TcSimplify.lhs 61.3 KB
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\begin{code}
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{-# OPTIONS -fno-warn-tabs #-}
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and
-- detab the module (please do the detabbing in a separate patch). See
--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
-- for details

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module TcSimplify( 
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       simplifyInfer, simplifyAmbiguityCheck,
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       simplifyDefault, simplifyDeriv, 
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       simplifyRule, simplifyTop, simplifyInteractive
  ) where
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#include "HsVersions.h"
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import TcRnMonad
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import TcErrors
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import TcMType
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import TcType 
import TcSMonad 
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import TcInteract 
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import Inst
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import Unify	( niFixTvSubst, niSubstTvSet )
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import Var
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import VarSet
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import VarEnv 
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import TcEvidence
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import TypeRep
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import Name
import NameEnv	( emptyNameEnv )
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import Bag
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import ListSetOps
import Util
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import PrelInfo
import PrelNames
import Class		( classKey )
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import BasicTypes       ( RuleName )
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import Control.Monad    ( when )
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import Outputable
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import FastString
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import TrieMap () -- DV: for now
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import DynFlags
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\end{code}


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*********************************************************************************
*                                                                               * 
*                           External interface                                  *
*                                                                               *
*********************************************************************************
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\begin{code}
simplifyTop :: WantedConstraints -> TcM (Bag EvBind)
-- Simplify top-level constraints
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-- Usually these will be implications,
-- but when there is nothing to quantify we don't wrap
-- in a degenerate implication, so we do that here instead
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simplifyTop wanteds 
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  = simplifyCheck (SimplCheck (ptext (sLit "top level"))) wanteds
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------------------
simplifyAmbiguityCheck :: Name -> WantedConstraints -> TcM (Bag EvBind)
simplifyAmbiguityCheck name wanteds
  = simplifyCheck (SimplCheck (ptext (sLit "ambiguity check for") <+> ppr name)) wanteds
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------------------
simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind)
simplifyInteractive wanteds 
  = simplifyCheck SimplInteractive wanteds

------------------
simplifyDefault :: ThetaType	-- Wanted; has no type variables in it
                -> TcM ()	-- Succeeds iff the constraint is soluble
simplifyDefault theta
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  = do { wanted <- newFlatWanteds DefaultOrigin theta
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       ; _ignored_ev_binds <- simplifyCheck (SimplCheck (ptext (sLit "defaults"))) 
                                            (mkFlatWC wanted)
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       ; return () }
\end{code}
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***********************************************************************************
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*                                                                                 * 
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*                            Deriving                                             *
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*                                                                                 *
***********************************************************************************
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\begin{code}
simplifyDeriv :: CtOrigin
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              -> PredType
	      -> [TyVar]	
	      -> ThetaType		-- Wanted
	      -> TcM ThetaType	-- Needed
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-- Given  instance (wanted) => C inst_ty 
-- Simplify 'wanted' as much as possibles
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-- Fail if not possible
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simplifyDeriv orig pred tvs theta 
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  = do { (skol_subst, tvs_skols) <- tcInstSkolTyVars tvs -- Skolemize
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      	 	-- The constraint solving machinery 
		-- expects *TcTyVars* not TyVars.  
		-- We use *non-overlappable* (vanilla) skolems
		-- See Note [Overlap and deriving]
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       ; let subst_skol = zipTopTvSubst tvs_skols $ map mkTyVarTy tvs
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             skol_set   = mkVarSet tvs_skols
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	     doc = parens $ ptext (sLit "deriving") <+> parens (ppr pred)
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       ; wanted <- newFlatWanteds orig (substTheta skol_subst theta)

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       ; traceTc "simplifyDeriv" (pprTvBndrs tvs $$ ppr theta $$ ppr wanted)
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       ; (residual_wanted, _ev_binds1)
             <- runTcS (SimplInfer doc) NoUntouchables emptyInert emptyWorkList $
                solveWanteds $ mkFlatWC wanted
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       ; let (good, bad) = partitionBagWith get_good (wc_flat residual_wanted)
                         -- See Note [Exotic derived instance contexts]
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             get_good :: Ct -> Either PredType Ct
             get_good ct | validDerivPred skol_set p = Left p
                         | otherwise                 = Right ct
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                         where p = ctPred ct
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       -- We never want to defer these errors because they are errors in the
       -- compiler! Hence the `False` below
       ; _ev_binds2 <- reportUnsolved False (residual_wanted { wc_flat = bad })
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       ; let min_theta = mkMinimalBySCs (bagToList good)
       ; return (substTheta subst_skol min_theta) }
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\end{code}
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Note [Overlap and deriving]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider some overlapping instances:
  data Show a => Show [a] where ..
  data Show [Char] where ...

Now a data type with deriving:
  data T a = MkT [a] deriving( Show )

We want to get the derived instance
  instance Show [a] => Show (T a) where...
and NOT
  instance Show a => Show (T a) where...
so that the (Show (T Char)) instance does the Right Thing

It's very like the situation when we're inferring the type
of a function
   f x = show [x]
and we want to infer
   f :: Show [a] => a -> String

BOTTOM LINE: use vanilla, non-overlappable skolems when inferring
             the context for the derived instance. 
