rhs rhs_expr wanteds
- | otherwise = ASSERT( null wanteds ) NoRhs
+ avail = Rhs rhs_expr wanteds
addGiven :: Avails -> Inst -> TcM Avails
-addGiven avails given = addAvailAndSCs avails given (Given (instToId given) False)
+addGiven avails given = addAvailAndSCs AddSCs avails given (Given (instToId given) False)
+ -- Always add superclasses for 'givens'
+ --
-- No ASSERT( not (given `elemFM` avails) ) because in an instance
-- decl for Ord t we can add both Ord t and Eq t as 'givens',
-- so the assert isn't true
addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
-addIrred NoSCs avails irred = returnM (addToFM avails irred Irred)
-addIrred AddSCs avails irred = ASSERT2( not (irred `elemFM` avails), ppr irred $$ ppr avails )
- addAvailAndSCs avails irred Irred
-
-addAvailAndSCs :: Avails -> Inst -> Avail -> TcM Avails
-addAvailAndSCs avails inst avail
- | not (isClassDict inst) = returnM avails1
- | otherwise = traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps]) `thenM_`
- addSCs is_loop avails1 inst
+addIrred want_scs avails irred = ASSERT2( not (irred `elemFM` avails), ppr irred $$ ppr avails )
+ addAvailAndSCs want_scs avails irred Irred
+
+addAvailAndSCs :: WantSCs -> Avails -> Inst -> Avail -> TcM Avails
+addAvailAndSCs want_scs avails inst avail
+ | not (isClassDict inst) = return avails_with_inst
+ | NoSCs <- want_scs = return avails_with_inst
+ | otherwise = do { traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps])
+ ; addSCs is_loop avails_with_inst inst }
where
- avails1 = addToFM avails inst avail
- is_loop inst = any (`tcEqType` idType (instToId inst)) dep_tys
+ avails_with_inst = addToFM avails inst avail
+
+ is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys
-- Note: this compares by *type*, not by Unique
- deps = findAllDeps emptyVarSet avail
+ deps = findAllDeps (unitVarSet (instToId inst)) avail
dep_tys = map idType (varSetElems deps)
findAllDeps :: IdSet -> Avail -> IdSet
-- Find all the Insts that this one depends on
- -- See Note [SUPERCLASS-LOOP]
+ -- See Note [SUPERCLASS-LOOP 2]
-- Watch out, though. Since the avails may contain loops
-- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far
- findAllDeps so_far (Rhs _ kids)
- = foldl findAllDeps
- (extendVarSetList so_far (map instToId kids)) -- Add the kids to so_far
- [a | Just a <- map (lookupFM avails) kids] -- Find the kids' Avail
- findAllDeps so_far other = so_far
-
+ findAllDeps so_far (Rhs _ kids) = foldl find_all so_far kids
+ findAllDeps so_far other = so_far
+
+ find_all :: IdSet -> Inst -> IdSet
+ find_all so_far kid
+ | kid_id `elemVarSet` so_far = so_far
+ | Just avail <- lookupFM avails kid = findAllDeps so_far' avail
+ | otherwise = so_far'
+ where
+ so_far' = extendVarSet so_far kid_id -- Add the new kid to so_far
+ kid_id = instToId kid
-addSCs :: (Inst -> Bool) -> Avails -> Inst -> TcM Avails
+addSCs :: (TcPredType -> Bool) -> Avails -> Inst -> TcM Avails
-- Add all the superclasses of the Inst to Avails
-- The first param says "dont do this because the original thing
-- depends on this one, so you'd build a loop"
-- Invariant: the Inst is already in Avails.
