TcErrors.lhs 30.6 KB
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\begin{code}
module TcErrors( 
       reportUnsolved, reportUnsolvedImplication, reportUnsolvedDeriv,
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       reportUnsolvedWantedEvVars, warnDefaulting, 
       unifyCtxt, typeExtraInfoMsg, 
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       kindErrorTcS, misMatchErrorTcS, flattenForAllErrorTcS,
       occursCheckErrorTcS, solverDepthErrorTcS
  ) where

#include "HsVersions.h"

import TcRnMonad
import TcMType
import TcSMonad
import TcType
import Inst
import InstEnv

import TyCon
import Name
import NameEnv
import Id	( idType )
import HsExpr	( pprMatchContext )
import Var
import VarSet
import VarEnv
import SrcLoc
import Bag
import ListSetOps( equivClasses )
import Util
import Unique
import FastString
import Outputable
import DynFlags
import StaticFlags( opt_PprStyle_Debug )
import Data.List( partition )
import Control.Monad( unless )
\end{code}

%************************************************************************
%*									*
\section{Errors and contexts}
%*									*
%************************************************************************

ToDo: for these error messages, should we note the location as coming
from the insts, or just whatever seems to be around in the monad just
now?

\begin{code}
reportUnsolved :: (CanonicalCts, Bag Implication) -> TcM ()
reportUnsolved (unsolved_flats, unsolved_implics)
  | isEmptyBag unsolved
  = return ()
  | otherwise
  = do { env0 <- tcInitTidyEnv
       ; let tidy_env      = tidyFreeTyVars env0 (tyVarsOfWanteds unsolved)
             tidy_unsolved = tidyWanteds tidy_env unsolved
             err_ctxt = CEC { cec_encl = [] 
                            , cec_extra = empty
                            , cec_tidy = tidy_env } 
       ; traceTc "reportUnsolved" (ppr unsolved)
       ; reportTidyWanteds err_ctxt tidy_unsolved }
  where
    unsolved = mkWantedConstraints unsolved_flats unsolved_implics

reportUnsolvedWantedEvVars :: Bag WantedEvVar -> TcM ()
reportUnsolvedWantedEvVars wanteds
  | isEmptyBag wanteds 
  = return ()
  | otherwise
  = do { env0 <- tcInitTidyEnv
       ; let tidy_env      = tidyFreeTyVars env0 (tyVarsOfWantedEvVars wanteds)
             tidy_unsolved = tidyWantedEvVars tidy_env wanteds
             err_ctxt = CEC { cec_encl  = [] 
                            , cec_extra = empty
                            , cec_tidy  = tidy_env } 
       ; groupErrs (reportFlat err_ctxt) (bagToList tidy_unsolved) }

reportUnsolvedDeriv :: [PredType] -> WantedLoc -> TcM ()
reportUnsolvedDeriv unsolved loc
  | null unsolved
  = return ()
  | otherwise
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  = setCtLoc loc $
    do { env0 <- tcInitTidyEnv
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       ; let tidy_env      = tidyFreeTyVars env0 (tyVarsOfTheta unsolved)
             tidy_unsolved = map (tidyPred tidy_env) unsolved
             err_ctxt = CEC { cec_encl  = [] 
                            , cec_extra = alt_fix
                            , cec_tidy  = tidy_env } 
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       ; reportFlat err_ctxt tidy_unsolved (ctLocOrigin loc) }
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  where
    alt_fix = vcat [ptext (sLit "Alternatively, use a standalone 'deriving instance' declaration,"),
                    nest 2 $ ptext (sLit "so you can specify the instance context yourself")]

reportUnsolvedImplication :: Implication -> TcM ()
reportUnsolvedImplication implic
  = do { env0 <- tcInitTidyEnv
       ; let tidy_env    = tidyFreeTyVars env0 (tyVarsOfImplication implic)
             tidy_implic = tidyImplication tidy_env implic
             new_tidy_env = foldNameEnv add tidy_env (ic_env implic)
             err_ctxt = CEC { cec_encl = [tidy_implic]
                            , cec_extra = empty
                            , cec_tidy = new_tidy_env } 
       ; reportTidyWanteds err_ctxt (ic_wanted tidy_implic) }
  where
    -- Extend the tidy env with a mapping from tyvars to the
    -- names the user originally used.  At the moment we do this
    -- from the type env, but it might be better to record the
    -- scoped type variable in the Implication.  Urgh.
    add (ATyVar name ty) (occ_env, var_env)
       | Just tv <- tcGetTyVar_maybe ty
       , not (getUnique name `elemVarEnvByKey` var_env)
       = case tidyOccName occ_env (nameOccName name) of
	    (occ_env', occ') ->  (occ_env', extendVarEnv var_env tv tv')
		where
		  tv'   = setTyVarName tv name'
		  name' = tidyNameOcc name occ'
    add _ tidy_env = tidy_env      