	     Hence tcInstSkolTyVars not tcInstSuperSkolTyVars

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Note [Exotic derived instance contexts]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In a 'derived' instance declaration, we *infer* the context.  It's a
bit unclear what rules we should apply for this; the Haskell report is
silent.  Obviously, constraints like (Eq a) are fine, but what about
	data T f a = MkT (f a) deriving( Eq )
where we'd get an Eq (f a) constraint.  That's probably fine too.
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One could go further: consider
	data T a b c = MkT (Foo a b c) deriving( Eq )
	instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
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Notice that this instance (just) satisfies the Paterson termination 
conditions.  Then we *could* derive an instance decl like this:
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	instance (C Int a, Eq b, Eq c) => Eq (T a b c) 
even though there is no instance for (C Int a), because there just
*might* be an instance for, say, (C Int Bool) at a site where we
need the equality instance for T's.  
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However, this seems pretty exotic, and it's quite tricky to allow
this, and yet give sensible error messages in the (much more common)
case where we really want that instance decl for C.
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So for now we simply require that the derived instance context
should have only type-variable constraints.
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Here is another example:
	data Fix f = In (f (Fix f)) deriving( Eq )
Here, if we are prepared to allow -XUndecidableInstances we
could derive the instance
	instance Eq (f (Fix f)) => Eq (Fix f)
but this is so delicate that I don't think it should happen inside
'deriving'. If you want this, write it yourself!

NB: if you want to lift this condition, make sure you still meet 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)).

The bottom line
~~~~~~~~~~~~~~~
Allow constraints which consist only of type variables, with no repeats.

*********************************************************************************
*                                                                                 * 
*                            Inference
*                                                                                 *
***********************************************************************************
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Note [Which variables to quantify]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose the inferred type of a function is
   T kappa (alpha:kappa) -> Int
where alpha is a type unification variable and 
      kappa is a kind unification variable
Then we want to quantify over *both* alpha and kappa.  But notice that
kappa appears "at top level" of the type, as well as inside the kind
of alpha.  So it should be fine to just look for the "top level"
kind/type variables of the type, without looking transitively into the
kinds of those type variables.

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\begin{code}
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simplifyInfer :: Bool
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              -> Bool                  -- Apply monomorphism restriction
              -> [(Name, TcTauType)]   -- Variables to be generalised,
                                       -- and their tau-types
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              -> WantedConstraints
              -> TcM ([TcTyVar],    -- Quantify over these type variables
                      [EvVar],      -- ... and these constraints
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		      Bool,	    -- The monomorphism restriction did something
		      		    --   so the results type is not as general as
				    --   it could be
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                      TcEvBinds)    -- ... binding these evidence variables
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simplifyInfer _top_lvl apply_mr name_taus wanteds
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  | isEmptyWC wanteds
  = do { gbl_tvs     <- tcGetGlobalTyVars            -- Already zonked
       ; zonked_taus <- zonkTcTypes (map snd name_taus)
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       ; let tvs_to_quantify = varSetElems (tyVarsOfTypes zonked_taus `minusVarSet` gbl_tvs)
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       	     		       -- tvs_to_quantify can contain both kind and type vars
       	                       -- See Note [Which variables to quantify]
       ; qtvs <- zonkQuantifiedTyVars tvs_to_quantify
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       ; return (qtvs, [], False, emptyTcEvBinds) }
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  | otherwise
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  = do { zonked_wanteds <- zonkWC wanteds
       ; gbl_tvs        <- tcGetGlobalTyVars
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       ; zonked_tau_tvs <- zonkTyVarsAndFV (tyVarsOfTypes (map snd name_taus))
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       ; runtimeCoercionErrors <- doptM Opt_DeferTypeErrors
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       ; traceTc "simplifyInfer {"  $ vcat
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             [ ptext (sLit "names =") <+> ppr (map fst name_taus)
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             , ptext (sLit "taus =") <+> ppr (map snd name_taus)
             , ptext (sLit "tau_tvs (zonked) =") <+> ppr zonked_tau_tvs
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             , ptext (sLit "gbl_tvs =") <+> ppr gbl_tvs
             , ptext (sLit "closed =") <+> ppr _top_lvl
             , ptext (sLit "apply_mr =") <+> ppr apply_mr
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             , ptext (sLit "wanted =") <+> ppr zonked_wanteds
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             ]

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             -- Step 1
             -- Make a guess at the quantified type variables
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	     -- Then split the constraints on the baisis of those tyvars
	     -- to avoid unnecessarily simplifying a class constraint
	     -- See Note [Avoid unecessary constraint simplification]
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       ; let proto_qtvs = growWanteds gbl_tvs zonked_wanteds $
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                          zonked_tau_tvs `minusVarSet` gbl_tvs
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             (perhaps_bound, surely_free)
                        = partitionBag (quantifyMe proto_qtvs) (wc_flat zonked_wanteds)

       ; traceTc "simplifyInfer proto"  $ vcat
             [ ptext (sLit "zonked_tau_tvs =") <+> ppr zonked_tau_tvs
             , ptext (sLit "proto_qtvs =") <+> ppr proto_qtvs
             , ptext (sLit "surely_fref =") <+> ppr surely_free
             ]

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       ; emitFlats surely_free
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       ; traceTc "sinf"  $ vcat
             [ ptext (sLit "perhaps_bound =") <+> ppr perhaps_bound
             , ptext (sLit "surely_free   =") <+> ppr surely_free
             ]
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            -- Step 2 
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            -- Now simplify the possibly-bound constraints
       ; let ctxt = SimplInfer (ppr (map fst name_taus))
       ; (simpl_results, tc_binds)
             <- runTcS ctxt NoUntouchables emptyInert emptyWorkList $ 
                simplifyWithApprox (zonked_wanteds { wc_flat = perhaps_bound })

            -- Fail fast if there is an insoluble constraint,
            -- unless we are deferring errors to runtime
       ; when (not runtimeCoercionErrors && insolubleWC simpl_results) $ 
         do { _ev_binds <- reportUnsolved False simpl_results 
            ; failM }
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            -- Step 3 
            -- Split again simplified_perhaps_bound, because some unifications 
            -- may have happened, and emit the free constraints. 