addSCs is_loop avails dict
- = newDictsFromOld dict sc_theta' `thenM` \ sc_dicts ->
- foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels)
+ = do { sc_dicts <- newDictsAtLoc (instLoc dict) sc_theta'
+ ; foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels) }
where
(clas, tys) = getDictClassTys dict
(tyvars, sc_theta, sc_sels, _) = classBigSig clas
- sc_theta' = substTheta (mkTopTyVarSubst tyvars tys) sc_theta
+ sc_theta' = substTheta (zipTopTvSubst tyvars tys) sc_theta
- add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses
- | add_me sc_dict = addSCs is_loop avails' sc_dict
- | otherwise = returnM avails
+ add_sc avails (sc_dict, sc_sel)
+ | is_loop (dictPred sc_dict) = return avails -- See Note [SUPERCLASS-LOOP 2]
+ | is_given sc_dict = return avails
+ | otherwise = addSCs is_loop avails' sc_dict
where
sc_sel_rhs = mkHsDictApp (mkHsTyApp (L (instSpan dict) (HsVar sc_sel)) tys) [instToId dict]
avails' = addToFM avails sc_dict (Rhs sc_sel_rhs [dict])
- add_me :: Inst -> Bool
- add_me sc_dict
- | is_loop sc_dict = False -- See Note [SUPERCLASS-LOOP]
- | otherwise = case lookupFM avails sc_dict of
- Just (Given _ _) -> False -- Given is cheaper than superclass selection
- other -> True
+ is_given :: Inst -> Bool
+ is_given sc_dict = case lookupFM avails sc_dict of
+ Just (Given _ _) -> True -- Given is cheaper than superclass selection
+ other -> False
\end{code}
-Note [SUPERCLASS-LOOP]: Checking for loops
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-We have to be careful here. If we are *given* d1:Ord a,
+Note [SUPERCLASS-LOOP 2]
+~~~~~~~~~~~~~~~~~~~~~~~~
+But the above isn't enough. Suppose we are *given* d1:Ord a,
and want to deduce (d2:C [a]) where
class Ord a => C a where
- instance 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
+Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the
superclasses of C [a] to avails. But we must not overwrite the binding
-for d1:Ord a (which is given) with a superclass selection or we'll just
+for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just
build a loop!
Here's another variant, immortalised in tcrun020
\begin{code}
tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
-tcSimplifyTop wanteds = tc_simplify_top False {- Not interactive loop -} wanteds
-tcSimplifyInteractive wanteds = tc_simplify_top True {- Interactive loop -} wanteds
+tcSimplifyTop wanteds
+ = tc_simplify_top doc False {- Not interactive loop -} AddSCs wanteds
+ where
+ doc = text "tcSimplifyTop"
+tcSimplifyInteractive wanteds
+ = tc_simplify_top doc True {- Interactive loop -} AddSCs wanteds
+ where
+ doc = text "tcSimplifyTop"
-- The TcLclEnv should be valid here, solely to improve
-- error message generation for the monomorphism restriction
-tc_simplify_top is_interactive wanteds
- = getLclEnv `thenM` \ lcl_env ->
- traceTc (text "tcSimplifyTop" <+> ppr (lclEnvElts lcl_env)) `thenM_`
- simpleReduceLoop (text "tcSimplTop") reduceMe wanteds `thenM` \ (frees, binds, irreds) ->
- ASSERT( null frees )
+tc_simplify_top doc is_interactive want_scs wanteds
+ = do { lcl_env <- getLclEnv
+ ; traceTc (text "tcSimplifyTop" <+> ppr (lclEnvElts lcl_env))
- let
- -- All the non-std ones are definite errors
- (stds, non_stds) = partition isStdClassTyVarDict irreds
+ ; let try_me inst = ReduceMe want_scs
+ ; (frees, binds, irreds) <- simpleReduceLoop doc try_me wanteds
+
+ ; let
+ -- First get rid of implicit parameters
+ (non_ips, bad_ips) = partition isClassDict irreds
+
+ -- All the non-tv or multi-param ones are definite errors
+ (unary_tv_dicts, non_tvs) = partition is_unary_tyvar_dict non_ips
+ bad_tyvars = unionVarSets (map tyVarsOfInst non_tvs)
-- Group by type variable
- std_groups = equivClasses cmp_by_tyvar stds
+ tv_groups = equivClasses cmp_by_tyvar unary_tv_dicts
-- Pick the ones which its worth trying to disambiguate
- -- namely, the onese whose type variable isn't bound
- -- up with one of the non-standard classes
- (std_oks, std_bads) = partition worth_a_try std_groups
- worth_a_try group@(d:_) = not (non_std_tyvars `intersectsVarSet` tyVarsOfInst d)
- non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
-
- -- Collect together all the bad guys
- bad_guys = non_stds ++ concat std_bads
- (bad_ips, non_ips) = partition isIPDict bad_guys
- (no_insts, ambigs) = partition no_inst non_ips
- no_inst d = not (isTyVarDict d)
- -- Previously, there was a more elaborate no_inst definition:
- -- no_inst d = not (isTyVarDict d) || tyVarsOfInst d `subVarSet` fixed_tvs
- -- fixed_tvs = oclose (fdPredsOfInsts tidy_dicts) emptyVarSet
- -- But that seems over-elaborate to me; it only bites for class decls with
- -- fundeps like this: class C a b | -> b where ...
