% % (c) The University of Glasgow 2006 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 % Type subsumption and unification \begin{code}

    module TcUnify (
            -- Full-blown subsumption
      tcWrapResult, tcSubType, tcGen,
      checkConstraints, newImplication, sigCtxt,

            -- Various unifications
      unifyType, unifyTypeList, unifyTheta, unifyKind,

      --------------------------------
      -- Holes
      tcInfer,
      matchExpectedListTy, matchExpectedPArrTy,
      matchExpectedTyConApp, matchExpectedAppTy,
      matchExpectedFunTys, matchExpectedFunKind,
      wrapFunResCoercion
      ) where

    #include "HsVersions.h"

    import HsSyn
    import TypeRep
    import CoreUtils( mkPiTypes )
    import TcErrors ( unifyCtxt )
    import TcMType
    import TcIface
    import TcRnMonad
    import TcType
    import Type
    import Coercion
    import Inst
    import TyCon
    import TysWiredIn
    import Var
    import VarSet
    import VarEnv
    import Name
    import ErrUtils
    import BasicTypes

    import Maybes ( allMaybes )
    import Util
    import Outputable
    import FastString

    import Control.Monad

\end{code} %************************************************************************ %* * matchExpected functions %* * %************************************************************************ Note [Herald for matchExpectedFunTys] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The 'herald' always looks like: "The equation(s) for 'f' have" "The abstraction (\x.e) takes" "The section (+ x) expects" "The function 'f' is applied to" This is used to construct a message of form The abstraction `\Just 1 -> ...' takes two arguments but its type `Maybe a -> a' has only one The equation(s) for `f' have two arguments but its type `Maybe a -> a' has only one The section `(f 3)' requires 'f' to take two arguments but its type `Int -> Int' has only one The function 'f' is applied to two arguments but its type `Int -> Int' has only one Note [matchExpectedFunTys] ~~~~~~~~~~~~~~~~~~~~~~~~~~ matchExpectedFunTys checks that an (Expected rho) has the form of an n-ary function. It passes the decomposed type to the thing_inside, and returns a wrapper to coerce between the two types It's used wherever a language construct must have a functional type, namely: A lambda expression A function definition An operator section This is not (currently) where deep skolemisation occurs; matchExpectedFunTys does not skolmise nested foralls in the expected type, becuase it expects that to have been done already \begin{code}

    matchExpectedFunTys :: SDoc 	-- See Note [Herald for matchExpectedFunTys]
                	    -> Arity
                	    -> TcRhoType
                	    -> TcM (CoercionI, [TcSigmaType], TcRhoType)

    -- If    matchExpectFunTys n ty = (co, [t1,..,tn], ty_r)
    -- then  co : ty ~ (t1 -> ... -> tn -> ty_r)
    --
    -- Does not allocate unnecessary meta variables: if the input already is
    -- a function, we just take it apart.  Not only is this efficient,
    -- it's important for higher rank: the argument might be of form
    --		(forall a. ty) -> other
    -- If allocated (fresh-meta-var1 -> fresh-meta-var2) and unified, we'd
    -- hide the forall inside a meta-variable

    matchExpectedFunTys herald arity orig_ty
      = go arity orig_ty
      where
        -- If     go n ty = (co, [t1,..,tn], ty_r)
        -- then   co : ty ~ t1 -> .. -> tn -> ty_r

        go n_req ty
          | n_req == 0 = return (IdCo ty, [], ty)

        go n_req ty
          | Just ty' <- tcView ty = go n_req ty'

        go n_req (FunTy arg_ty res_ty)
          | not (isPredTy arg_ty)
          = do { (coi, tys, ty_r) <- go (n_req-1) res_ty
               ; return (mkFunTyCoI (IdCo arg_ty) coi, arg_ty:tys, ty_r) }

        go _ (TyConApp tc _)	      -- A common case
          | not (isSynFamilyTyCon tc)
          = do { (env,msg) <- mk_ctxt emptyTidyEnv
               ; failWithTcM (env,msg) }

        go n_req ty@(TyVarTy tv)
          | ASSERT( isTcTyVar tv) isMetaTyVar tv
          = do { cts <- readMetaTyVar tv
    	   ; case cts of
    	       Indirect ty' -> go n_req ty'
    	       Flexi        -> defer n_req ty }

           -- In all other cases we bale out into ordinary unification
        go n_req ty = defer n_req ty

        ------------
        defer n_req fun_ty
          = addErrCtxtM mk_ctxt $
            do { arg_tys <- newFlexiTyVarTys n_req argTypeKind
               ; res_ty  <- newFlexiTyVarTy openTypeKind
               ; coi     <- unifyType fun_ty (mkFunTys arg_tys res_ty)
               ; return (coi, arg_tys, res_ty) }

        ------------
        mk_ctxt :: TidyEnv -> TcM (TidyEnv, Message)
        mk_ctxt env = do { orig_ty1 <- zonkTcType orig_ty
                         ; let (env', orig_ty2) = tidyOpenType env orig_ty1
                               (args, _) = tcSplitFunTys orig_ty2
                               n_actual = length args
                         ; return (env', mk_msg orig_ty2 n_actual) }

        mk_msg ty n_args
          = herald <+> speakNOf arity (ptext (sLit "argument")) <> comma $$
    	sep [ptext (sLit "but its type") <+> quotes (pprType ty),
    	     if n_args == 0 then ptext (sLit "has none")
    	     else ptext (sLit "has only") <+> speakN n_args]

\end{code} \begin{code}

    ----------------------
    matchExpectedListTy :: TcRhoType -> TcM (CoercionI, TcRhoType)
    -- Special case for lists
    matchExpectedListTy exp_ty
     = do { (coi, [elt_ty]) <- matchExpectedTyConApp listTyCon exp_ty
          ; return (coi, elt_ty) }

    ----------------------
    matchExpectedPArrTy :: TcRhoType -> TcM (CoercionI, TcRhoType)
    -- Special case for parrs
    matchExpectedPArrTy exp_ty
      = do { (coi, [elt_ty]) <- matchExpectedTyConApp parrTyCon exp_ty
           ; return (coi, elt_ty) }

    ----------------------
    matchExpectedTyConApp :: TyCon                -- T :: k1 -> ... -> kn -> *
                          -> TcRhoType 	      -- orig_ty
                          -> TcM (CoercionI,      -- T a b c ~ orig_ty
                                  [TcSigmaType])  -- Element types, a b c