       ; gbl_tvs        <- tcGetGlobalTyVars
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       ; zonked_tau_tvs <- zonkTyVarsAndFV zonked_tau_tvs
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       ; zonked_flats <- zonkCts (wc_flat simpl_results)
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       ; let init_tvs 	     = zonked_tau_tvs `minusVarSet` gbl_tvs
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             poly_qtvs       = growWantedEVs gbl_tvs zonked_flats init_tvs
	     (pbound, pfree) = partitionBag (quantifyMe poly_qtvs) zonked_flats
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	     -- Monomorphism restriction
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             mr_qtvs  	     = init_tvs `minusVarSet` constrained_tvs
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             constrained_tvs = tyVarsOfCts zonked_flats
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	     mr_bites        = apply_mr && not (isEmptyBag pbound)

             (qtvs, (bound, free))
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                | mr_bites  = (mr_qtvs,   (emptyBag, zonked_flats))
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                | otherwise = (poly_qtvs, (pbound,   pfree))
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       ; emitFlats free
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       ; if isEmptyVarSet qtvs && isEmptyBag bound
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         then ASSERT( isEmptyBag (wc_insol simpl_results) )
              do { traceTc "} simplifyInfer/no quantification" empty
                 ; emitImplications (wc_impl simpl_results)
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                 ; return ([], [], mr_bites, EvBinds tc_binds) }
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         else do

            -- Step 4, zonk quantified variables 
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       { let minimal_flat_preds = mkMinimalBySCs $ 
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                                  map ctPred $ bagToList bound
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             skol_info = InferSkol [ (name, mkSigmaTy [] minimal_flat_preds ty)
                                   | (name, ty) <- name_taus ]
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                        -- Don't add the quantified variables here, because
                        -- they are also bound in ic_skols and we want them to be
                        -- tidied uniformly

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       ; qtvs_to_return <- zonkQuantifiedTyVars (varSetElems qtvs)
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            -- Step 5
            -- Minimize `bound' and emit an implication
       ; minimal_bound_ev_vars <- mapM TcMType.newEvVar minimal_flat_preds
       ; ev_binds_var <- newTcEvBinds
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       ; mapBagM_ (\(EvBind evar etrm) -> addTcEvBind ev_binds_var evar etrm) 
           tc_binds
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       ; lcl_env <- getLclTypeEnv
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       ; gloc <- getCtLoc skol_info
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       ; let implic = Implic { ic_untch    = NoUntouchables
                             , ic_env      = lcl_env
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                             , ic_skols    = qtvs_to_return
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                             , ic_given    = minimal_bound_ev_vars
                             , ic_wanted   = simpl_results { wc_flat = bound }
                             , ic_insol    = False
                             , ic_binds    = ev_binds_var
                             , ic_loc      = gloc }
       ; emitImplication implic
       ; traceTc "} simplifyInfer/produced residual implication for quantification" $
             vcat [ ptext (sLit "implic =") <+> ppr implic
                       -- ic_skols, ic_given give rest of result
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                  , ptext (sLit "qtvs =") <+> ppr qtvs_to_return
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                  , ptext (sLit "spb =") <+> ppr zonked_flats
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                  , ptext (sLit "bound =") <+> ppr bound ]



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       ; return ( qtvs_to_return, minimal_bound_ev_vars
                , mr_bites,  TcEvBinds ev_binds_var) } }
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\end{code}
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Note [Minimize by Superclasses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
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When we quantify over a constraint, in simplifyInfer we need to
quantify over a constraint that is minimal in some sense: For
instance, if the final wanted constraint is (Eq alpha, Ord alpha),
we'd like to quantify over Ord alpha, because we can just get Eq alpha
from superclass selection from Ord alpha. This minimization is what
mkMinimalBySCs does. Then, simplifyInfer uses the minimal constraint
to check the original wanted.
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\begin{code}
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simplifyWithApprox :: WantedConstraints -> TcS WantedConstraints
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-- Post: returns only wanteds (no deriveds)
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simplifyWithApprox wanted
 = do { traceTcS "simplifyApproxLoop" (ppr wanted)
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      ; let all_flats = wc_flat wanted `unionBags` keepWanted (wc_insol wanted) 
      ; solveInteractCts $ bagToList all_flats
      ; unsolved_implics <- simpl_loop 1 (wc_impl wanted)

      ; let (residual_implics,floats) = approximateImplications unsolved_implics

      -- Solve extra stuff for real: notice that all the extra unsolved constraints will 
      -- be in the inerts of the monad, so we are OK
      ; traceTcS "simplifyApproxLoop" $ text "Calling solve_wanteds!"