- in
+ -- namely, the ones whose type variable isn't bound
+ -- up with one of the non-tyvar classes
+ (default_gps, non_default_gps) = partition defaultable_group tv_groups
+ defaultable_group ds
+ = not (bad_tyvars `intersectsVarSet` tyVarsOfInst (head ds))
+ && defaultable_classes (map get_clas ds)
+ defaultable_classes clss
+ | is_interactive = any isInteractiveClass clss
+ | otherwise = all isStandardClass clss && any isNumericClass clss
+
+ isInteractiveClass cls = isNumericClass cls
+ || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
+ -- In interactive mode, we default Show a to Show ()
+ -- to avoid graututious errors on "show []"
+
+
+ -- Collect together all the bad guys
+ bad_guys = non_tvs ++ concat non_default_gps
+ (ambigs, no_insts) = partition isTyVarDict bad_guys
+ -- If the dict has no type constructors involved, it must be ambiguous,
+ -- except I suppose that another error with fundeps maybe should have
+ -- constrained those type variables
-- Report definite errors
- groupErrs (addNoInstanceErrs Nothing []) no_insts `thenM_`
- addTopIPErrs bad_ips `thenM_`
+ ; ASSERT( null frees )
+ groupErrs (addNoInstanceErrs Nothing []) no_insts
+ ; strangeTopIPErrs bad_ips
-- Deal with ambiguity errors, but only if
- -- if there has not been an error so far; errors often
- -- give rise to spurious ambiguous Insts
- ifErrsM (returnM []) (
-
- -- Complain about the ones that don't fall under
- -- the Haskell rules for disambiguation
- -- This group includes both non-existent instances
- -- e.g. Num (IO a) and Eq (Int -> Int)
- -- and ambiguous dictionaries
- -- e.g. Num a
- addTopAmbigErrs ambigs `thenM_`
-
- -- Disambiguate the ones that look feasible
- mappM (disambigGroup is_interactive) std_oks
- ) `thenM` \ binds_ambig ->
-
- returnM (binds `unionBags` unionManyBags binds_ambig)
+ -- if there has not been an error so far:
+ -- errors often give rise to spurious ambiguous Insts.
+ -- For example:
+ -- f = (*) -- Monomorphic
+ -- g :: Num a => a -> a
+ -- g x = f x x
+ -- Here, we get a complaint when checking the type signature for g,
+ -- that g isn't polymorphic enough; but then we get another one when
+ -- dealing with the (Num a) context arising from f's definition;
+ -- we try to unify a with Int (to default it), but find that it's
+ -- already been unified with the rigid variable from g's type sig
+ ; binds_ambig <- ifErrsM (returnM []) $
+ do { -- Complain about the ones that don't fall under
+ -- the Haskell rules for disambiguation
+ -- This group includes both non-existent instances
+ -- e.g. Num (IO a) and Eq (Int -> Int)
+ -- and ambiguous dictionaries
+ -- e.g. Num a
+ addTopAmbigErrs ambigs
+
+ -- Disambiguate the ones that look feasible
+ ; mappM disambigGroup default_gps }
+
+ ; return (binds `unionBags` unionManyBags binds_ambig) }
----------------------------------
d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
+is_unary_tyvar_dict :: Inst -> Bool -- Dicts of form (C a)
+ -- Invariant: argument is a ClassDict, not IP or method
+is_unary_tyvar_dict d = case getDictClassTys d of
+ (_, [ty]) -> tcIsTyVarTy ty
+ other -> False
+
get_tv d = case getDictClassTys d of
(clas, [ty]) -> tcGetTyVar "tcSimplify" ty
get_clas d = case getDictClassTys d of
- (clas, [ty]) -> clas
+ (clas, _) -> clas
\end{code}
If a dictionary constrains a type variable which is
@void@.