data ReportErrCtxt 
    = CEC { cec_encl :: [Implication]  -- Enclosing implications
                	       	       --   (innermost first)
          , cec_tidy :: TidyEnv
          , cec_extra :: SDoc	       -- Add this to each error message
      }

reportTidyImplic :: ReportErrCtxt -> Implication -> TcM ()
reportTidyImplic ctxt implic
  = reportTidyWanteds ctxt' (ic_wanted implic)
  where
    ctxt' = ctxt { cec_encl = implic : cec_encl ctxt }
  
reportTidyWanteds :: ReportErrCtxt -> WantedConstraints -> TcM ()
reportTidyWanteds ctxt unsolved
  = do { let (flats, implics) = splitWanteds unsolved
             (ambigs, others) = partition is_ambiguous (bagToList flats)
       ; groupErrs (reportFlat ctxt) others
       ; mapBagM_ (reportTidyImplic ctxt) implics
       ; ifErrsM (return ()) $
       	   -- Only report ambiguity if no other errors happened
	   -- See Note [Avoiding spurious errors]
         reportAmbigErrs ctxt skols ambigs }
  where
    skols = foldr (unionVarSet . ic_skols) emptyVarSet (cec_encl ctxt)
 
	-- Treat it as "ambiguous" if 
	--   (a) it is a class constraint
        --   (b) it constrains only type variables
	--       (else we'd prefer to report it as "no instance for...")
        --   (c) it mentions type variables that are not skolems
    is_ambiguous d = isTyVarClassPred pred
                  && not (tyVarsOfPred pred `subVarSet` skols)
		  where   
                     pred = wantedEvVarPred d

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reportFlat :: ReportErrCtxt -> [PredType] -> CtOrigin -> TcM ()
reportFlat ctxt flats origin
  = do { unless (null dicts) $ reportDictErrs ctxt dicts origin
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       ; unless (null eqs)   $ reportEqErrs   ctxt eqs   origin
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       ; unless (null ips)   $ reportIPErrs   ctxt ips   origin
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       ; ASSERT( null others ) return () }
  where
    (dicts, non_dicts) = partition isClassPred flats
    (eqs, non_eqs)     = partition isEqPred    non_dicts
    (ips, others)      = partition isIPPred    non_eqs

--------------------------------------------
--      Support code 
--------------------------------------------

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groupErrs :: ([PredType] -> CtOrigin -> TcM ()) -- Deal with one group
	  -> [WantedEvVar]	                -- Unsolved wanteds
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          -> TcM ()
-- Group together insts with the same origin
-- We want to report them together in error messages

groupErrs _ [] 
  = return ()
groupErrs report_err (wanted : wanteds)
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  = do	{ setCtLoc the_loc $ 
          report_err the_vars (ctLocOrigin the_loc)
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	; groupErrs report_err others }
  where
   the_loc           = wantedEvVarLoc wanted
   the_key	     = mk_key the_loc
   the_vars          = map wantedEvVarPred (wanted:friends)
   (friends, others) = partition is_friend wanteds
   is_friend friend  = mk_key (wantedEvVarLoc friend) == the_key

   mk_key :: WantedLoc -> (SrcSpan, String)
   mk_key loc = (ctLocSpan loc, showSDoc (ppr (ctLocOrigin loc)))
	-- It may seem crude to compare the error messages,
	-- but it makes sure that we combine just what the user sees,
	-- and it avoids need equality on InstLocs.

-- Add the "arising from..." part to a message about bunch of dicts
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addArising :: CtOrigin -> SDoc -> SDoc
addArising orig msg = msg $$ nest 2 (pprArising orig)
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pprWithArising :: [WantedEvVar] -> (WantedLoc, SDoc)
-- Print something like
--    (Eq a) arising from a use of x at y
--    (Show a) arising froma use of p at q
-- Also return a location for the erroe message
pprWithArising [] 
  = panic "pprWithArising"
pprWithArising [WantedEvVar ev loc] 
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  = (loc, pprEvVarTheta [ev] <+> pprArising (ctLocOrigin loc))
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pprWithArising ev_vars
  = (first_loc, vcat (map ppr_one ev_vars))
  where
    first_loc = wantedEvVarLoc (head ev_vars)
    ppr_one (WantedEvVar v loc) 
       = parens (pprPred (evVarPred v)) <+> pprArisingAt loc

