+type TcHoleType = TcSigmaType -- Either a TcSigmaType,
+ -- or else a hole
+
+tcSubExp :: TcHoleType -> TcSigmaType -> TcM ExprCoFn
+tcSubOff :: TcSigmaType -> TcHoleType -> TcM ExprCoFn
+tcSub :: TcSigmaType -> TcSigmaType -> TcM ExprCoFn
+\end{code}
+
+These two check for holes
+
+\begin{code}
+tcSubExp expected_ty offered_ty
+ = traceTc (text "tcSubExp" <+> (ppr expected_ty $$ ppr offered_ty)) `thenM_`
+ checkHole expected_ty offered_ty tcSub
+
+tcSubOff expected_ty offered_ty
+ = checkHole offered_ty expected_ty (\ off exp -> tcSub exp off)
+
+-- checkHole looks for a hole in its first arg;
+-- If so, and it is uninstantiated, it fills in the hole
+-- with its second arg
+-- Otherwise it calls thing_inside, passing the two args, looking
+-- through any instantiated hole
+
+checkHole (TyVarTy tv) other_ty thing_inside
+ | isHoleTyVar tv
+ = getTcTyVar tv `thenM` \ maybe_ty ->
+ case maybe_ty of
+ Just ty -> thing_inside ty other_ty
+ Nothing -> traceTc (text "checkHole" <+> ppr tv) `thenM_`
+ putTcTyVar tv other_ty `thenM_`
+ returnM idCoercion
+
+checkHole ty other_ty thing_inside
+ = thing_inside ty other_ty
+\end{code}
+
+No holes expected now. Add some error-check context info.
+
+\begin{code}
+tcSub expected_ty actual_ty
+ = traceTc (text "tcSub" <+> details) `thenM_`
+ addErrCtxtM (unifyCtxt "type" expected_ty actual_ty)
+ (tc_sub expected_ty expected_ty actual_ty actual_ty)
+ where
+ details = vcat [text "Expected:" <+> ppr expected_ty,
+ text "Actual: " <+> ppr actual_ty]
+\end{code}
+
+tc_sub carries the types before and after expanding type synonyms
+
+\begin{code}
+tc_sub :: TcSigmaType -- expected_ty, before expanding synonyms
+ -> TcSigmaType -- ..and after
+ -> TcSigmaType -- actual_ty, before
+ -> TcSigmaType -- ..and after
+ -> TcM ExprCoFn
+
+-----------------------------------
+-- Expand synonyms
+tc_sub exp_sty (NoteTy _ exp_ty) act_sty act_ty = tc_sub exp_sty exp_ty act_sty act_ty
+tc_sub exp_sty exp_ty act_sty (NoteTy _ act_ty) = tc_sub exp_sty exp_ty act_sty act_ty
+
+-----------------------------------
+-- Generalisation case
+-- actual_ty: d:Eq b => b->b
+-- expected_ty: forall a. Ord a => a->a
+-- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
+
+-- It is essential to do this *before* the specialisation case
+-- Example: f :: (Eq a => a->a) -> ...
+-- g :: Ord b => b->b
+-- Consider f g !
+
+tc_sub exp_sty expected_ty act_sty actual_ty
+ | isSigmaTy expected_ty
+ = tcGen expected_ty (tyVarsOfType actual_ty) (
+ -- It's really important to check for escape wrt the free vars of
+ -- both expected_ty *and* actual_ty
+ \ body_exp_ty -> tc_sub body_exp_ty body_exp_ty act_sty actual_ty
+ ) `thenM` \ (gen_fn, co_fn) ->
+ returnM (gen_fn <.> co_fn)
+
+-----------------------------------
+-- Specialisation case:
+-- actual_ty: forall a. Ord a => a->a
+-- expected_ty: Int -> Int
+-- co_fn e = e Int dOrdInt
+
+tc_sub exp_sty expected_ty act_sty actual_ty
+ | isSigmaTy actual_ty
+ = tcInstCall Rank2Origin actual_ty `thenM` \ (inst_fn, body_ty) ->
+ tc_sub exp_sty expected_ty body_ty body_ty `thenM` \ co_fn ->
+ returnM (co_fn <.> mkCoercion inst_fn)
+
+-----------------------------------
+-- Function case
+
+tc_sub _ (FunTy exp_arg exp_res) _ (FunTy act_arg act_res)
+ = tcSub_fun exp_arg exp_res act_arg act_res
+
+-----------------------------------
+-- Type variable meets function: imitate
+--
+-- NB 1: we can't just unify the type variable with the type
+-- because the type might not be a tau-type, and we aren't
+-- allowed to instantiate an ordinary type variable with
+-- a sigma-type
+--
+-- NB 2: can we short-cut to an error case?
