Preliminary monad-comprehension patch (Trac #4370)

[ghc-hetmet.git] / compiler / typecheck / TcExpr.lhs

diff --git a/compiler/typecheck/TcExpr.lhs b/compiler/typecheck/TcExpr.lhs
index 7205287..a821f25 100644 (file)
--- a/compiler/typecheck/TcExpr.lhs
+++ b/compiler/typecheck/TcExpr.lhs
@@ -49,6 +49,7 @@ import TysWiredIn
 import TysPrim( intPrimTy )
 import PrimOp( tagToEnumKey )
 import PrelNames
+import Module
 import DynFlags
 import SrcLoc
 import Util
@@ -82,7 +83,7 @@ tcPolyExpr expr res_ty
 
 tcPolyExprNC expr res_ty
   = do { traceTc "tcPolyExprNC" (ppr res_ty)
-       ; (gen_fn, expr') <- tcGen (GenSkol res_ty) res_ty $ \ _ rho ->
+       ; (gen_fn, expr') <- tcGen GenSigCtxt res_ty $ \ _ rho ->
                            tcMonoExprNC expr rho
        ; return (mkLHsWrap gen_fn expr') }
 
@@ -191,7 +192,7 @@ tcExpr (ExprWithTySig expr sig_ty) res_ty
 
       -- Remember to extend the lexical type-variable environment
       ; (gen_fn, expr') 
-            <- tcGen (SigSkol ExprSigCtxt) sig_tc_ty $ \ skol_tvs res_ty ->
+            <- tcGen ExprSigCtxt sig_tc_ty $ \ skol_tvs res_ty ->
               tcExtendTyVarEnv2 (hsExplicitTvs sig_ty `zip` mkTyVarTys skol_tvs) $
                                -- See Note [More instantiated than scoped] in TcBinds
               tcMonoExprNC expr res_ty
@@ -392,14 +393,30 @@ tcExpr (HsCase scrut matches) exp_ty
     match_ctxt = MC { mc_what = CaseAlt,
                      mc_body = tcBody }
 
-tcExpr (HsIf pred b1 b2) res_ty
-  = do { pred' <- tcMonoExpr pred boolTy
-       ; b1' <- tcMonoExpr b1 res_ty
-       ; b2' <- tcMonoExpr b2 res_ty
-       ; return (HsIf pred' b1' b2') }
-
-tcExpr (HsDo do_or_lc stmts body _) res_ty
-  = tcDoStmts do_or_lc stmts body res_ty
+tcExpr (HsIf Nothing pred b1 b2) res_ty           -- Ordinary 'if'
+  = do { pred' <- tcMonoExpr pred boolTy
+       ; b1' <- tcMonoExpr b1 res_ty
+       ; b2' <- tcMonoExpr b2 res_ty
+       ; return (HsIf Nothing pred' b1' b2') }
+
+tcExpr (HsIf (Just fun) pred b1 b2) res_ty   -- Note [Rebindable syntax for if]
+  = do { pred_ty <- newFlexiTyVarTy openTypeKind
+       ; b1_ty   <- newFlexiTyVarTy openTypeKind
+       ; b2_ty   <- newFlexiTyVarTy openTypeKind
+       ; let if_ty = mkFunTys [pred_ty, b1_ty, b2_ty] res_ty
+       ; fun'  <- tcSyntaxOp IfOrigin fun if_ty
+       ; pred' <- tcMonoExpr pred pred_ty
+       ; b1'   <- tcMonoExpr b1 b1_ty
+       ; b2'   <- tcMonoExpr b2 b2_ty
+       -- Fundamentally we are just typing (ifThenElse e1 e2 e3)
+       -- so maybe we should use the code for function applications
+       -- (which would allow ifThenElse to be higher rank).
+       -- But it's a little awkward, so I'm leaving it alone for now
+       -- and it maintains uniformity with other rebindable syntax
+       ; return (HsIf (Just fun') pred' b1' b2') }
+
+tcExpr (HsDo do_or_lc stmts body return_op _) res_ty
+  = tcDoStmts do_or_lc stmts body return_op res_ty
 
 tcExpr (HsProc pat cmd) res_ty
   = do { (pat', cmd', coi) <- tcProc pat cmd res_ty
@@ -414,6 +431,22 @@ tcExpr e@(HsArrForm _ _ _) _
                       ptext (sLit "was found where an expression was expected")])
 \end{code}
 
+Note [Rebindable syntax for if]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The rebindable syntax for 'if' uses the most flexible possible type
+for conditionals:
+  ifThenElse :: p -> b1 -> b2 -> res
+to support expressions like this:
+
+ ifThenElse :: Maybe a -> (a -> b) -> b -> b
+ ifThenElse (Just a) f _ = f a  ifThenElse Nothing  _ e = e
+
+ example :: String
+ example = if Just 2
+              then \v -> show v
+              else "No value"
+
+
 %************************************************************************
 %*                                                                     *
                Record construction and update
@@ -705,7 +738,7 @@ tcExpr (PArrSeq _ seq@(FromTo expr1 expr2)) res_ty
        ; expr1' <- tcPolyExpr expr1 elt_ty
        ; expr2' <- tcPolyExpr expr2 elt_ty
        ; enum_from_to <- newMethodFromName (PArrSeqOrigin seq) 
-                                enumFromToPName elt_ty 
+                                (enumFromToPName basePackageId) elt_ty    -- !!!FIXME: chak
        ; return $ mkHsWrapCoI coi 
                      (PArrSeq enum_from_to (FromTo expr1' expr2')) }
 
@@ -715,7 +748,7 @@ tcExpr (PArrSeq _ seq@(FromThenTo expr1 expr2 expr3)) res_ty
        ; expr2' <- tcPolyExpr expr2 elt_ty
        ; expr3' <- tcPolyExpr expr3 elt_ty
        ; eft <- newMethodFromName (PArrSeqOrigin seq)
-                     enumFromThenToPName elt_ty
+                     (enumFromThenToPName basePackageId) elt_ty        -- !!!FIXME: chak
        ; return $ mkHsWrapCoI coi 
                      (PArrSeq eft (FromThenTo expr1' expr2' expr3')) }
 
@@ -787,7 +820,8 @@ tcApp fun args res_ty
        -- Typecheck the result, thereby propagating 
         -- info (if any) from result into the argument types
         -- Both actual_res_ty and res_ty are deeply skolemised
-        ; co_res <- unifyType actual_res_ty res_ty
+        ; co_res <- addErrCtxt (funResCtxt fun) $
+                    unifyType actual_res_ty res_ty
 
        -- Typecheck the arguments
        ; args1 <- tcArgs fun args expected_arg_tys
@@ -1352,6 +1386,10 @@ funAppCtxt fun arg arg_no
                    quotes (ppr fun) <> text ", namely"])
        2 (quotes (ppr arg))
 
+funResCtxt :: LHsExpr Name -> SDoc
+funResCtxt fun
+  = ptext (sLit "In the return type of a call of") <+> quotes (ppr fun)
+
 badFieldTypes :: [(Name,TcType)] -> SDoc
 badFieldTypes prs
   = hang (ptext (sLit "Record update for insufficiently polymorphic field")