addErrorReport :: ReportErrCtxt -> SDoc -> TcM ()
addErrorReport ctxt msg = addErrTcM (cec_tidy ctxt, msg $$ cec_extra ctxt)

pprErrCtxtLoc :: ReportErrCtxt -> SDoc
pprErrCtxtLoc ctxt 
  = case map (ctLocOrigin . ic_loc) (cec_encl ctxt) of
       []           -> ptext (sLit "the top level")	-- Should not happen
       (orig:origs) -> ppr_skol orig $$ 
                       vcat [ ptext (sLit "or") <+> ppr_skol orig | orig <- origs ]
  where
    ppr_skol (PatSkol dc _) = ptext (sLit "the data constructor") <+> quotes (ppr dc)
    ppr_skol skol_info      = pprSkolInfo skol_info

couldNotDeduce :: [EvVar] -> [PredType] -> SDoc
couldNotDeduce givens wanteds
  = sep [ ptext (sLit "Could not deduce") <+> pprTheta wanteds
        , nest 2 $ ptext (sLit "from the context") 
                     <+> pprEvVarTheta givens]

getUserGivens :: ReportErrCtxt -> Maybe [EvVar]
-- Just gs => Say "could not deduce ... from gs"
-- Nothing => No interesting givens, say something else
getUserGivens (CEC {cec_encl = ctxt})
  | null user_givens = Nothing
  | otherwise        = Just user_givens
  where 
    givens = foldl (\gs ic -> ic_given ic ++ gs) [] ctxt
    user_givens | opt_PprStyle_Debug = givens
                | otherwise          = filterOut isSelfDict givens
       -- In user mode, don't show the "self-dict" given
       -- which is only added to do co-inductive solving
       -- Rather an awkward hack, but there we are
       -- This is the only use of isSelfDict, so it's not in an inner loop
\end{code}


%************************************************************************
%*									*
                Implicit parameter errors
%*									*
%************************************************************************

\begin{code}
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reportIPErrs :: ReportErrCtxt -> [PredType] -> CtOrigin -> TcM ()
reportIPErrs ctxt ips orig
  = addErrorReport ctxt $ addArising orig msg
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  where
    msg | Just givens <- getUserGivens ctxt
        = couldNotDeduce givens ips
        | otherwise
        = sep [ ptext (sLit "Unbound implicit parameter") <> plural ips
              , nest 2 (pprTheta ips) ] 
\end{code}


%************************************************************************
%*									*
                Equality errors
%*									*
%************************************************************************

\begin{code}
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reportEqErrs :: ReportErrCtxt -> [PredType] -> CtOrigin -> TcM ()
reportEqErrs ctxt eqs orig
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  = mapM_ report_one eqs 
  where
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    env0 = cec_tidy ctxt
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    report_one (EqPred ty1 ty2) 
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      = getWantedEqExtra emptyTvSubst env0 orig ty1 ty2 $ \ env1 extra ->
        let ctxt' = ctxt { cec_tidy = env1
                         , cec_extra = cec_extra ctxt $$ extra }
        in reportEqErr ctxt' ty1 ty2 
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    report_one pred 
      = pprPanic "reportEqErrs" (ppr pred)    
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reportEqErr :: ReportErrCtxt -> TcType -> TcType -> TcM ()
reportEqErr ctxt ty1 ty2
  | Just tv1 <- tcGetTyVar_maybe ty1 = reportTyVarEqErr ctxt tv1 ty2
  | Just tv2 <- tcGetTyVar_maybe ty2 = reportTyVarEqErr ctxt tv2 ty1
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  | otherwise	-- Neither side is a type variable
    		-- Since the unsolved constraint is canonical, 
		-- it must therefore be of form (F tys ~ ty)
  = addErrorReport ctxt (msg $$ mkTyFunInfoMsg ty1 ty2)
  where
    msg = case getUserGivens ctxt of
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            Just givens -> couldNotDeduce givens [EqPred ty1 ty2]
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            Nothing     -> misMatchMsg ty1 ty2

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reportTyVarEqErr :: ReportErrCtxt -> TcTyVar -> TcType -> TcM ()
reportTyVarEqErr ctxt tv1 ty2
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  | not is_meta1
  , Just tv2 <- tcGetTyVar_maybe ty2
  , isMetaTyVar tv2
  = -- sk ~ alpha: swap
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    reportTyVarEqErr ctxt tv2 ty1
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  | not is_meta1
  = -- sk ~ ty, where ty isn't a meta-tyvar: mis-match
    addErrTcM (misMatchMsgWithExtras (cec_tidy ctxt) ty1 ty2)