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      ; wants_or_ders <- solve_wanteds (WC { wc_flat  = floats -- They are floated so they are not in the evvar cache
                                           , wc_impl  = residual_implics
                                           , wc_insol = emptyBag })
      ; return $ 
        wants_or_ders { wc_flat = keepWanted (wc_flat wants_or_ders) } }
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approximateImplications :: Bag Implication -> (Bag Implication, Cts)
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-- Extracts any nested constraints that don't mention the skolems
approximateImplications impls
  = do_bag (float_implic emptyVarSet) impls
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  where 
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    do_bag :: forall a b c. (a -> (Bag b, Bag c)) -> Bag a -> (Bag b, Bag c)
    do_bag f = foldrBag (plus . f) (emptyBag, emptyBag)
    plus :: forall b c. (Bag b, Bag c) -> (Bag b, Bag c) -> (Bag b, Bag c)
    plus (a1,b1) (a2,b2) = (a1 `unionBags` a2, b1 `unionBags` b2)

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    float_implic :: TyVarSet -> Implication -> (Bag Implication, Cts)
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    float_implic skols imp
      = (unitBag (imp { ic_wanted = wanted' }), floats)
      where
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        (wanted', floats) = float_wc (skols `extendVarSetList` ic_skols imp) (ic_wanted imp)
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    float_wc skols wc@(WC { wc_flat = flat, wc_impl = implic })
      = (wc { wc_flat = flat', wc_impl = implic' }, floats1 `unionBags` floats2)
      where
        (flat',   floats1) = do_bag (float_flat   skols) flat
        (implic', floats2) = do_bag (float_implic skols) implic

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    float_flat :: TcTyVarSet -> Ct -> (Cts, Cts)
    float_flat skols ct
      | tyVarsOfCt ct `disjointVarSet` skols = (emptyBag, unitBag ct)
      | otherwise                            = (unitBag ct, emptyBag)
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\end{code}
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\begin{code}
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-- (growX gbls wanted tvs) grows a seed 'tvs' against the 
-- X-constraint 'wanted', nuking the 'gbls' at each stage
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-- It's conservative in that if the seed could *possibly*
-- grow to include a type variable, then it does
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growWanteds :: TyVarSet -> WantedConstraints -> TyVarSet -> TyVarSet
growWanteds gbl_tvs wc = fixVarSet (growWC gbl_tvs wc)

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growWantedEVs :: TyVarSet -> Cts -> TyVarSet -> TyVarSet
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growWantedEVs gbl_tvs ws tvs
  | isEmptyBag ws = tvs
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  | otherwise     = fixVarSet (growPreds gbl_tvs ctPred ws) tvs
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--------  Helper functions, do not do fixpoint ------------------------
growWC :: TyVarSet -> WantedConstraints -> TyVarSet -> TyVarSet
growWC gbl_tvs wc = growImplics gbl_tvs             (wc_impl wc) .
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                    growPreds   gbl_tvs ctPred (wc_flat wc) .
                    growPreds   gbl_tvs ctPred (wc_insol wc)
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growImplics :: TyVarSet -> Bag Implication -> TyVarSet -> TyVarSet
growImplics gbl_tvs implics tvs
  = foldrBag grow_implic tvs implics
  where
    grow_implic implic tvs
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      = grow tvs `delVarSetList` ic_skols implic
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      where
        grow = growWC gbl_tvs (ic_wanted implic) .
               growPreds gbl_tvs evVarPred (listToBag (ic_given implic))
               -- We must grow from givens too; see test IPRun

growPreds :: TyVarSet -> (a -> PredType) -> Bag a -> TyVarSet -> TyVarSet
growPreds gbl_tvs get_pred items tvs
  = foldrBag extend tvs items
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  where
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    extend item tvs = tvs `unionVarSet`
                      (growPredTyVars (get_pred item) tvs `minusVarSet` gbl_tvs)
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--------------------
quantifyMe :: TyVarSet      -- Quantifying over these
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	   -> Ct
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	   -> Bool	    -- True <=> quantify over this wanted
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quantifyMe qtvs ct
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  | isIPPred pred = True  -- Note [Inheriting implicit parameters]
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  | otherwise	  = tyVarsOfType pred `intersectsVarSet` qtvs
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  where
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    pred = ctPred ct
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\end{code}
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Note [Avoid unecessary constraint simplification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When inferring the type of a let-binding, with simplifyInfer,
try to avoid unnecessariliy simplifying class constraints.
Doing so aids sharing, but it also helps with delicate 
situations like
   instance C t => C [t] where ..
   f :: C [t] => ....
   f x = let g y = ...(constraint C [t])... 
         in ...
When inferring a type for 'g', we don't want to apply the
instance decl, because then we can't satisfy (C t).  So we
just notice that g isn't quantified over 't' and partition
the contraints before simplifying.

This only half-works, but then let-generalisation only half-works.


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Note [Inheriting implicit parameters]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Consider this:

	f x = (x::Int) + ?y
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where f is *not* a top-level binding.
From the RHS of f we'll get the constraint (?y::Int).
There are two types we might infer for f:
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	f :: Int -> Int

(so we get ?y from the context of f's definition), or
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	f :: (?y::Int) => Int -> Int

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At first you might think the first was better, becuase then
?y behaves like a free variable of the definition, rather than
having to be passed at each call site.  But of course, the WHOLE
IDEA is that ?y should be passed at each call site (that's what
dynamic binding means) so we'd better infer the second.

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BOTTOM LINE: when *inferring types* you *must* quantify 
over implicit parameters. See the predicate isFreeWhenInferring.