    -- It's used for wired-in tycons, so we call checkWiredInTyCon
    -- Precondition: never called with FunTyCon
    -- Precondition: input type :: *

    matchExpectedTyConApp tc orig_ty
      = do  { checkWiredInTyCon tc
            ; go (tyConArity tc) orig_ty [] }
      where
        go :: Int -> TcRhoType -> [TcSigmaType] -> TcM (CoercionI, [TcSigmaType])
        -- If     go n ty tys = (co, [t1..tn] ++ tys)
        -- then   co : T t1..tn ~ ty

        go n_req ty tys
          | Just ty' <- tcView ty = go n_req ty' tys

        go n_req ty@(TyVarTy tv) tys
          | ASSERT( isTcTyVar tv) isMetaTyVar tv
          = do { cts <- readMetaTyVar tv
               ; case cts of
                   Indirect ty -> go n_req ty tys
                   Flexi       -> defer n_req ty tys }

        go n_req ty@(TyConApp tycon args) tys
          | tc == tycon
          = ASSERT( n_req == length args)   -- ty::*
            return (IdCo ty, args ++ tys)

        go n_req (AppTy fun arg) tys
          | n_req > 0
          = do { (coi, args) <- go (n_req - 1) fun (arg : tys)
               ; return (mkAppTyCoI coi (IdCo arg), args) }

        go n_req ty tys = defer n_req ty tys

        ----------
        defer n_req ty tys
          = do { tau_tys <- mapM newFlexiTyVarTy arg_kinds
               ; coi <- unifyType (mkTyConApp tc tau_tys) ty
               ; return (coi, tau_tys ++ tys) }
          where
            (arg_kinds, _) = splitKindFunTysN n_req (tyConKind tc)

    ----------------------
    matchExpectedAppTy :: TcRhoType                         -- orig_ty
                       -> TcM (CoercionI,                   -- m a ~ orig_ty
                               (TcSigmaType, TcSigmaType))  -- Returns m, a
    -- If the incoming type is a mutable type variable of kind k, then
    -- matchExpectedAppTy returns a new type variable (m: * -> k); note the *.

    matchExpectedAppTy orig_ty
      = go orig_ty
      where
        go ty
          | Just ty' <- tcView ty = go ty'

          | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
          = return (IdCo orig_ty, (fun_ty, arg_ty))

        go (TyVarTy tv)
          | ASSERT( isTcTyVar tv) isMetaTyVar tv
          = do { cts <- readMetaTyVar tv
               ; case cts of
                   Indirect ty -> go ty
                   Flexi       -> defer }

        go _ = defer

        -- Defer splitting by generating an equality constraint
        defer = do { ty1 <- newFlexiTyVarTy kind1
                   ; ty2 <- newFlexiTyVarTy kind2
                   ; coi <- unifyType (mkAppTy ty1 ty2) orig_ty
                   ; return (coi, (ty1, ty2)) }

        orig_kind = typeKind orig_ty
        kind1 = mkArrowKind liftedTypeKind (defaultKind orig_kind)
        kind2 = liftedTypeKind    -- m :: * -> k
                                  -- arg type :: *
            -- The defaultKind is a bit smelly.  If you remove it,
            -- try compiling        f x = do { x }
            -- and you'll get a kind mis-match.  It smells, but
            -- not enough to lose sleep over.

\end{code} %************************************************************************ %* * Subsumption checking %* * %************************************************************************ All the tcSub calls have the form tcSub actual_ty expected_ty which checks actual_ty <= expected_ty That is, that a value of type actual_ty is acceptable in a place expecting a value of type expected_ty. It returns a coercion function co_fn :: actual_ty ~ expected_ty which takes an HsExpr of type actual_ty into one of type expected_ty. \begin{code}

    tcSubType :: CtOrigin -> UserTypeCtxt -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
    -- Check that ty_actual is more polymorphic than ty_expected
    -- Both arguments might be polytypes, so we must instantiate and skolemise
    -- Returns a wrapper of shape   ty_actual ~ ty_expected
    tcSubType origin ctxt ty_actual ty_expected
      | isSigmaTy ty_actual
      = do { (sk_wrap, inst_wrap)
                <- tcGen ctxt ty_expected $ \ _ sk_rho -> do
                { (in_wrap, in_rho) <- deeplyInstantiate origin ty_actual
                ; coi <- unifyType in_rho sk_rho
                ; return (coiToHsWrapper coi <.> in_wrap) }
           ; return (sk_wrap <.> inst_wrap) }

      | otherwise	-- Urgh!  It seems deeply weird to have equality
        		-- when actual is not a polytype, and it makes a big
    		-- difference e.g. tcfail104
      = do { coi <- unifyType ty_actual ty_expected
           ; return (coiToHsWrapper coi) }

    tcInfer :: (TcType -> TcM a) -> TcM (a, TcType)
    tcInfer tc_infer = do { ty  <- newFlexiTyVarTy openTypeKind
                          ; res <- tc_infer ty
    		      ; return (res, ty) }

    -----------------
    tcWrapResult :: HsExpr TcId -> TcRhoType -> TcRhoType -> TcM (HsExpr TcId)
    tcWrapResult expr actual_ty res_ty
      = do { coi <- unifyType actual_ty res_ty
           	        -- Both types are deeply skolemised
           ; return (mkHsWrapCoI coi expr) }

    -----------------------------------
    wrapFunResCoercion
            :: [TcType]     -- Type of args
            -> HsWrapper    -- HsExpr a -> HsExpr b
            -> TcM HsWrapper        -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
    wrapFunResCoercion arg_tys co_fn_res
      | isIdHsWrapper co_fn_res
      = return idHsWrapper
      | null arg_tys
      = return co_fn_res
      | otherwise
      = do  { arg_ids <- newSysLocalIds (fsLit "sub") arg_tys
            ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpEvVarApps arg_ids) }

\end{code} %************************************************************************ %* * \subsection{Generalisation} %* * %************************************************************************ \begin{code}

    tcGen :: UserTypeCtxt -> TcType
          -> ([TcTyVar] -> TcRhoType -> TcM result)
          -> TcM (HsWrapper, result)
            -- The expression has type: spec_ty -> expected_ty

    tcGen ctxt expected_ty thing_inside
       -- We expect expected_ty to be a forall-type
       -- If not, the call is a no-op
      = do  { traceTc "tcGen" empty
            ; (wrap, tvs', given, rho') <- deeplySkolemise expected_ty

            ; when debugIsOn $
                  traceTc "tcGen" $ vcat [
                               text "expected_ty" <+> ppr expected_ty,
                               text "inst ty" <+> ppr tvs' <+> ppr rho' ]