  -- So tv is a meta tyvar, and presumably it is
  -- an *untouchable* meta tyvar, else it'd have been unified
  | not (k2 `isSubKind` k1)   	 -- Kind error
  = addErrorReport ctxt $ (kindErrorMsg (mkTyVarTy tv1) ty2)

  -- Check for skolem escape
  | (implic:_) <- cec_encl ctxt   -- Get the innermost context
  , let esc_skols = varSetElems (tyVarsOfType ty2 `intersectVarSet` ic_skols implic)
        implic_loc = ic_loc implic
  , not (null esc_skols)
  = setCtLoc implic_loc $	-- Override the error message location from the
    	     			-- place the equality arose to the implication site
    do { (env1, env_sigs) <- findGlobals ctxt (unitVarSet tv1)
       ; let msg = misMatchMsg ty1 ty2
             esc_doc | isSingleton esc_skols 
                     = ptext (sLit "because this skolem type variable would escape:")
                     | otherwise
                     = ptext (sLit "because these skolem type variables would escape:")
             extra1 = vcat [ nest 2 $ esc_doc <+> pprQuotedList esc_skols
                           , sep [ (if isSingleton esc_skols 
                                      then ptext (sLit "This skolem is")
                                      else ptext (sLit "These skolems are"))
                                   <+> ptext (sLit "bound by")
                                 , nest 2 $ pprSkolInfo (ctLocOrigin implic_loc) ] ]
       ; addErrTcM (env1, msg $$ extra1 $$ mkEnvSigMsg (ppr tv1) env_sigs) }

  -- Nastiest case: attempt to unify an untouchable variable
  | (implic:_) <- cec_encl ctxt   -- Get the innermost context
  , let implic_loc = ic_loc implic
        given      = ic_given implic
  = setCtLoc (ic_loc implic) $
    do { let (env1, msg) = misMatchMsgWithExtras (cec_tidy ctxt) ty1 ty2
             extra = vcat [ ptext (sLit "because") <+> ppr tv1 <+> ptext (sLit "is untouchable")
                          , ptext (sLit "inside the constraints") <+> pprEvVarTheta given 
                          , nest 2 (ptext (sLit "bound at")
                             <+> pprSkolInfo (ctLocOrigin implic_loc)) ]
       ; addErrTcM (env1, msg $$ extra) }

  | otherwise	   -- I'm not sure how this can happen!
  = addErrTcM (misMatchMsgWithExtras (cec_tidy ctxt) ty1 ty2)
  where         
    is_meta1 = isMetaTyVar tv1
    k1 	     = tyVarKind tv1
    k2 	     = typeKind ty2
    ty1      = mkTyVarTy tv1

mkTyFunInfoMsg :: TcType -> TcType -> SDoc
-- See Note [Non-injective type functions]
mkTyFunInfoMsg ty1 ty2
  | Just (tc1,_) <- tcSplitTyConApp_maybe ty1
  , Just (tc2,_) <- tcSplitTyConApp_maybe ty2
  , tc1 == tc2, isSynFamilyTyCon tc1
  = ptext (sLit "NB:") <+> quotes (ppr tc1) 
    <+> ptext (sLit "is a type function") <> (pp_inj tc1)
  | otherwise = empty
  where       
    pp_inj tc | isInjectiveTyCon tc = empty
              | otherwise = ptext (sLit (", and may not be injective"))

misMatchMsgWithExtras :: TidyEnv -> TcType -> TcType -> (TidyEnv, SDoc)
-- This version is used by TcSimplify too, which doesn't track the
-- expected/acutal thing, so we just have ty1 ty2 here
-- NB: The types are already tidied
misMatchMsgWithExtras env ty1 ty2
  = (env2, sep [ misMatchMsg ty1 ty2, nest 2 (extra1 $$ extra2) ])
  where
    (env1, extra1) = typeExtraInfoMsg env ty1
    (env2, extra2) = typeExtraInfoMsg env1 ty2

misMatchMsg :: TcType -> TcType -> SDoc	   -- Types are already tidy
misMatchMsg ty1 ty2 = sep [ ptext (sLit "Couldn't match type") <+> quotes (ppr ty1)
	                  , nest 15 $ ptext (sLit "with") <+> quotes (ppr ty2)]

