X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcExpr.lhs;h=297b4e884e924411667c44dd283f13a92c481604;hb=ef6d82a4e1d4ba4884c322be85cff291e017f0e6;hp=03e06874c8430b5c89d7ee0a2f406eaad3e15dce;hpb=d2ce0f52d42edf32bb9f13796e6ba6edba8bd516;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcExpr.lhs b/compiler/typecheck/TcExpr.lhs
index 03e0687..297b4e8 100644
--- a/compiler/typecheck/TcExpr.lhs
+++ b/compiler/typecheck/TcExpr.lhs
@@ -82,7 +82,7 @@ tcPolyExpr expr res_ty
 
 tcPolyExprNC expr res_ty
   = do { traceTc "tcPolyExprNC" (ppr res_ty)
-       ; (gen_fn, expr') <- tcGen (GenSkol res_ty) emptyVarSet res_ty $ \ _ rho ->
+       ; (gen_fn, expr') <- tcGen (GenSkol res_ty) res_ty $ \ _ rho ->
 			    tcMonoExprNC expr rho
        ; return (mkLHsWrap gen_fn expr') }
 
@@ -191,7 +191,7 @@ tcExpr (ExprWithTySig expr sig_ty) res_ty
 
       -- Remember to extend the lexical type-variable environment
       ; (gen_fn, expr') 
-            <- tcGen (SigSkol ExprSigCtxt) emptyVarSet sig_tc_ty $ \ skol_tvs res_ty ->
+            <- tcGen (SigSkol 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
@@ -311,7 +311,7 @@ tcExpr (SectionR op arg2) res_ty
 tcExpr (SectionL arg1 op) res_ty
   = do { (op', op_ty) <- tcInferFun op
        ; dflags <- getDOpts	    -- Note [Left sections]
-       ; let n_reqd_args | dopt Opt_PostfixOperators dflags = 1
+       ; let n_reqd_args | xopt Opt_PostfixOperators dflags = 1
                          | otherwise                        = 2
 
        ; (co_fn, (arg1_ty:arg_tys), op_res_ty) <- unifyOpFunTys op n_reqd_args op_ty
@@ -392,11 +392,27 @@ 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 (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 _) res_ty
   = tcDoStmts do_or_lc stmts body res_ty
@@ -414,6 +430,22 @@ 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"
+
+
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