    	-- Generally we must check that the "forall_tvs" havn't been constrained
            -- The interesting bit here is that we must include the free variables
            -- of the expected_ty.  Here's an example:
            --       runST (newVar True)
            -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
            -- for (newVar True), with s fresh.  Then we unify with the runST's arg type
            -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
            -- So now s' isn't unconstrained because it's linked to a.
            --
    	-- However [Oct 10] now that the untouchables are a range of
            -- TcTyVars, all this is handled automatically with no need for
    	-- extra faffing around

            -- Use the *instantiated* type in the SkolemInfo
            -- so that the names of displayed type variables line up
            ; let skol_info = SigSkol ctxt (mkPiTypes given rho')

            ; (ev_binds, result) <- checkConstraints skol_info tvs' given $
                                    thing_inside tvs' rho'

            ; return (wrap <.> mkWpLet ev_binds, result) }
    	  -- The ev_binds returned by checkConstraints is very
    	  -- often empty, in which case mkWpLet is a no-op

    checkConstraints :: SkolemInfo
    		 -> [TcTyVar]		-- Skolems
    		 -> [EvVar]             -- Given
    		 -> TcM result
    		 -> TcM (TcEvBinds, result)

    checkConstraints skol_info skol_tvs given thing_inside
      | null skol_tvs && null given
      = do { res <- thing_inside; return (emptyTcEvBinds, res) }
          -- Just for efficiency.  We check every function argument with
          -- tcPolyExpr, which uses tcGen and hence checkConstraints.

      | otherwise
      = newImplication skol_info skol_tvs given thing_inside

    newImplication :: SkolemInfo -> [TcTyVar]
    	       -> [EvVar] -> TcM result
                   -> TcM (TcEvBinds, result)
    newImplication skol_info skol_tvs given thing_inside
      = ASSERT2( all isTcTyVar skol_tvs, ppr skol_tvs )
        ASSERT2( all isSkolemTyVar skol_tvs, ppr skol_tvs )
        do { ((result, untch), wanted) <- captureConstraints  $
                                          captureUntouchables $
                                          thing_inside

           ; if isEmptyWC wanted && not (hasEqualities given)
           	    -- Optimisation : if there are no wanteds, and the givens
           	    -- are sufficiently simple, don't generate an implication
           	    -- at all.  Reason for the hasEqualities test:
    	    -- we don't want to lose the "inaccessible alternative"
    	    -- error check
             then
                return (emptyTcEvBinds, result)
             else do
           { ev_binds_var <- newTcEvBinds
           ; lcl_env <- getLclTypeEnv
           ; loc <- getCtLoc skol_info
           ; emitImplication $ Implic { ic_untch = untch
                 		     	  , ic_env = lcl_env
                 		     	  , ic_skols = mkVarSet skol_tvs
                                 	  , ic_given = given
                                      , ic_wanted = wanted
                                      , ic_insol  = insolubleWC wanted
                                      , ic_binds = ev_binds_var
                 		     	  , ic_loc = loc }

           ; return (TcEvBinds ev_binds_var, result) } }

\end{code} %************************************************************************ %* * Boxy unification %* * %************************************************************************ The exported functions are all defined as versions of some non-exported generic functions. \begin{code}

    ---------------
    unifyType :: TcTauType -> TcTauType -> TcM CoercionI
    -- Actual and expected types
    -- Returns a coercion : ty1 ~ ty2
    unifyType ty1 ty2 = uType [] ty1 ty2

    ---------------
    unifyPred :: PredType -> PredType -> TcM CoercionI
    -- Actual and expected types
    unifyPred p1 p2 = uPred [UnifyOrigin (mkPredTy p1) (mkPredTy p2)] p1 p2

    ---------------
    unifyTheta :: TcThetaType -> TcThetaType -> TcM [CoercionI]
    -- Actual and expected types
    unifyTheta theta1 theta2
      = do  { checkTc (equalLength theta1 theta2)
                      (vcat [ptext (sLit "Contexts differ in length"),
                             nest 2 $ parens $ ptext (sLit "Use -XRelaxedPolyRec to allow this")])
            ; zipWithM unifyPred theta1 theta2 }

\end{code} @unifyTypeList@ takes a single list of @TauType@s and unifies them all together. It is used, for example, when typechecking explicit lists, when all the elts should be of the same type. \begin{code}

    unifyTypeList :: [TcTauType] -> TcM ()
    unifyTypeList []                 = return ()
    unifyTypeList [_]                = return ()
    unifyTypeList (ty1:tys@(ty2:_)) = do { _ <- unifyType ty1 ty2
                                         ; unifyTypeList tys }

\end{code} %************************************************************************ %* * uType and friends %* * %************************************************************************ uType is the heart of the unifier. Each arg occurs twice, because we want to report errors in terms of synomyms if possible. The first of the pair is used in error messages only; it is always the same as the second, except that if the first is a synonym then the second may be a de-synonym'd version. This way we get better error messages. \begin{code}

    data SwapFlag
      = NotSwapped	-- Args are: actual,   expected
      | IsSwapped   -- Args are: expected, actual

    instance Outputable SwapFlag where
      ppr IsSwapped  = ptext (sLit "Is-swapped")
      ppr NotSwapped = ptext (sLit "Not-swapped")

    unSwap :: SwapFlag -> (a->a->b) -> a -> a -> b
    unSwap NotSwapped f a b = f a b
    unSwap IsSwapped  f a b = f b a

    ------------
    uType, uType_np, uType_defer
      :: [EqOrigin]
      -> TcType    -- ty1 is the *actual* type
      -> TcType    -- ty2 is the *expected* type
      -> TcM CoercionI

    --------------
    -- It is always safe to defer unification to the main constraint solver
    -- See Note [Deferred unification]
    uType_defer (item : origin) ty1 ty2
      = wrapEqCtxt origin $
        do { co_var <- newCoVar ty1 ty2
           ; loc <- getCtLoc (TypeEqOrigin item)
           ; emitFlat (mkEvVarX co_var loc)

           -- Error trace only
           ; ctxt <- getErrCtxt
           ; doc <- mkErrInfo emptyTidyEnv ctxt
           ; traceTc "utype_defer" (vcat [ppr co_var, ppr ty1, ppr ty2, ppr origin, doc])

           ; return $ ACo $ mkTyVarTy co_var }
    uType_defer [] _ _
      = panic "uType_defer"

    --------------
    -- Push a new item on the origin stack (the most common case)
    uType origin ty1 ty2  -- Push a new item on the origin stack
      = uType_np (pushOrigin ty1 ty2 origin) ty1 ty2