kindErrorMsg :: TcType -> TcType -> SDoc   -- Types are already tidy
kindErrorMsg ty1 ty2
  = vcat [ ptext (sLit "Kind incompatibility when matching types:")
         , nest 2 (vcat [ ppr ty1 <+> dcolon <+> ppr k1
                        , ppr ty2 <+> dcolon <+> ppr k2 ]) ]
  where
    k1 = typeKind ty1
    k2 = typeKind ty2

typeExtraInfoMsg :: TidyEnv -> Type -> (TidyEnv, SDoc)
-- Shows a bit of extra info about skolem constants
typeExtraInfoMsg env ty 
  | Just tv <- tcGetTyVar_maybe ty
  , isTcTyVar tv
  , isSkolemTyVar tv || isSigTyVar tv
  , not (isUnk tv)
  , let (env1, tv1) = tidySkolemTyVar env tv
  = (env1, pprSkolTvBinding tv1)
  where
typeExtraInfoMsg env _ty = (env, empty)		-- Normal case
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--------------------
unifyCtxt :: EqOrigin -> TidyEnv -> TcM (TidyEnv, SDoc)
unifyCtxt (UnifyOrigin { uo_actual = act_ty, uo_expected = exp_ty }) tidy_env
  = do  { act_ty' <- zonkTcType act_ty
        ; exp_ty' <- zonkTcType exp_ty
        ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
              (env2, act_ty'') = tidyOpenType env1     act_ty'
        ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }

mkExpectedActualMsg :: Type -> Type -> SDoc
mkExpectedActualMsg act_ty exp_ty
  = vcat [ text "Expected type" <> colon <+> ppr exp_ty
         , text "  Actual type" <> colon <+> ppr act_ty ]
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\end{code}

Note [Non-injective type functions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It's very confusing to get a message like
     Couldn't match expected type `Depend s'
            against inferred type `Depend s1'
so mkTyFunInfoMsg adds:
       NB: `Depend' is type function, and hence may not be injective

Warn of loopy local equalities that were dropped.


%************************************************************************
%*									*
                 Type-class errors
%*									*
%************************************************************************

\begin{code}
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reportDictErrs :: ReportErrCtxt -> [PredType] -> CtOrigin -> TcM ()	
reportDictErrs ctxt wanteds orig
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  = do { inst_envs <- tcGetInstEnvs
       ; let (others, overlaps) = partitionWith (check_overlap inst_envs) wanteds
       ; unless (null others) $
         addErrorReport ctxt (mk_no_inst_err others) 
       ; mapM_ (addErrorReport ctxt) overlaps }
  where
    check_overlap :: (InstEnv,InstEnv) -> PredType -> Either PredType SDoc
	-- Right msg  => overlap message
	-- Left  inst => no instance
    check_overlap inst_envs pred@(ClassP clas tys)
	= case lookupInstEnv inst_envs clas tys of
		([], _) -> Left pred		-- No match
		-- 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
		([_],[])
		 | debugIsOn -> pprPanic "check_overlap" (ppr pred)
		res -> Right (mk_overlap_msg pred res)
    check_overlap _ _ = panic "check_overlap"

    mk_overlap_msg pred (matches, unifiers)
      = ASSERT( not (null matches) )
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        vcat [	addArising orig (ptext (sLit "Overlapping instances for") 
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				<+> pprPred pred)
    	     ,	sep [ptext (sLit "Matching instances") <> colon,
    		     nest 2 (vcat [pprInstances ispecs, pprInstances unifiers])]
	     ,	if not (isSingleton matches)
    		then 	-- Two or more matches
		     empty
    		else 	-- One match, plus some unifiers
		ASSERT( not (null unifiers) )
		parens (vcat [ptext (sLit "The choice depends on the instantiation of") <+>
	    		         quotes (pprWithCommas ppr (varSetElems (tyVarsOfPred pred))),
			      ptext (sLit "To pick the first instance above, use -XIncoherentInstances"),
			      ptext (sLit "when compiling the other instance declarations")])]
      where
    	ispecs = [ispec | (ispec, _) <- matches]

    mk_no_inst_err :: [PredType] -> SDoc
    mk_no_inst_err wanteds
      | Just givens <- getUserGivens ctxt
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      = vcat [ addArising orig $ couldNotDeduce givens wanteds
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	     , show_fixes (fix1 : fixes2) ]

      | otherwise	-- Top level 
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      = vcat [ addArising orig $
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	       ptext (sLit "No instance") <> plural wanteds
		    <+> ptext (sLit "for") <+> pprTheta wanteds
	     , show_fixes fixes2 ]

      where
    	fix1 = sep [ ptext (sLit "add") <+> pprTheta wanteds 
                          <+> ptext (sLit "to the context of")
	           , nest 2 $ pprErrCtxtLoc ctxt ]

    	fixes2 | null instance_dicts = []
	       | otherwise	     = [sep [ptext (sLit "add an instance declaration for"),
				        pprTheta instance_dicts]]
	instance_dicts = filterOut isTyVarClassPred wanteds
		-- Insts for which it is worth suggesting an adding an 
		-- instance declaration.  Exclude tyvar dicts.