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*********************************************************************************
*                                                                                 * 
*                             RULES                                               *
*                                                                                 *
***********************************************************************************
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Note [Simplifying RULE lhs constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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On the LHS of transformation rules we only simplify only equalities,
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but not dictionaries.  We want to keep dictionaries unsimplified, to
serve as the available stuff for the RHS of the rule.  We *do* want to
simplify equalities, however, to detect ill-typed rules that cannot be
applied.
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Implementation: the TcSFlags carried by the TcSMonad controls the
amount of simplification, so simplifyRuleLhs just sets the flag
appropriately.
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Example.  Consider the following left-hand side of a rule
	f (x == y) (y > z) = ...
If we typecheck this expression we get constraints
	d1 :: Ord a, d2 :: Eq a
We do NOT want to "simplify" to the LHS
	forall x::a, y::a, z::a, d1::Ord a.
	  f ((==) (eqFromOrd d1) x y) ((>) d1 y z) = ...
Instead we want	
	forall x::a, y::a, z::a, d1::Ord a, d2::Eq a.
	  f ((==) d2 x y) ((>) d1 y z) = ...
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Here is another example:
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	fromIntegral :: (Integral a, Num b) => a -> b
	{-# RULES "foo"  fromIntegral = id :: Int -> Int #-}
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In the rule, a=b=Int, and Num Int is a superclass of Integral Int. But
we *dont* want to get
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	forall dIntegralInt.
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	   fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
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because the scsel will mess up RULE matching.  Instead we want
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	forall dIntegralInt, dNumInt.
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	  fromIntegral Int Int dIntegralInt dNumInt = id Int
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Even if we have 
	g (x == y) (y == z) = ..
where the two dictionaries are *identical*, we do NOT WANT
	forall x::a, y::a, z::a, d1::Eq a
	  f ((==) d1 x y) ((>) d1 y z) = ...
because that will only match if the dict args are (visibly) equal.
Instead we want to quantify over the dictionaries separately.
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In short, simplifyRuleLhs must *only* squash equalities, leaving
all dicts unchanged, with absolutely no sharing.  
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HOWEVER, under a nested implication things are different
Consider
  f :: (forall a. Eq a => a->a) -> Bool -> ...
  {-# RULES "foo" forall (v::forall b. Eq b => b->b).
       f b True = ...
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    #-}
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Here we *must* solve the wanted (Eq a) from the given (Eq a)
resulting from skolemising the agument type of g.  So we 
revert to SimplCheck when going under an implication.  
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\begin{code}
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simplifyRule :: RuleName 
             -> [TcTyVar]		-- Explicit skolems
             -> WantedConstraints	-- Constraints from LHS
             -> WantedConstraints	-- Constraints from RHS
             -> TcM ([EvVar], 		-- LHS dicts
                     TcEvBinds,		-- Evidence for LHS
                     TcEvBinds)		-- Evidence for RHS
-- See Note [Simplifying RULE lhs constraints]
simplifyRule name tv_bndrs lhs_wanted rhs_wanted
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  = do { loc        <- getCtLoc (RuleSkol name)
       ; zonked_lhs <- zonkWC lhs_wanted
       ; let untch = NoUntouchables
	     	 -- We allow ourselves to unify environment 
		 -- variables; hence *no untouchables*

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-- DV: SPJ and I discussed a new plan here:
-- Step 1: Simplify everything in the same bag
-- Step 2: zonk away the lhs constraints and get only the non-trivial ones
-- Step 3: do the implications with these constraints
-- This will have the advantage that (i) we no longer need simplEqsOnly flag
-- and (ii) will fix problems appearing from touchable unification variables
-- in the givens, manifested by #5853
-- TODO ... 

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       ; (lhs_results, lhs_binds)
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              <- runTcS (SimplRuleLhs name) untch emptyInert emptyWorkList $
                 solveWanteds zonked_lhs
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       ; traceTc "simplifyRule" $
         vcat [ text "zonked_lhs"   <+> ppr zonked_lhs 
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              , text "lhs_results"  <+> ppr lhs_results
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              , text "lhs_binds"    <+> ppr lhs_binds 
              , text "rhs_wanted"   <+> ppr rhs_wanted ]

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       -- Don't quantify over equalities (judgement call here)
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       ; let (eqs, dicts) = partitionBag (isEqPred . ctPred)
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                                         (wc_flat lhs_results)
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             lhs_dicts    = map ctId (bagToList dicts)
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                                 -- Dicts and implicit parameters
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                                 -- NB: dicts come from lhs_results which
                                 -- are all Wanted, hence have ids, hence
                                 -- it's fine to call ctId on them
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           -- Fail if we have not got down to unsolved flats
       ; ev_binds_var <- newTcEvBinds
       ; emitImplication $ Implic { ic_untch  = untch
                                  , ic_env    = emptyNameEnv
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                                  , ic_skols  = tv_bndrs
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                                  , ic_given  = lhs_dicts
                                  , ic_wanted = lhs_results { wc_flat = eqs }
                                  , ic_insol  = insolubleWC lhs_results
                                  , ic_binds  = ev_binds_var
                                  , ic_loc    = loc }
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	     -- Notice that we simplify the RHS with only the explicitly
	     -- introduced skolems, allowing the RHS to constrain any 
	     -- unification variables.