+-- when the arg/res is not a tau-type?
+-- NO! e.g. f :: ((forall a. a->a) -> Int) -> Int
+-- then x = (f,f)
+-- is perfectly fine, because we can instantiat f's type to a monotype
+--
+-- However, we get can get jolly unhelpful error messages.
+-- e.g. foo = id runST
+--
+-- Inferred type is less polymorphic than expected
+-- Quantified type variable `s' escapes
+-- Expected type: ST s a -> t
+-- Inferred type: (forall s1. ST s1 a) -> a
+-- In the first argument of `id', namely `runST'
+-- In a right-hand side of function `foo': id runST
+--
+-- I'm not quite sure what to do about this!
+
+tc_sub exp_sty exp_ty@(FunTy exp_arg exp_res) _ (TyVarTy tv)
+ = ASSERT( not (isHoleTyVar tv) )
+ getTcTyVar tv `thenM` \ maybe_ty ->
+ case maybe_ty of
+ Just ty -> tc_sub exp_sty exp_ty ty ty
+ Nothing -> imitateFun tv exp_sty `thenM` \ (act_arg, act_res) ->
+ tcSub_fun exp_arg exp_res act_arg act_res
+
+tc_sub _ (TyVarTy tv) act_sty act_ty@(FunTy act_arg act_res)
+ = ASSERT( not (isHoleTyVar tv) )
+ getTcTyVar tv `thenM` \ maybe_ty ->
+ case maybe_ty of
+ Just ty -> tc_sub ty ty act_sty act_ty
+ Nothing -> imitateFun tv act_sty `thenM` \ (exp_arg, exp_res) ->
+ tcSub_fun exp_arg exp_res act_arg act_res
+
+-----------------------------------
+-- Unification case
+-- If none of the above match, we revert to the plain unifier
+tc_sub exp_sty expected_ty act_sty actual_ty
+ = uTys exp_sty expected_ty act_sty actual_ty `thenM_`
+ returnM idCoercion
+\end{code}
+
+%************************************************************************
+%* *
+\subsection{Functions}
+%* *
+%************************************************************************
+
+\begin{code}
+tcSub_fun exp_arg exp_res act_arg act_res
+ = tc_sub act_arg act_arg exp_arg exp_arg `thenM` \ co_fn_arg ->
+ tc_sub exp_res exp_res act_res act_res `thenM` \ co_fn_res ->
+ newUnique `thenM` \ uniq ->
+ let
+ -- co_fn_arg :: HsExpr exp_arg -> HsExpr act_arg
+ -- co_fn_res :: HsExpr act_res -> HsExpr exp_res
+ -- co_fn :: HsExpr (act_arg -> act_res) -> HsExpr (exp_arg -> exp_res)
+ arg_id = mkSysLocal FSLIT("sub") uniq exp_arg
+ coercion | isIdCoercion co_fn_arg,
+ isIdCoercion co_fn_res = idCoercion
+ | otherwise = mkCoercion co_fn
+
+ co_fn e = DictLam [arg_id]
+ (co_fn_res <$> (HsApp e (co_fn_arg <$> (HsVar arg_id))))
+ -- Slight hack; using a "DictLam" to get an ordinary simple lambda
+ -- HsVar arg_id :: HsExpr exp_arg
+ -- co_fn_arg $it :: HsExpr act_arg
+ -- HsApp e $it :: HsExpr act_res
+ -- co_fn_res $it :: HsExpr exp_res
+ in
+ returnM coercion
+
+imitateFun :: TcTyVar -> TcType -> TcM (TcType, TcType)
+imitateFun tv ty
+ = ASSERT( not (isHoleTyVar tv) )
+ -- NB: tv is an *ordinary* tyvar and so are the new ones
+
+ -- Check that tv isn't a type-signature type variable
+ -- (This would be found later in checkSigTyVars, but
+ -- we get a better error message if we do it here.)
+ checkM (not (isSkolemTyVar tv))
+ (failWithTcM (unifyWithSigErr tv ty)) `thenM_`
+
+ newTyVarTy openTypeKind `thenM` \ arg ->
+ newTyVarTy openTypeKind `thenM` \ res ->
+ putTcTyVar tv (mkFunTy arg res) `thenM_`
+ returnM (arg,res)