	show_fixes :: [SDoc] -> SDoc
	show_fixes []     = empty
	show_fixes (f:fs) = sep [ptext (sLit "Possible fix:"), 
				 nest 2 (vcat (f : map (ptext (sLit "or") <+>) fs))]

reportAmbigErrs :: ReportErrCtxt -> TcTyVarSet -> [WantedEvVar] -> TcM ()
reportAmbigErrs ctxt skols ambigs 
-- Divide into groups that share a common set of ambiguous tyvars
  = mapM_ report (equivClasses cmp ambigs_w_tvs)
  where
    ambigs_w_tvs = [ (d, varSetElems (tyVarsOfWantedEvVar d `minusVarSet` skols))
                   | d <- ambigs ]
    cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2

    report :: [(WantedEvVar, [TcTyVar])] -> TcM ()
    report pairs
       = setCtLoc loc $
         do { let main_msg = sep [ text "Ambiguous type variable" <> plural tvs
	         	           <+> pprQuotedList tvs
                                   <+> text "in the constraint" <> plural pairs <> colon
                                 , nest 2 pp_wanteds ]
             ; (tidy_env, mono_msg) <- mkMonomorphismMsg ctxt tvs
            ; addErrTcM (tidy_env, main_msg $$ mono_msg) }
       where
         (_, tvs) : _ = pairs
         (loc, pp_wanteds) = pprWithArising (map fst pairs)

mkMonomorphismMsg :: ReportErrCtxt -> [TcTyVar] -> TcM (TidyEnv, SDoc)
-- 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 ctxt inst_tvs
  = do	{ dflags <- getDOpts
	; (tidy_env, docs) <- findGlobals ctxt (mkVarSet inst_tvs)
	; return (tidy_env, mk_msg dflags docs) }
  where
    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 dflags docs 
	= vcat [ptext (sLit "Possible cause: the monomorphism restriction applied to the following:"),
		nest 2 (vcat docs),
		monomorphism_fix dflags]

monomorphism_fix :: DynFlags -> SDoc
monomorphism_fix dflags
  = ptext (sLit "Probable fix:") <+> vcat
	[ptext (sLit "give these definition(s) an explicit type signature"),
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	 if xopt Opt_MonomorphismRestriction dflags
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           then ptext (sLit "or use -XNoMonomorphismRestriction")
           else empty]	-- Only suggest adding "-XNoMonomorphismRestriction"
			-- if it is not already set!


-----------------------
-- findGlobals looks at the value environment and finds values whose
-- types mention any of the offending type variables.  It has to be
-- careful to zonk the Id's type first, so it has to be in the monad.
-- We must be careful to pass it a zonked type variable, too.

mkEnvSigMsg :: SDoc -> [SDoc] -> SDoc
mkEnvSigMsg what env_sigs
 | null env_sigs = empty
 | otherwise = vcat [ ptext (sLit "The following variables have types that mention") <+> what
                    , nest 2 (vcat env_sigs) ]

findGlobals :: ReportErrCtxt
            -> TcTyVarSet
            -> TcM (TidyEnv, [SDoc])

findGlobals ctxt tvs 
  = do { lcl_ty_env <- case cec_encl ctxt of 
                        []    -> getLclTypeEnv
                        (i:_) -> return (ic_env i)
       ; go (cec_tidy ctxt) [] (nameEnvElts lcl_ty_env) }
  where
    go tidy_env acc [] = return (tidy_env, acc)
    go tidy_env acc (thing : things) = do
        (tidy_env1, maybe_doc) <- find_thing tidy_env ignore_it thing
	case maybe_doc of
	  Just d  -> go tidy_env1 (d:acc) things
	  Nothing -> go tidy_env1 acc     things