    --------------
    -- unify_np (short for "no push" on the origin stack) does the work
    uType_np origin orig_ty1 orig_ty2
      = do { traceTc "u_tys " $ vcat
                  [ sep [ ppr orig_ty1, text "~", ppr orig_ty2]
                  , ppr origin]
           ; coi <- go origin orig_ty1 orig_ty2
           ; case coi of
                ACo co -> traceTc "u_tys yields coercion:" (ppr co)
                IdCo _ -> traceTc "u_tys yields no coercion" empty
           ; return coi }
      where
        bale_out :: [EqOrigin] -> TcM a
        bale_out origin = failWithMisMatch origin

        go :: [EqOrigin] -> TcType -> TcType -> TcM CoercionI
    	-- The arguments to 'go' are always semantically identical
    	-- to orig_ty{1,2} except for looking through type synonyms

            -- Variables; go for uVar
    	-- Note that we pass in *original* (before synonym expansion),
            -- so that type variables tend to get filled in with
            -- the most informative version of the type
        go origin (TyVarTy tyvar1) ty2 = uVar origin NotSwapped tyvar1 ty2
        go origin ty1 (TyVarTy tyvar2) = uVar origin IsSwapped  tyvar2 ty1

            -- Expand synonyms:
    	--      see Note [Unification and synonyms]
    	-- Do this after the variable case so that we tend to unify
    	-- variables with un-expended type synonym
        go origin ty1 ty2
          | Just ty1' <- tcView ty1 = uType origin ty1' ty2
          | Just ty2' <- tcView ty2 = uType origin ty1  ty2'

            -- Predicates
        go origin (PredTy p1) (PredTy p2) = uPred origin p1 p2

            -- Coercion functions: (t1a ~ t1b) => t1c  ~  (t2a ~ t2b) => t2c
        go origin ty1 ty2
          | Just (t1a,t1b,t1c) <- splitCoPredTy_maybe ty1,
            Just (t2a,t2b,t2c) <- splitCoPredTy_maybe ty2
          = do { co1 <- uType origin t1a t2a
               ; co2 <- uType origin t1b t2b
               ; co3 <- uType origin t1c t2c
               ; return $ mkCoPredCoI co1 co2 co3 }

            -- Functions (or predicate functions) just check the two parts
        go origin (FunTy fun1 arg1) (FunTy fun2 arg2)
          = do { coi_l <- uType origin fun1 fun2
               ; coi_r <- uType origin arg1 arg2
               ; return $ mkFunTyCoI coi_l coi_r }

            -- Always defer if a type synonym family (type function)
          	-- is involved.  (Data families behave rigidly.)
        go origin ty1@(TyConApp tc1 _) ty2
          | isSynFamilyTyCon tc1 = uType_defer origin ty1 ty2
        go origin ty1 ty2@(TyConApp tc2 _)
          | isSynFamilyTyCon tc2 = uType_defer origin ty1 ty2

        go origin (TyConApp tc1 tys1) (TyConApp tc2 tys2)
          | tc1 == tc2	   -- See Note [TyCon app]
          = do { cois <- uList origin uType tys1 tys2
               ; return $ mkTyConAppCoI tc1 cois }

    	-- See Note [Care with type applications]
        go origin (AppTy s1 t1) ty2
          | Just (s2,t2) <- tcSplitAppTy_maybe ty2
          = do { coi_s <- uType_np origin s1 s2  -- See Note [Unifying AppTy]
               ; coi_t <- uType origin t1 t2
               ; return $ mkAppTyCoI coi_s coi_t }

        go origin ty1 (AppTy s2 t2)
          | Just (s1,t1) <- tcSplitAppTy_maybe ty1
          = do { coi_s <- uType_np origin s1 s2
               ; coi_t <- uType origin t1 t2
               ; return $ mkAppTyCoI coi_s coi_t }

        go _ ty1 ty2
          | tcIsForAllTy ty1 || tcIsForAllTy ty2
          = unifySigmaTy origin ty1 ty2

            -- Anything else fails
        go origin _ _ = bale_out origin

    unifySigmaTy :: [EqOrigin] -> TcType -> TcType -> TcM CoercionI
    unifySigmaTy origin ty1 ty2
      = do { let (tvs1, body1) = tcSplitForAllTys ty1
                 (tvs2, body2) = tcSplitForAllTys ty2
           ; unless (equalLength tvs1 tvs2) (failWithMisMatch origin)
           ; skol_tvs <- tcInstSkolTyVars tvs1
                      -- Get location from monad, not from tvs1
           ; let tys      = mkTyVarTys skol_tvs
                 in_scope = mkInScopeSet (mkVarSet skol_tvs)
                 phi1     = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
                 phi2     = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
    --             untch = tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2

           ; ((coi, _untch), lie) <- captureConstraints $
                                     captureUntouchables $
                           		 uType origin phi1 phi2
              -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
              -- VERY UNSATISFACTORY; the constraint might be fine, but
    	  -- we fail eagerly because we don't have any place to put
    	  -- the bindings from an implication constraint
    	  -- This only works because most constraints get solved on the fly
    	  -- See Note [Avoid deferring]
             ; when (any (`elemVarSet` tyVarsOfWC lie) skol_tvs)
                  (failWithMisMatch origin)	-- ToDo: give details from bad_lie

           ; emitConstraints lie
           ; return (foldr mkForAllTyCoI coi skol_tvs) }

    ----------
    uPred :: [EqOrigin] -> PredType -> PredType -> TcM CoercionI
    uPred origin (IParam n1 t1) (IParam n2 t2)
      | n1 == n2
      = do { coi <- uType origin t1 t2
           ; return $ mkIParamPredCoI n1 coi }
    uPred origin (ClassP c1 tys1) (ClassP c2 tys2)
      | c1 == c2
      = do { cois <- uList origin uType tys1 tys2
              -- Guaranteed equal lengths because the kinds check
           ; return $ mkClassPPredCoI c1 cois }
    uPred origin (EqPred ty1a ty1b) (EqPred ty2a ty2b)
      = do { coia <- uType origin ty1a ty2a
           ; coib <- uType origin ty1b ty2b
           ; return $ mkEqPredCoI coia coib }

    uPred origin _ _ = failWithMisMatch origin

    ---------------
    uList :: [EqOrigin]
          -> ([EqOrigin] -> a -> a -> TcM b)
          -> [a] -> [a] -> TcM [b]
    -- Unify corresponding elements of two lists of types, which
    -- should be of equal length.  We charge down the list explicitly so that
    -- we can complain if their lengths differ.
    uList _       _     []         []        = return []
    uList origin unify (ty1:tys1) (ty2:tys2) = do { x  <- unify origin ty1 ty2;
                                                  ; xs <- uList origin unify tys1 tys2
                                                  ; return (x:xs) }
    uList origin _ _ _ = failWithMisMatch origin
           -- See Note [Mismatched type lists and application decomposition]