	     -- Then, and only then, we call zonkQuantifiedTypeVariables
	     -- Example   foo :: Ord a => a -> a
	     --		  foo_spec :: Int -> Int
	     --		  {-# RULE "foo"  foo = foo_spec #-}
	     --	    Here, it's the RHS that fixes the type variable

	     -- So we don't want to make untouchable the type
	     -- variables in the envt of the RHS, because they include
	     -- the template variables of the RULE

	     -- Hence the rather painful ad-hoc treatement here
       ; rhs_binds_var@(EvBindsVar evb_ref _)  <- newTcEvBinds
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       ; let doc = ptext (sLit "rhs of rule") <+> doubleQuotes (ftext name)
       ; rhs_binds1 <- simplifyCheck (SimplCheck doc) $
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            WC { wc_flat = emptyBag
               , wc_insol = emptyBag
               , wc_impl = unitBag $
                    Implic { ic_untch   = NoUntouchables
                            , ic_env    = emptyNameEnv
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                            , ic_skols  = tv_bndrs
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                            , ic_given  = lhs_dicts
                            , ic_wanted = rhs_wanted
                            , ic_insol  = insolubleWC rhs_wanted
                            , ic_binds  = rhs_binds_var
                            , ic_loc    = loc } }
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       ; rhs_binds2 <- readTcRef evb_ref

       ; return ( lhs_dicts
                , EvBinds lhs_binds 
                , EvBinds (rhs_binds1 `unionBags` evBindMapBinds rhs_binds2)) }
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\end{code}


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*********************************************************************************
*                                                                                 * 
*                                 Main Simplifier                                 *
*                                                                                 *
***********************************************************************************
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\begin{code}
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simplifyCheck :: SimplContext
	      -> WantedConstraints	-- Wanted
              -> TcM (Bag EvBind)
-- Solve a single, top-level implication constraint
-- e.g. typically one created from a top-level type signature
-- 	    f :: forall a. [a] -> [a]
--          f x = rhs
-- We do this even if the function has no polymorphism:
--    	    g :: Int -> Int

--          g y = rhs
-- (whereas for *nested* bindings we would not create
--  an implication constraint for g at all.)
--
-- Fails if can't solve something in the input wanteds
simplifyCheck ctxt wanteds
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  = do { wanteds <- zonkWC wanteds
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       ; traceTc "simplifyCheck {" (vcat
             [ ptext (sLit "wanted =") <+> ppr wanteds ])

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       ; (unsolved, eb1)
           <- runTcS ctxt NoUntouchables emptyInert emptyWorkList $ 
              solveWanteds wanteds

       ; traceTc "simplifyCheck }" $ ptext (sLit "unsolved =") <+> ppr unsolved

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       ; traceTc "reportUnsolved {" empty
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       -- See Note [Deferring coercion errors to runtime]
       ; runtimeCoercionErrors <- doptM Opt_DeferTypeErrors
       ; eb2 <- reportUnsolved runtimeCoercionErrors unsolved 
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       ; traceTc "reportUnsolved }" empty

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       ; return (eb1 `unionBags` eb2) }
\end{code}

Note [Deferring coercion errors to runtime]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

While developing, sometimes it is desirable to allow compilation to succeed even
if there are type errors in the code. Consider the following case:

  module Main where
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  a :: Int
  a = 'a'
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  main = print "b"
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Even though `a` is ill-typed, it is not used in the end, so if all that we're
interested in is `main` it is handy to be able to ignore the problems in `a`.
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Since we treat type equalities as evidence, this is relatively simple. Whenever
we run into a type mismatch in TcUnify, we normally just emit an error. But it
is always safe to defer the mismatch to the main constraint solver. If we do
that, `a` will get transformed into

  co :: Int ~ Char
  co = ...

  a :: Int
  a = 'a' `cast` co

The constraint solver would realize that `co` is an insoluble constraint, and
emit an error with `reportUnsolved`. But we can also replace the right-hand side
of `co` with `error "Deferred type error: Int ~ Char"`. This allows the program
to compile, and it will run fine unless we evaluate `a`. This is what
`deferErrorsToRuntime` does.

It does this by keeping track of which errors correspond to which coercion
in TcErrors (with ErrEnv). TcErrors.reportTidyWanteds does not print the errors
and does not fail if -fwarn-type-errors is on, so that we can continue
compilation. The errors are turned into warnings in `reportUnsolved`.

\begin{code}
solveWanteds :: WantedConstraints -> TcS WantedConstraints
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-- Returns: residual constraints, plus evidence bindings 
-- NB: When we are called from TcM there are no inerts to pass down to TcS
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solveWanteds wanted
  = do { wc_out <- solve_wanteds wanted
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       ; let wc_ret = wc_out { wc_flat = keepWanted (wc_flat wc_out) } 
                      -- Discard Derived
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       ; return wc_ret }
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solve_wanteds :: WantedConstraints
              -> TcS WantedConstraints  -- NB: wc_flats may be wanted *or* derived now
solve_wanteds wanted@(WC { wc_flat = flats, wc_impl = implics, wc_insol = insols }) 
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  = do { traceTcS "solveWanteds {" (ppr wanted)

                 -- Try the flat bit
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                 -- Discard from insols all the derived/given constraints
                 -- because they will show up again when we try to solve
                 -- everything else.  Solving them a second time is a bit
                 -- of a waste, but the code is simple, and the program is
                 -- wrong anyway!
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       ; let all_flats = flats `unionBags` keepWanted insols
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       ; solveInteractCts $ bagToList all_flats
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       -- solve_wanteds iterates when it is able to float equalities 
       -- out of one or more of the implications. 