    ignore_it ty = tvs `disjointVarSet` tyVarsOfType ty

-----------------------
find_thing :: TidyEnv -> (TcType -> Bool)
           -> TcTyThing -> TcM (TidyEnv, Maybe SDoc)
find_thing tidy_env ignore_it (ATcId { tct_id = id })
  = do { id_ty <- zonkTcType  (idType id)
       ; if ignore_it id_ty then
	   return (tidy_env, Nothing)
         else do 
       { let (tidy_env', tidy_ty) = tidyOpenType tidy_env id_ty
	     msg = sep [ ppr id <+> dcolon <+> ppr tidy_ty
		       , nest 2 (parens (ptext (sLit "bound at") <+>
			 	   ppr (getSrcLoc id)))]
       ; return (tidy_env', Just msg) } }

find_thing tidy_env ignore_it (ATyVar tv ty)
  = do { tv_ty <- zonkTcType ty
       ; if ignore_it tv_ty then
	    return (tidy_env, Nothing)
         else do
       { let -- The name tv is scoped, so we don't need to tidy it
	    (tidy_env1, tidy_ty) = tidyOpenType  tidy_env tv_ty
            msg = sep [ ptext (sLit "Scoped type variable") <+> quotes (ppr tv) <+> eq_stuff
                      , nest 2 bound_at]

	    eq_stuff | Just tv' <- tcGetTyVar_maybe tv_ty 
		     , getOccName tv == getOccName tv' = empty
		     | otherwise = equals <+> ppr tidy_ty
		-- It's ok to use Type.getTyVar_maybe because ty is zonked by now
	    bound_at = parens $ ptext (sLit "bound at:") <+> ppr (getSrcLoc tv)
 
       ; return (tidy_env1, Just msg) } }

find_thing _ _ thing = pprPanic "find_thing" (ppr thing)

warnDefaulting :: [WantedEvVar] -> Type -> TcM ()
warnDefaulting wanteds default_ty
  = do { warn_default <- doptM Opt_WarnTypeDefaults
       ; setCtLoc loc $ warnTc warn_default warn_msg }
  where
	-- Tidy them first
    warn_msg  = vcat [ ptext (sLit "Defaulting the following constraint(s) to type") <+>
				quotes (ppr default_ty),
		      nest 2 ppr_wanteds ]
    (loc, ppr_wanteds) = pprWithArising wanteds
\end{code}

%************************************************************************
%*									*
                 Error from the canonicaliser
%*									*
%************************************************************************

\begin{code}
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kindErrorTcS :: CtFlavor -> TcType -> TcType -> TcS ()
-- If there's a kind error, we don't want to blindly say "kind error"
-- We might, say, be unifying a skolem 'a' with a type 'Int', 
-- in which case that's the error to report.  So we set things
-- up to call reportEqErr, which does the business properly
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kindErrorTcS fl ty1 ty2
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  = wrapEqErrTcS fl ty1 ty2 $ \ env0 extra -> 
    do { let (env1, ty1') = tidyOpenType env0 ty1
             (env2, ty2') = tidyOpenType env1 ty2
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             ctxt = CEC { cec_encl = []
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                        , cec_extra = extra
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                        , cec_tidy = env2 }
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       ; reportEqErr ctxt ty1' ty2' }
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misMatchErrorTcS :: CtFlavor -> TcType -> TcType -> TcS a
misMatchErrorTcS fl ty1 ty2
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  = wrapEqErrTcS fl ty1 ty2 $ \ env0 extra -> 
    do { let (env1, ty1') = tidyOpenType env0 ty1
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             (env2, ty2') = tidyOpenType env1 ty2
             (env3, msg)  = misMatchMsgWithExtras env2 ty1' ty2'
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       ; failWithTcM (env3, inaccessible_msg $$ msg $$ extra) }
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  where
    inaccessible_msg 
      = case fl of 
          Given loc -> hang (ptext (sLit "Inaccessible code in"))
                          2 (mk_what loc)
          _         -> empty
    mk_what loc 
      = case ctLocOrigin loc of
          PatSkol dc mc -> sep [ ptext (sLit "a pattern with constructor") 
                                   <+> quotes (ppr dc) <> comma
                               , ptext (sLit "in") <+> pprMatchContext mc ]
	  other_skol -> pprSkolInfo other_skol

occursCheckErrorTcS :: CtFlavor -> TcTyVar -> TcType -> TcS a
occursCheckErrorTcS fl tv ty
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  = wrapEqErrTcS fl (mkTyVarTy tv) ty $ \ env0 extra2 -> 
    do	{ let (env1, tv') = tidyOpenTyVar env0 tv
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	      (env2, ty') = tidyOpenType env1 ty
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	      extra1 = sep [ppr tv', char '=', ppr ty']
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	; failWithTcM (env2, hang msg 2 (extra1 $$ extra2)) }
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  where
    msg = text $ "Occurs check: cannot construct the infinite type:"