\end{code} Note [Care with type applications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Note: type applications need a bit of care! They can match FunTy and TyConApp, so use splitAppTy_maybe NB: we've already dealt with type variables and Notes, so if one type is an App the other one jolly well better be too Note [Unifying AppTy] ~~~~~~~~~~~~~~~~~~~~~ Considerm unifying (m Int) ~ (IO Int) where m is a unification variable that is now bound to (say) (Bool ->). Then we want to report "Can't unify (Bool -> Int) with (IO Int) and not "Can't unify ((->) Bool) with IO" That is why we use the "_np" variant of uType, which does not alter the error message. Note [TyCon app] ~~~~~~~~~~~~~~~~ When we find two TyConApps, the argument lists are guaranteed equal length. Reason: intially the kinds of the two types to be unified is the same. The only way it can become not the same is when unifying two AppTys (f1 a1)~(f2 a2). In that case there can't be a TyConApp in the f1,f2 (because it'd absorb the app). If we unify f1~f2 first, which we do, that ensures that f1,f2 have the same kind; and that means a1,a2 have the same kind. And now the argument repeats. Note [Mismatched type lists and application decomposition] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we find two TyConApps, you might think that the argument lists are guaranteed equal length. But they aren't. Consider matching w (T x) ~ Foo (T x y) We do match (w ~ Foo) first, but in some circumstances we simply create a deferred constraint; and then go ahead and match (T x ~ T x y). This came up in Trac #3950. So either (a) either we must check for identical argument kinds when decomposing applications, (b) or we must be prepared for ill-kinded unification sub-problems Currently we adopt (b) since it seems more robust -- no need to maintain a global invariant. Note [Unification and synonyms] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If you are tempted to make a short cut on synonyms, as in this pseudocode... uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2) = if (con1 == con2) then -- Good news! Same synonym constructors, so we can shortcut -- by unifying their arguments and ignoring their expansions. unifyTypepeLists args1 args2 else -- Never mind. Just expand them and try again uTys ty1 ty2 then THINK AGAIN. Here is the whole story, as detected and reported by Chris Okasaki: Here's a test program that should detect the problem: type Bogus a = Int x = (1 :: Bogus Char) :: Bogus Bool The problem with [the attempted shortcut code] is that con1 == con2 is not a sufficient condition to be able to use the shortcut! You also need to know that the type synonym actually USES all its arguments. For example, consider the following type synonym which does not use all its arguments. type Bogus a = Int If you ever tried unifying, say, (Bogus Char) with )Bogus Bool), the unifier would blithely try to unify Char with Bool and would fail, even though the expanded forms (both Int) should match. Similarly, unifying (Bogus Char) with (Bogus t) would unnecessarily bind t to Char. ... You could explicitly test for the problem synonyms and mark them somehow as needing expansion, perhaps also issuing a warning to the user. Note [Deferred Unification] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ We may encounter a unification ty1 ~ ty2 that cannot be performed syntactically, and yet its consistency is undetermined. Previously, there was no way to still make it consistent. So a mismatch error was issued. Now these unfications are deferred until constraint simplification, where type family instances and given equations may (or may not) establish the consistency. Deferred unifications are of the form F ... ~ ... or x ~ ... where F is a type function and x is a type variable. E.g. id :: x ~ y => x -> y id e = e involves the unfication x = y. It is deferred until we bring into account the context x ~ y to establish that it holds. If available, we defer original types (rather than those where closed type synonyms have already been expanded via tcCoreView). This is, as usual, to improve error messages. %************************************************************************ %* * uVar and friends %* * %************************************************************************ @uVar@ is called when at least one of the types being unified is a variable. It does {\em not} assume that the variable is a fixed point of the substitution; rather, notice that @uVar@ (defined below) nips back into @uTys@ if it turns out that the variable is already bound. \begin{code}

    uVar :: [EqOrigin] -> SwapFlag -> TcTyVar -> TcTauType -> TcM CoercionI
    uVar origin swapped tv1 ty2
      = do  { traceTc "uVar" (vcat [ ppr origin
                                    , ppr swapped
                                    , ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1)
                           		, nest 2 (ptext (sLit " ~ "))
                           		, ppr ty2 <+> dcolon <+> ppr (typeKind ty2)])
            ; details <- lookupTcTyVar tv1
            ; case details of
                Filled ty1  -> unSwap swapped (uType_np origin) ty1 ty2
                Unfilled details1 -> uUnfilledVar origin swapped tv1 details1 ty2
            }

    ----------------
    uUnfilledVar :: [EqOrigin]
                 -> SwapFlag
                 -> TcTyVar -> TcTyVarDetails       -- Tyvar 1
                 -> TcTauType  			-- Type 2
                 -> TcM CoercionI
    -- "Unfilled" means that the variable is definitely not a filled-in meta tyvar
    --            It might be a skolem, or untouchable, or meta

    uUnfilledVar origin swapped tv1 details1 (TyVarTy tv2)
      | tv1 == tv2  -- Same type variable => no-op
      = return (IdCo (mkTyVarTy tv1))

      | otherwise  -- Distinct type variables
      = do  { lookup2 <- lookupTcTyVar tv2
            ; case lookup2 of
                Filled ty2'       -> uUnfilledVar origin swapped tv1 details1 ty2'
                Unfilled details2 -> uUnfilledVars origin swapped tv1 details1 tv2 details2
            }

    uUnfilledVar origin swapped tv1 details1 non_var_ty2  -- ty2 is not a type variable
      = case details1 of
          MetaTv TauTv ref1
            -> do { mb_ty2' <- checkTauTvUpdate tv1 non_var_ty2
                  ; case mb_ty2' of
                      Nothing   -> do { traceTc "Occ/kind defer" (ppr tv1); defer }
                      Just ty2' -> updateMeta tv1 ref1 ty2'
                  }

          _other -> do { traceTc "Skolem defer" (ppr tv1); defer }	-- Skolems of all sorts
      where
        defer | Just ty2' <- tcView non_var_ty2	-- Note [Avoid deferring]
        	    	         	   		-- non_var_ty2 isn't expanded yet
              = uUnfilledVar origin swapped tv1 details1 ty2'
              | otherwise
              = unSwap swapped (uType_defer origin) (mkTyVarTy tv1) non_var_ty2
              -- Occurs check or an untouchable: just defer
    	  -- NB: occurs check isn't necessarily fatal:
    	  --     eg tv1 occured in type family parameter