\begin{code}
-disambigGroup :: Bool -- True <=> simplifying at top-level interactive loop
- -> [Inst] -- All standard classes of form (C a)
+disambigGroup :: [Inst] -- All standard classes of form (C a)
-> TcM TcDictBinds
-disambigGroup is_interactive dicts
- | any std_default_class classes -- Guaranteed all standard classes
+disambigGroup dicts
= -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT
-- SO, TRY DEFAULT TYPES IN ORDER
case mb_ty of
Left _ -> bomb_out
Right chosen_default_ty -> choose_default chosen_default_ty
-
- | otherwise -- No defaults
- = bomb_out
-
where
tyvar = get_tv (head dicts) -- Should be non-empty
classes = map get_clas dicts
- std_default_class cls
- = isNumericClass cls
- || (is_interactive &&
- classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
- -- In interactive mode, we default Show a to Show ()
- -- to avoid graututious errors on "show []"
-
choose_default default_ty -- Commit to tyvar = default_ty
= -- Bind the type variable
- unifyTauTy default_ty (mkTyVarTy tyvar) `thenM_`
+ unifyType default_ty (mkTyVarTy tyvar) `thenM_`
-- and reduce the context, for real this time
simpleReduceLoop (text "disambig" <+> ppr dicts)
- reduceMe dicts `thenM` \ (frees, binds, ambigs) ->
+ reduceMe dicts `thenM` \ (frees, binds, ambigs) ->
WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs )
warnDefault dicts default_ty `thenM_`
returnM binds
Nothing -> -- No use-supplied default;
-- use [Integer, Double]
do { integer_ty <- tcMetaTy integerTyConName
+ ; checkWiredInTyCon doubleTyCon
; return [integer_ty, doubleTy] } }
\end{code}
When typechecking _ccall_s, TcExpr ensures that the external
function is only passed arguments (and in the other direction,
-results) of a restricted set of 'native' types. This is
-implemented via the help of the pseudo-type classes,
-@CReturnable@ (CR) and @CCallable@ (CC.)
+results) of a restricted set of 'native' types.
The interaction between the defaulting mechanism for numeric
values and CC & CR can be a bit puzzling to the user at times.
Haskell 1.4's default-default of (Int, Double), 'x' has type
Int.
-To try to minimise the potential for surprises here, the
-defaulting mechanism is turned off in the presence of
-CCallable and CReturnable.
-
End of aside]
instance declarations.
\begin{code}
-tcSimplifyDeriv :: [TyVar]
+tcSimplifyDeriv :: TyCon
+ -> [TyVar]
-> ThetaType -- Wanted
-> TcM ThetaType -- Needed
-tcSimplifyDeriv tyvars theta
- = tcInstTyVars VanillaTv tyvars `thenM` \ (tvs, _, tenv) ->
+tcSimplifyDeriv tc tyvars theta
+ = tcInstTyVars tyvars `thenM` \ (tvs, _, tenv) ->
-- The main loop may do unification, and that may crash if
-- it doesn't see a TcTyVar, so we have to instantiate. Sigh
-- ToDo: what if two of them do get unified?
- newDicts DataDeclOrigin (substTheta tenv theta) `thenM` \ wanteds ->
+ newDicts DerivOrigin (substTheta tenv theta) `thenM` \ wanteds ->
simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
ASSERT( null frees ) -- reduceMe never returns Free
+ doptM Opt_GlasgowExts `thenM` \ gla_exts ->
doptM Opt_AllowUndecidableInstances `thenM` \ undecidable_ok ->
let
tv_set = mkVarSet tvs
= let pred = dictPred dict -- reduceMe squashes all non-dicts
in isEmptyVarSet (tyVarsOfPred pred)
-- Things like (Eq T) are bad
- || (not undecidable_ok && not (isTyVarClassPred pred))
- -- The returned dictionaries should be of form (C a b)
- -- (where a, b are type variables).
- -- We allow non-tyvar dicts if we had -fallow-undecidable-instances,
- -- but note that risks non-termination in the 'deriving' context-inference
- -- fixpoint loop. It is useful for situations like
- -- data Min h a = E | M a (h a)
- -- which gives the instance decl
- -- instance (Eq a, Eq (h a)) => Eq (Min h a)
+ || (not gla_exts && not (isTyVarClassPred pred))
simpl_theta = map dictPred ok_insts
weird_preds = [pred | pred <- simpl_theta
-- 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 = mkTopTyVarSubst tvs (mkTyVarTys tyvars)
+ rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
-- This reverse-mapping is a Royal Pain,
-- but the result should mention TyVars not TcTyVars
+
+ head_ty = TyConApp tc (map TyVarTy tvs)
in
addNoInstanceErrs Nothing [] bad_insts `thenM_`
mapM_ (addErrTc . badDerivedPred) weird_preds `thenM_`
checkAmbiguity tvs simpl_theta tv_set `thenM_`
+ -- Check instance termination as for user-declared instances.