solverDepthErrorTcS :: Int -> [CanonicalCt] -> TcS a
solverDepthErrorTcS depth stack
  | null stack	    -- Shouldn't happen unless you say -fcontext-stack=0
  = wrapErrTcS $ failWith msg
  | otherwise
  = wrapErrTcS $ 
    setCtFlavorLoc (cc_flavor top_item) $
    do { env0 <- tcInitTidyEnv
       ; let ev_vars  = map cc_id stack
             env1     = tidyFreeTyVars env0 free_tvs
             free_tvs = foldr (unionVarSet . tyVarsOfEvVar) emptyVarSet ev_vars
             extra    = pprEvVars (map (tidyEvVar env1) ev_vars)
       ; failWithTcM (env1, hang msg 2 extra) }
  where
    top_item = head stack
    msg = vcat [ ptext (sLit "Context reduction stack overflow; size =") <+> int depth
               , ptext (sLit "Use -fcontext-stack=N to increase stack size to N") ]

flattenForAllErrorTcS :: CtFlavor -> TcType -> Bag CanonicalCt -> TcS a
flattenForAllErrorTcS fl ty _bad_eqs
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  = wrapErrTcS        $ 
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    setCtFlavorLoc fl $ 
    do { env0 <- tcInitTidyEnv
       ; let (env1, ty') = tidyOpenType env0 ty 
             msg = sep [ ptext (sLit "Cannot deal with a type function under a forall type:")
                       , ppr ty' ]
       ; failWithTcM (env1, msg) }
\end{code}
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%************************************************************************
%*									*
                 Setting the context
%*									*
%************************************************************************

\begin{code}
setCtFlavorLoc :: CtFlavor -> TcM a -> TcM a
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setCtFlavorLoc (Wanted  loc) thing = setCtLoc loc thing
setCtFlavorLoc (Derived loc) thing = setCtLoc loc thing
setCtFlavorLoc (Given   loc) thing = setCtLoc loc thing

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wrapEqErrTcS :: CtFlavor -> TcType -> TcType
             -> (TidyEnv -> SDoc -> TcM a)
             -> TcS a
wrapEqErrTcS fl ty1 ty2 thing_inside
  = do { ty_binds_var <- getTcSTyBinds
       ; wrapErrTcS $ setCtFlavorLoc fl $ 
    do { env0 <- tcInitTidyEnv 
       ; ty_binds_bag <- readTcRef ty_binds_var
       ; let subst = mkOpenTvSubst (mkVarEnv (bagToList ty_binds_bag))
       ; case fl of
           Wanted  loc -> getWantedEqExtra subst env0 (ctLocOrigin loc) ty1 ty2 thing_inside
           Derived loc -> getWantedEqExtra subst env0 (ctLocOrigin loc) ty1 ty2 thing_inside
           Given {}    -> thing_inside env0 empty  -- We could print more info, but it
                                                   -- seems to be coming out already
       } }  

getWantedEqExtra :: TvSubst -> TidyEnv -> CtOrigin -> TcType -> TcType
                 -> (TidyEnv -> SDoc -> TcM a)
                 -> TcM a
getWantedEqExtra subst env0 (TypeEqOrigin item) ty1 ty2 thing_inside
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  -- If the types in the error message are the same 
  -- as the types we are unifying (remember to zonk the latter)
  -- don't add the extra expected/actual message
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  --
  -- The complication is that the types in the TypeEqOrigin must
  --   (a) be zonked
  --   (b) have any TcS-monad pending equalities applied to them 
  --   	   	(hence the passed-in substitution)
  = do { act0 <- zonkTcType (uo_actual item)
       ; exp0 <- zonkTcType (uo_expected item)
       ; let act1 = substTy subst act0
             exp1 = substTy subst exp0
             (env1, exp2) = tidyOpenType env0 exp1
             (env2, act2) = tidyOpenType env1 act1
       ; if (act1 `tcEqType` ty1 && exp1 `tcEqType` ty2)
         || (exp1 `tcEqType` ty1 && act1 `tcEqType` ty2)
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         then	
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            thing_inside env0 empty
         else 
            thing_inside env2 (mkExpectedActualMsg act2 exp2) }
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getWantedEqExtra _ env0 orig _ _ thing_inside
  = thing_inside env0 (pprArising orig)
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\end{code}