       ; unsolved_implics <- simpl_loop 1 implics
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       ; (insoluble_flats,unsolved_flats) <- extractUnsolvedTcS 

       ; bb <- getTcEvBindsMap
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       ; tb <- getTcSTyBindsMap
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       ; traceTcS "solveWanteds }" $
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                 vcat [ text "unsolved_flats   =" <+> ppr unsolved_flats
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                      , text "unsolved_implics =" <+> ppr unsolved_implics
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                      , text "current evbinds  =" <+> ppr (evBindMapBinds bb)
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                      , text "current tybinds  =" <+> vcat (map ppr (varEnvElts tb))
                      ]

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       ; (subst, remaining_unsolved_flats) <- solveCTyFunEqs unsolved_flats
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                -- See Note [Solving Family Equations]
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                -- NB: remaining_flats has already had subst applied

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       ; traceTcS "solveWanteds finished with" $
                 vcat [ text "remaining_unsolved_flats =" <+> ppr remaining_unsolved_flats
                      , text "subst =" <+> ppr subst
                      ]

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       ; return $ 
         WC { wc_flat  = mapBag (substCt subst) remaining_unsolved_flats
            , wc_impl  = mapBag (substImplication subst) unsolved_implics
            , wc_insol = mapBag (substCt subst) insoluble_flats }
       }

simpl_loop :: Int
           -> Bag Implication
           -> TcS (Bag Implication)
simpl_loop n implics
  | n > 10 
  = traceTcS "solveWanteds: loop!" empty >> return implics
  | otherwise 
  = do { (implic_eqs, unsolved_implics) <- solveNestedImplications implics
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       ; inerts <- getTcSInerts
       ; let ((_,unsolved_flats),_) = extractUnsolved inerts
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       ; improve_eqs <- if not (isEmptyBag implic_eqs)
                        then return implic_eqs
                        else applyDefaultingRules unsolved_flats
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       ; traceTcS "solveWanteds: simpl_loop end" $
             vcat [ text "improve_eqs      =" <+> ppr improve_eqs
                  , text "unsolved_flats   =" <+> ppr unsolved_flats
                  , text "unsolved_implics =" <+> ppr unsolved_implics ]
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       ; if isEmptyBag improve_eqs then return unsolved_implics 
         else do { solveInteractCts $ bagToList improve_eqs
                 ; simpl_loop (n+1) unsolved_implics } }
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solveNestedImplications :: Bag Implication
                        -> TcS (Cts, Bag Implication)
-- Precondition: the TcS inerts may contain unsolved flats which have 
-- to be converted to givens before we go inside a nested implication.
solveNestedImplications implics
  | isEmptyBag implics
  = return (emptyBag, emptyBag)
  | otherwise 
  = do { inerts <- getTcSInerts
       ; let ((_insoluble_flats, unsolved_flats),thinner_inerts) = extractUnsolved inerts 
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       ; (implic_eqs, unsolved_implics)
           <- doWithInert thinner_inerts $ 
              do { let pushed_givens = givens_from_wanteds unsolved_flats
                       tcs_untouchables = filterVarSet isFlexiTcsTv $ 
                                          tyVarsOfCts unsolved_flats
                 -- See Note [Preparing inert set for implications]
	         -- Push the unsolved wanteds inwards, but as givens
                 ; traceTcS "solveWanteds: preparing inerts for implications {" $ 
                   vcat [ppr tcs_untouchables, ppr pushed_givens]
                 ; solveInteractCts pushed_givens 
                 ; traceTcS "solveWanteds: } now doing nested implications {" empty
                 ; flatMapBagPairM (solveImplication tcs_untouchables) implics }

       -- ... and we are back in the original TcS inerts 
       -- Notice that the original includes the _insoluble_flats so it was safe to ignore
       -- them in the beginning of this function.