+ -- unless we had -fallow-undecidable-instances (which risks
+ -- non-termination in the 'deriving' context-inference fixpoint
+ -- loop).
+ ifM (gla_exts && not undecidable_ok)
+ (checkInstTermination simpl_theta [head_ty]) `thenM_`
returnM (substTheta rev_env simpl_theta)
where
doc = ptext SLIT("deriving classes for a data type")
-> TcM ()
tcSimplifyDefault theta
- = newDicts DataDeclOrigin theta `thenM` \ wanteds ->
+ = newDicts DefaultOrigin theta `thenM` \ wanteds ->
simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
ASSERT( null frees ) -- try_me never returns Free
addNoInstanceErrs Nothing [] irreds `thenM_`
addInstLoc :: [Inst] -> Message -> Message
addInstLoc insts msg = msg $$ nest 2 (pprInstLoc (instLoc (head insts)))
-plural [x] = empty
-plural xs = char 's'
-
-addTopIPErrs dicts
+addTopIPErrs :: [Name] -> [Inst] -> TcM ()
+addTopIPErrs bndrs []
+ = return ()
+addTopIPErrs bndrs ips
+ = addErrTcM (tidy_env, mk_msg tidy_ips)
+ where
+ (tidy_env, tidy_ips) = tidyInsts ips
+ mk_msg ips = vcat [sep [ptext SLIT("Implicit parameters escape from"),
+ nest 2 (ptext SLIT("the monomorphic top-level binding(s) of")
+ <+> pprBinders bndrs <> colon)],
+ nest 2 (vcat (map ppr_ip ips)),
+ monomorphism_fix]
+ ppr_ip ip = pprPred (dictPred ip) <+> pprInstLoc (instLoc ip)
+
+strangeTopIPErrs :: [Inst] -> TcM ()
+strangeTopIPErrs dicts -- Strange, becuase addTopIPErrs should have caught them all
= groupErrs report tidy_dicts
where
(tidy_env, tidy_dicts) = tidyInsts dicts
addNoInstanceErrs mb_what givens dicts
= -- Some of the dicts are here because there is no instances
-- and some because there are too many instances (overlap)
- -- The first thing we do is separate them
- getDOpts `thenM` \ dflags ->
tcGetInstEnvs `thenM` \ inst_envs ->
let
(tidy_env1, tidy_givens) = tidyInsts givens
check_overlap (overlap_doc, no_inst_dicts) dict
| not (isClassDict dict) = (overlap_doc, dict : no_inst_dicts)
| otherwise
- = case lookupInstEnv dflags inst_envs clas tys of
+ = 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.
+ -- a successful lookup. That can't happen here, becuase
+ -- dicts only end up here if they didn't match in Inst.lookupInst
#ifdef DEBUG
([m],[]) -> pprPanic "addNoInstanceErrs" (ppr dict)
#endif
in
-- Now generate a good message for the no-instance bunch
- mk_probable_fix tidy_env2 mb_what no_inst_dicts `thenM` \ (tidy_env3, probable_fix) ->
+ mk_probable_fix tidy_env2 no_inst_dicts `thenM` \ (tidy_env3, probable_fix) ->
let
no_inst_doc | null no_inst_dicts = empty
| otherwise = vcat [addInstLoc no_inst_dicts heading, probable_fix]
= vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for")
<+> pprPred (dictPred dict))),
sep [ptext SLIT("Matching instances") <> colon,
- nest 2 (pprDFuns (dfuns ++ unifiers))],
+ nest 2 (vcat [pprInstances ispecs, pprInstances unifiers])],
ASSERT( not (null matches) )
if not (isSingleton matches)
then -- Two or more matches
quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
ptext SLIT("Use -fallow-incoherent-instances to use the first choice above")])]
where
- dfuns = [df | (_, (_,_,df)) <- matches]
+ ispecs = [ispec | (_, ispec) <- matches]
- mk_probable_fix tidy_env Nothing dicts -- Top level
- = mkMonomorphismMsg tidy_env dicts
- mk_probable_fix tidy_env (Just what) dicts -- Nested (type signatures, instance decls)
- = returnM (tidy_env, sep [ptext SLIT("Probable fix:"), nest 2 fix1, nest 2 fix2])
+ mk_probable_fix tidy_env dicts
+ = returnM (tidy_env, sep [ptext SLIT("Possible fix:"), nest 2 (vcat fixes)])