    ----------------
    uUnfilledVars :: [EqOrigin]
                  -> SwapFlag
                  -> TcTyVar -> TcTyVarDetails      -- Tyvar 1
                  -> TcTyVar -> TcTyVarDetails      -- Tyvar 2
                  -> TcM CoercionI
    -- Invarant: The type variables are distinct,
    --           Neither is filled in yet

    uUnfilledVars origin swapped tv1 details1 tv2 details2
      = case (details1, details2) of
          (MetaTv i1 ref1, MetaTv i2 ref2)
              | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1 i1 i2
                             then updateMeta tv1 ref1 ty2
                             else updateMeta tv2 ref2 ty1
              | k2_sub_k1 -> updateMeta tv1 ref1 ty2

          (_, MetaTv _ ref2) | k1_sub_k2 -> updateMeta tv2 ref2 ty1
          (MetaTv _ ref1, _) | k2_sub_k1 -> updateMeta tv1 ref1 ty2

          (_, _) -> unSwap swapped (uType_defer origin) ty1 ty2
          	        -- Defer for skolems of all sorts
      where
        k1 	      = tyVarKind tv1
        k2 	      = tyVarKind tv2
        k1_sub_k2 = k1 `isSubKind` k2
        k2_sub_k1 = k2 `isSubKind` k1
        ty1       = mkTyVarTy tv1
        ty2       = mkTyVarTy tv2

        nicer_to_update_tv1 _         (SigTv _) = True
        nicer_to_update_tv1 (SigTv _) _         = False
        nicer_to_update_tv1 _         _         = isSystemName (Var.varName tv1)
            -- Try not to update SigTvs; and try to update sys-y type
            -- variables in preference to ones gotten (say) by
            -- instantiating a polymorphic function with a user-written
            -- type sig

    ----------------
    checkTauTvUpdate :: TcTyVar -> TcType -> TcM (Maybe TcType)
    --    (checkTauTvUpdate tv ty)
    -- We are about to update the TauTv tv with ty.
    -- Check (a) that tv doesn't occur in ty (occurs check)
    --	 (b) that kind(ty) is a sub-kind of kind(tv)
    --       (c) that ty does not contain any type families, see Note [Type family sharing]
    --
    -- We have two possible outcomes:
    -- (1) Return the type to update the type variable with,
    --        [we know the update is ok]
    -- (2) Return Nothing,
    --        [the update might be dodgy]
    --
    -- Note that "Nothing" does not mean "definite error".  For example
    --   type family F a
    --   type instance F Int = Int
    -- consider
    --   a ~ F a
    -- This is perfectly reasonable, if we later get a ~ Int.  For now, though,
    -- we return Nothing, leaving it to the later constraint simplifier to
    -- sort matters out.

    checkTauTvUpdate tv ty
      = do { ty' <- zonkTcType ty
           ; if typeKind ty' `isSubKind` tyVarKind tv then
               case ok ty' of
                 Nothing -> return Nothing
                 Just ty'' -> return (Just ty'')
             else return Nothing }

      where ok :: TcType -> Maybe TcType
            ok (TyVarTy tv') | not (tv == tv') = Just (TyVarTy tv')
            ok this_ty@(TyConApp tc tys)
              | not (isSynFamilyTyCon tc), Just tys' <- allMaybes (map ok tys)
              = Just (TyConApp tc tys')
              | isSynTyCon tc, Just ty_expanded <- tcView this_ty
              = ok ty_expanded -- See Note [Type synonyms and the occur check]
            ok (PredTy sty) | Just sty' <- ok_pred sty = Just (PredTy sty')
            ok (FunTy arg res) | Just arg' <- ok arg, Just res' <- ok res
                               = Just (FunTy arg' res')
            ok (AppTy fun arg) | Just fun' <- ok fun, Just arg' <- ok arg
                               = Just (AppTy fun' arg')
            ok (ForAllTy tv1 ty1) | Just ty1' <- ok ty1 = Just (ForAllTy tv1 ty1')
            -- Fall-through
            ok _ty = Nothing

            ok_pred (IParam nm ty) | Just ty' <- ok ty = Just (IParam nm ty')
            ok_pred (ClassP cl tys)
              | Just tys' <- allMaybes (map ok tys)
              = Just (ClassP cl tys')
            ok_pred (EqPred ty1 ty2)
              | Just ty1' <- ok ty1, Just ty2' <- ok ty2
              = Just (EqPred ty1' ty2')
            -- Fall-through
            ok_pred _pty = Nothing

\end{code} Note [Avoid deferring] ~~~~~~~~~~~~~~~~~~~~~~ We try to avoid creating deferred constraints for two reasons. * First, efficiency. * Second, currently we can only defer some constraints under a forall. See unifySigmaTy. So expanding synonyms here is a good thing to do. Example (Trac #4917) a ~ Const a b where type Const a b = a. We can solve this immediately, even when 'a' is a skolem, just by expanding the synonym; and we should do so in case this unification happens inside unifySigmaTy (sigh). Note [Type synonyms and the occur check] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Generally speaking we try to update a variable with type synonyms not expanded, which improves later error messages, unless looking inside a type synonym may help resolve a spurious occurs check error. Consider: type A a = () f :: (A a -> a -> ()) -> () f = \ _ -> () x :: () x = f (\ x p -> p x) We will eventually get a constraint of the form t ~ A t. The ok function above will properly expand the type (A t) to just (), which is ok to be unified with t. If we had unified with the original type A t, we would lead the type checker into an infinite loop. Hence, if the occurs check fails for a type synonym application, then (and *only* then), the ok function expands the synonym to detect opportunities for occurs check success using the underlying definition of the type synonym. The same applies later on in the constraint interaction code; see TcInteract, function @occ_check_ok@. Note [Type family sharing] ~~~~~~~~~~~~~~ We must avoid eagerly unifying type variables to types that contain function symbols, because this may lead to loss of sharing, and in turn, in very poor performance of the constraint simplifier. Assume that we have a wanted constraint: { m1 ~ [F m2], m2 ~ [F m3], m3 ~ [F m4], D m1, D m2, D m3 } where D is some type class. If we eagerly unify m1 := [F m2], m2 := [F m3], m3 := [F m2], then, after zonking, our constraint simplifier will be faced with the following wanted constraint: { D [F [F [F m4]]], D [F [F m4]], D [F m4] } which has to be flattened by the constraint solver. However, because the sharing is lost, an polynomially larger number of flatten skolems will be created and the constraint sets we are working with will be polynomially larger. Instead, if we defer the unifications m1 := [F m2], etc. we will only be generating three flatten skolems, which is the maximum possible sharing arising from the original constraint. \begin{code}

