For single-method classes use newtypes

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

diff --git a/compiler/typecheck/TcExpr.lhs b/compiler/typecheck/TcExpr.lhs
index 7205287..297b4e8 100644 (file)
--- a/compiler/typecheck/TcExpr.lhs
+++ b/compiler/typecheck/TcExpr.lhs
@@ -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"
+
+
 %************************************************************************
 %*                                                                     *
                Record construction and update