where
- fix1 = sep [ptext SLIT("Add") <+> pprDictsTheta dicts,
- ptext SLIT("to the") <+> what]
+ fixes = add_ors (fix1 ++ fix2)
- fix2 | null instance_dicts = empty
- | otherwise = ptext SLIT("Or add an instance declaration for")
- <+> pprDictsTheta instance_dicts
+ fix1 = case mb_what of
+ Nothing -> [] -- Top level
+ Just what -> -- Nested (type signatures, instance decls)
+ [ sep [ ptext SLIT("add") <+> pprDictsTheta dicts,
+ ptext SLIT("to the") <+> what] ]
+
+ fix2 | null instance_dicts = []
+ | otherwise = [ ptext SLIT("add an instance declaration for")
+ <+> pprDictsTheta instance_dicts ]
instance_dicts = [d | d <- dicts, isClassDict d, not (isTyVarDict d)]
-- Insts for which it is worth suggesting an adding an instance declaration
-- Exclude implicit parameters, and tyvar dicts
+ add_ors :: [SDoc] -> [SDoc] -- The empty case should not happen
+ add_ors [] = [ptext SLIT("[No suggested fixes]")] -- Strange
+ add_ors (f1:fs) = f1 : map (ptext SLIT("or") <+>) fs
addTopAmbigErrs dicts
-- Divide into groups that share a common set of ambiguous tyvars
report :: [(Inst,[TcTyVar])] -> TcM ()
report pairs@((inst,tvs) : _) -- The pairs share a common set of ambiguous tyvars
- = mkMonomorphismMsg tidy_env dicts `thenM` \ (tidy_env, mono_msg) ->
- addSrcSpan (instLocSrcSpan (instLoc inst)) $
+ = mkMonomorphismMsg tidy_env tvs `thenM` \ (tidy_env, mono_msg) ->
+ setSrcSpan (instLocSrcSpan (instLoc inst)) $
-- the location of the first one will do for the err message
addErrTcM (tidy_env, msg $$ mono_msg)
where
dicts = map fst pairs
msg = sep [text "Ambiguous type variable" <> plural tvs <+>
- pprQuotedList tvs <+> in_msg,
+ pprQuotedList tvs <+> in_msg,
nest 2 (pprDictsInFull dicts)]
- in_msg | isSingleton dicts = text "in the top-level constraint:"
- | otherwise = text "in these top-level constraints:"
+ in_msg = text "in the constraint" <> plural dicts <> colon
-mkMonomorphismMsg :: TidyEnv -> [Inst] -> TcM (TidyEnv, Message)
+mkMonomorphismMsg :: TidyEnv -> [TcTyVar] -> TcM (TidyEnv, Message)
-- There's an error with these Insts; if they have free type variables
-- it's probably caused by the monomorphism restriction.
-- Try to identify the offending variable
-- ASSUMPTION: the Insts are fully zonked
-mkMonomorphismMsg tidy_env insts
- | isEmptyVarSet inst_tvs
- = returnM (tidy_env, empty)
- | otherwise
- = findGlobals inst_tvs tidy_env `thenM` \ (tidy_env, docs) ->
+mkMonomorphismMsg tidy_env inst_tvs
+ = findGlobals (mkVarSet inst_tvs) tidy_env `thenM` \ (tidy_env, docs) ->
returnM (tidy_env, mk_msg docs)
-
where
- inst_tvs = tyVarsOfInsts insts
-
- mk_msg [] = empty -- This happens in things like
- -- f x = show (read "foo")
- -- whre monomorphism doesn't play any role
+ mk_msg [] = ptext SLIT("Probable fix: add a type signature that fixes these type variable(s)")
+ -- This happens in things like
+ -- f x = show (read "foo")
+ -- whre monomorphism doesn't play any role
mk_msg docs = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
nest 2 (vcat docs),
- ptext SLIT("Probable fix: give these definition(s) an explicit type signature")]
+ monomorphism_fix
+ ]
+monomorphism_fix :: SDoc
+monomorphism_fix = ptext SLIT("Probable fix:") <+>
+ (ptext SLIT("give these definition(s) an explicit type signature")
+ $$ ptext SLIT("or use -fno-monomorphism-restriction"))
warnDefault dicts default_ty
= doptM Opt_WarnTypeDefaults `thenM` \ warn_flag ->
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