    data LookupTyVarResult	-- The result of a lookupTcTyVar call
      = Unfilled TcTyVarDetails	-- SkolemTv or virgin MetaTv
      | Filled   TcType

    lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
    lookupTcTyVar tyvar
      | MetaTv _ ref <- details
      = do { meta_details <- readMutVar ref
           ; case meta_details of
               Indirect ty -> return (Filled ty)
               Flexi -> do { is_untch <- isUntouchable tyvar
                           ; let    -- Note [Unifying untouchables]
                                 ret_details | is_untch  = vanillaSkolemTv
                                             | otherwise = details
           	               ; return (Unfilled ret_details) } }
      | otherwise
      = return (Unfilled details)
      where
        details = ASSERT2( isTcTyVar tyvar, ppr tyvar )
                  tcTyVarDetails tyvar

    updateMeta :: TcTyVar -> TcRef MetaDetails -> TcType -> TcM CoercionI
    updateMeta tv1 ref1 ty2
      = do { writeMetaTyVarRef tv1 ref1 ty2
           ; return (IdCo ty2) }

\end{code} Note [Unifying untouchables] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We treat an untouchable type variable as if it was a skolem. That ensures it won't unify with anything. It's a slight had, because we return a made-up TcTyVarDetails, but I think it works smoothly. %************************************************************************ %* * Errors and contexts %* * %************************************************************************ \begin{code}

    pushOrigin :: TcType -> TcType -> [EqOrigin] -> [EqOrigin]
    pushOrigin ty_act ty_exp origin
      = UnifyOrigin { uo_actual = ty_act, uo_expected = ty_exp } : origin

    ---------------
    wrapEqCtxt :: [EqOrigin] -> TcM a -> TcM a
    -- Build a suitable error context from the origin and do the thing inside
    -- The "couldn't match" error comes from the innermost item on the stack,
    -- and, if there is more than one item, the "Expected/inferred" part
    -- comes from the outermost item
    wrapEqCtxt []    thing_inside = thing_inside
    wrapEqCtxt items thing_inside = addErrCtxtM (unifyCtxt (last items)) thing_inside

    ---------------
    failWithMisMatch :: [EqOrigin] -> TcM a
    -- Generate the message when two types fail to match,
    -- going to some trouble to make it helpful.
    -- We take the failing types from the top of the origin stack
    -- rather than reporting the particular ones we are looking
    -- at right now
    failWithMisMatch (item:origin)
      = wrapEqCtxt origin $
        do	{ ty_act <- zonkTcType (uo_actual item)
            ; ty_exp <- zonkTcType (uo_expected item)
            ; env0 <- tcInitTidyEnv
            ; let (env1, pp_exp) = tidyOpenType env0 ty_exp
                  (env2, pp_act) = tidyOpenType env1 ty_act
            ; failWithTcM (env2, misMatchMsg pp_act pp_exp) }
    failWithMisMatch []
      = panic "failWithMisMatch"

    misMatchMsg :: TcType -> TcType -> SDoc
    misMatchMsg ty_act ty_exp
      = sep [ ptext (sLit "Couldn't match expected type") <+> quotes (ppr ty_exp)
            , nest 12 $   ptext (sLit "with actual type") <+> quotes (ppr ty_act)]

\end{code} ----------------------------------------- UNUSED FOR NOW ----------------------------------------- ---------------- ---------------- -- If an error happens we try to figure out whether the function -- function has been given too many or too few arguments, and say so. addSubCtxt :: InstOrigin -> TcType -> TcType -> TcM a -> TcM a addSubCtxt orig actual_res_ty expected_res_ty thing_inside = addErrCtxtM mk_err thing_inside where mk_err tidy_env = do { exp_ty' <- zonkTcType expected_res_ty ; act_ty' <- zonkTcType actual_res_ty ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty' (env2, act_ty'') = tidyOpenType env1 act_ty' (exp_args, _) = tcSplitFunTys exp_ty'' (act_args, _) = tcSplitFunTys act_ty'' len_act_args = length act_args len_exp_args = length exp_args message = case orig of OccurrenceOf fun | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun _ -> mkExpectedActualMsg act_ty'' exp_ty'' ; return (env2, message) } %************************************************************************ %* * Kind unification %* * %************************************************************************ Unifying kinds is much, much simpler than unifying types. \begin{code}

    matchExpectedFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
    -- Like unifyFunTy, but does not fail; instead just returns Nothing

    matchExpectedFunKind (TyVarTy kvar) = do
        maybe_kind <- readKindVar kvar
        case maybe_kind of
          Indirect fun_kind -> matchExpectedFunKind fun_kind
          Flexi ->
              do { arg_kind <- newKindVar
                 ; res_kind <- newKindVar
                 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
                 ; return (Just (arg_kind,res_kind)) }

    matchExpectedFunKind (FunTy arg_kind res_kind) = return (Just (arg_kind,res_kind))
    matchExpectedFunKind _                         = return Nothing

    -----------------
    unifyKind :: TcKind                 -- Expected
              -> TcKind                 -- Actual
              -> TcM ()

    unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
      | isSubKindCon kc2 kc1 = return ()

    unifyKind (FunTy a1 r1) (FunTy a2 r2)
      = do { unifyKind a2 a1; unifyKind r1 r2 }
                    -- Notice the flip in the argument,
                    -- so that the sub-kinding works right
    unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
    unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
    unifyKind k1 k2 = unifyKindMisMatch k1 k2

    ----------------
    uKVar :: Bool -> KindVar -> TcKind -> TcM ()
    uKVar swapped kv1 k2
      = do  { mb_k1 <- readKindVar kv1
            ; case mb_k1 of
                Flexi -> uUnboundKVar swapped kv1 k2
                Indirect k1 | swapped   -> unifyKind k2 k1
                            | otherwise -> unifyKind k1 k2 }

    ----------------
    uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
    uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
      | kv1 == kv2 = return ()
      | otherwise   -- Distinct kind variables
      = do  { mb_k2 <- readKindVar kv2
            ; case mb_k2 of
                Indirect k2 -> uUnboundKVar swapped kv1 k2
                Flexi -> writeKindVar kv1 k2 }

    uUnboundKVar swapped kv1 non_var_k2
      = do  { k2' <- zonkTcKind non_var_k2
            ; kindOccurCheck kv1 k2'
            ; k2'' <- kindSimpleKind swapped k2'
                    -- KindVars must be bound only to simple kinds
                    -- Polarities: (kindSimpleKind True ?) succeeds
                    -- returning *, corresponding to unifying
                    --      expected: ?
                    --      actual:   kind-ver
            ; writeKindVar kv1 k2'' }