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       ; traceTcS "solveWanteds: done nested implications }" $
                  vcat [ text "implic_eqs ="       <+> ppr implic_eqs
                       , text "unsolved_implics =" <+> ppr unsolved_implics ]

       ; return (implic_eqs, unsolved_implics) }

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  where givens_from_wanteds = foldrBag get_wanted []
        get_wanted cc rest_givens
            | pushable_wanted cc
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            = let fl = cc_flavor cc
                  wloc = flav_wloc fl
                  gfl = Given (mkGivenLoc wloc UnkSkol) (flav_evar fl) 
                  this_given = cc { cc_flavor = gfl }
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              in this_given : rest_givens
            | otherwise = rest_givens 

        pushable_wanted :: Ct -> Bool 
        pushable_wanted cc 
         | isWantedCt cc 
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         = isEqPred (ctPred cc) -- see Note [Preparing inert set for implications]
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         | otherwise = False 

solveImplication :: TcTyVarSet     -- Untouchable TcS unification variables
                 -> Implication    -- Wanted
                 -> TcS (Cts,      -- All wanted or derived floated equalities: var = type
                         Bag Implication) -- Unsolved rest (always empty or singleton)
-- Precondition: The TcS monad contains an empty worklist and given-only inerts 
-- which after trying to solve this implication we must restore to their original value
solveImplication tcs_untouchables
     imp@(Implic { ic_untch  = untch
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                 , ic_binds  = ev_binds
                 , ic_skols  = skols 
                 , ic_given  = givens
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                 , ic_wanted = wanteds
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                 , ic_loc    = loc })
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  = nestImplicTcS ev_binds (untch, tcs_untouchables) $
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    recoverTcS (return (emptyBag, emptyBag)) $
       -- Recover from nested failures.  Even the top level is
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       -- just a bunch of implications, so failing at the first one is bad
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    do { traceTcS "solveImplication {" (ppr imp) 

         -- Solve flat givens
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       ; solveInteractGiven loc givens 
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         -- Simplify the wanteds
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       ; WC { wc_flat = unsolved_flats
            , wc_impl = unsolved_implics
            , wc_insol = insols } <- solve_wanteds wanteds
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       ; let (res_flat_free, res_flat_bound)
                 = floatEqualities skols givens unsolved_flats
             final_flat = keepWanted res_flat_bound
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       ; let res_wanted = WC { wc_flat  = final_flat
                             , wc_impl  = unsolved_implics
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                             , wc_insol = insols }
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             res_implic = unitImplication $
                          imp { ic_wanted = res_wanted
                              , ic_insol  = insolubleWC res_wanted }
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       ; evbinds <- getTcEvBindsMap

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       ; traceTcS "solveImplication end }" $ vcat
             [ text "res_flat_free =" <+> ppr res_flat_free
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             , text "implication evbinds = " <+> ppr (evBindMapBinds evbinds)
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             , text "res_implic =" <+> ppr res_implic ]
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       ; return (res_flat_free, res_implic) }
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    -- and we are back to the original inerts
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floatEqualities :: [TcTyVar] -> [EvVar] -> Cts -> (Cts, Cts)
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-- Post: The returned FlavoredEvVar's are only Wanted or Derived
-- and come from the input wanted ev vars or deriveds 
floatEqualities skols can_given wantders
  | hasEqualities can_given = (emptyBag, wantders)
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          -- Note [Float Equalities out of Implications]
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  | otherwise = partitionBag is_floatable wantders
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-- TODO: Maybe we should try out /not/ floating constraints that contain touchables only, 
-- since they are inert and not going to interact with anything more in a more global scope.

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  where skol_set = mkVarSet skols
        is_floatable :: Ct -> Bool
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        is_floatable ct
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          | ct_predty <- ctPred ct
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          , isEqPred ct_predty
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          = skol_set `disjointVarSet` tvs_under_fsks ct_predty
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        is_floatable _ct = False
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        tvs_under_fsks :: Type -> TyVarSet
        -- ^ NB: for type synonyms tvs_under_fsks does /not/ expand the synonym
        tvs_under_fsks (TyVarTy tv)     
          | not (isTcTyVar tv)               = unitVarSet tv
          | FlatSkol ty <- tcTyVarDetails tv = tvs_under_fsks ty
          | otherwise                        = unitVarSet tv
        tvs_under_fsks (TyConApp _ tys) = unionVarSets (map tvs_under_fsks tys)
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        tvs_under_fsks (LitTy {})       = emptyVarSet
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        tvs_under_fsks (FunTy arg res)  = tvs_under_fsks arg `unionVarSet` tvs_under_fsks res
        tvs_under_fsks (AppTy fun arg)  = tvs_under_fsks fun `unionVarSet` tvs_under_fsks arg
        tvs_under_fsks (ForAllTy tv ty) -- The kind of a coercion binder 
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        	     	       	        -- can mention type variables!
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          | isTyVar tv		      = inner_tvs `delVarSet` tv
          | otherwise  {- Coercion -} = -- ASSERT( not (tv `elemVarSet` inner_tvs) )
                                        inner_tvs `unionVarSet` tvs_under_fsks (tyVarKind tv)
          where
            inner_tvs = tvs_under_fsks ty
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\end{code}
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Note [Preparing inert set for implications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Before solving the nested implications, we convert any unsolved flat wanteds
to givens, and add them to the inert set.  Reasons:
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  a) In checking mode, suppresses unnecessary errors.  We already have
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     on unsolved-wanted error; adding it to the givens prevents any 
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     consequential errors from showing up
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  b) More importantly, in inference mode, we are going to quantify over this
     constraint, and we *don't* want to quantify over any constraints that
     are deducible from it.

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  c) Flattened type-family equalities must be exposed to the nested
     constraints.  Consider
	F b ~ alpha, (forall c.  F b ~ alpha)
     Obviously this is soluble with [alpha := F b].  But the
     unification is only done by solveCTyFunEqs, right at the end of
     solveWanteds, and if we aren't careful we'll end up with an
     unsolved goal inside the implication.  We need to "push" the
     as-yes-unsolved (F b ~ alpha) inwards, as a *given*, so that it
     can be used to solve the inner (F b
     ~ alpha).  See Trac #4935.

  d) There are other cases where interactions between wanteds that can help
     to solve a constraint. For example

  	class C a b | a -> b

  	(C Int alpha), (forall d. C d blah => C Int a)

     If we push the (C Int alpha) inwards, as a given, it can produce
     a fundep (alpha~a) and this can float out again and be used to
     fix alpha.  (In general we can't float class constraints out just
     in case (C d blah) might help to solve (C Int a).)

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The unsolved wanteds are *canonical* but they may not be *inert*,
because when made into a given they might interact with other givens.
Hence the call to solveInteract.  Example:

 Original inert set = (d :_g D a) /\ (co :_w  a ~ [beta]) 

We were not able to solve (a ~w [beta]) but we can't just assume it as
given because the resulting set is not inert. Hence we have to do a