    ----------------
    kindOccurCheck :: TyVar -> Type -> TcM ()
    kindOccurCheck kv1 k2   -- k2 is zonked
      = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
      where
        not_in (TyVarTy kv2) = kv1 /= kv2
        not_in (FunTy a2 r2) = not_in a2 && not_in r2
        not_in _             = True

    kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
    -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
    -- If the flag is False, it requires k <: sk
    -- E.g.         kindSimpleKind False ?? = *
    -- What about (kv -> *) ~ ?? -> *
    kindSimpleKind orig_swapped orig_kind
      = go orig_swapped orig_kind
      where
        go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
                                 ; k2' <- go sw k2
                                 ; return (mkArrowKind k1' k2') }
        go True k
         | isOpenTypeKind k = return liftedTypeKind
         | isArgTypeKind k  = return liftedTypeKind
        go _ k
         | isLiftedTypeKind k   = return liftedTypeKind
         | isUnliftedTypeKind k = return unliftedTypeKind
        go _ k@(TyVarTy _) = return k -- KindVars are always simple
        go _ _ = failWithTc (ptext (sLit "Unexpected kind unification failure:")
                                      <+> ppr orig_swapped <+> ppr orig_kind)
            -- I think this can't actually happen

    -- T v = MkT v           v must be a type
    -- T v w = MkT (v -> w)  v must not be an umboxed tuple

    unifyKindMisMatch :: TcKind -> TcKind -> TcM ()
    unifyKindMisMatch ty1 ty2 = do
        ty1' <- zonkTcKind ty1
        ty2' <- zonkTcKind ty2
        let
    	msg = hang (ptext (sLit "Couldn't match kind"))
    		   2 (sep [quotes (ppr ty1'),
    			   ptext (sLit "against"),
    			   quotes (ppr ty2')])
        failWithTc msg

    ----------------
    kindOccurCheckErr :: Var -> Type -> SDoc
    kindOccurCheckErr tyvar ty
      = hang (ptext (sLit "Occurs check: cannot construct the infinite kind:"))
           2 (sep [ppr tyvar, char '=', ppr ty])

\end{code} %************************************************************************ %* * \subsection{Checking signature type variables} %* * %************************************************************************ @checkSigTyVars@ checks that a set of universally quantified type varaibles are not mentioned in the environment. In particular: (a) Not mentioned in the type of a variable in the envt eg the signature for f in this: g x = ... where f :: a->[a] f y = [x,y] Here, f is forced to be monorphic by the free occurence of x. (d) Not (unified with another type variable that is) in scope. eg f x :: (r->r) = (\y->y) :: forall a. a->r when checking the expression type signature, we find that even though there is nothing in scope whose type mentions r, nevertheless the type signature for the expression isn't right. Another example is in a class or instance declaration: class C a where op :: forall b. a -> b op x = x Here, b gets unified with a Before doing this, the substitution is applied to the signature type variable. -- \begin{code} checkSigTyVars :: [TcTyVar] -> TcM () checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM () -- The extra_tvs can include boxy type variables; -- e.g. TcMatches.tcCheckExistentialPat checkSigTyVarsWrt extra_tvs sig_tvs = do { extra_tvs' <- zonkTcTyVarsAndFV extra_tvs ; check_sig_tyvars extra_tvs' sig_tvs } check_sig_tyvars :: TcTyVarSet -- Global type variables. The universally quantified -- tyvars should not mention any of these -- Guaranteed already zonked. -> [TcTyVar] -- Universally-quantified type variables in the signature -- Guaranteed to be skolems -> TcM () check_sig_tyvars _ [] = return () check_sig_tyvars extra_tvs sig_tvs = ASSERT( all isTcTyVar sig_tvs && all isSkolemTyVar sig_tvs ) do { gbl_tvs <- tcGetGlobalTyVars ; traceTc "check_sig_tyvars" $ vcat [ text "sig_tys" <+> ppr sig_tvs , text "gbl_tvs" <+> ppr gbl_tvs , text "extra_tvs" <+> ppr extra_tvs] ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs ; when (any (`elemVarSet` env_tvs) sig_tvs) (bleatEscapedTvs env_tvs sig_tvs sig_tvs) } bleatEscapedTvs :: TcTyVarSet -- The global tvs -> [TcTyVar] -- The possibly-escaping type variables -> [TcTyVar] -- The zonked versions thereof -> TcM () -- Complain about escaping type variables -- We pass a list of type variables, at least one of which -- escapes. The first list contains the original signature type variable, -- while the second contains the type variable it is unified to (usually itself) bleatEscapedTvs globals sig_tvs zonked_tvs = do { env0 <- tcInitTidyEnv ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs) ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) } where main_msg = ptext (sLit "Inferred type is less polymorphic than expected") check (tidy_env, msgs) (sig_tv, zonked_tv) | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs) | otherwise = do { lcl_env <- getLclTypeEnv ; (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) lcl_env tidy_env ; return (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) } ----------------------- escape_msg :: Var -> Var -> [SDoc] -> SDoc escape_msg sig_tv zonked_tv globs | notNull globs = vcat [sep [msg, ptext (sLit "is mentioned in the environment:")], nest 2 (vcat globs)] | otherwise = msg <+> ptext (sLit "escapes") -- Sigh. It's really hard to give a good error message -- all the time. One bad case is an existential pattern match. -- We rely on the "When..." context to help. where msg = ptext (sLit "Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to is_bound_to | sig_tv == zonked_tv = empty | otherwise = ptext (sLit "is unified with") <+> quotes (ppr zonked_tv) <+> ptext (sLit "which") -- \end{code} These two context are used with checkSigTyVars \begin{code}

    sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
            -> TidyEnv -> TcM (TidyEnv, Message)
    sigCtxt id sig_tvs sig_theta sig_tau tidy_env = do
        actual_tau <- zonkTcType sig_tau
        let
            (env1, tidy_sig_tvs)    = tidyOpenTyVars tidy_env sig_tvs
            (env2, tidy_sig_rho)    = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
            (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
            sub_msg = vcat [ptext (sLit "Signature type:    ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
                            ptext (sLit "Type to generalise:") <+> pprType tidy_actual_tau
                       ]
            msg = vcat [ptext (sLit "When trying to generalise the type inferred for") <+> quotes (ppr id),
                        nest 2 sub_msg]

        return (env3, msg)

\end{code}