Make the type-defaulting in GHCi use () as the first default type

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

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index b4b0b73..7311ae0 100644 (file)
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -2252,17 +2252,7 @@ disambiguate interactive dflags insts
        ;     return () }
   | otherwise
   = do         {       -- Figure out what default types to use
-         mb_defaults <- getDefaultTys
-       ; default_tys <- case mb_defaults of
-                          Just tys -> return tys
-                          Nothing  ->  -- No use-supplied default;
-                                       -- use [Integer, Double]
-                               do { integer_ty <- tcMetaTy integerTyConName
-                                  ; checkWiredInTyCon doubleTyCon
-                                  ; string_ty <- tcMetaTy stringTyConName
-                                  ; if ovl_strings     -- Add String if -foverloaded-strings
-                                       then return [integer_ty,doubleTy,string_ty] 
-                                       else return [integer_ty,doubleTy] }
+       ; default_tys <- getDefaultTys extended_defaulting ovl_strings
 
        ; traceTc (text "disambigutate" <+> vcat [ppr unaries, ppr bad_tvs, ppr defaultable_groups])
        ; mapM_ (disambigGroup default_tys) defaultable_groups  }
@@ -2325,8 +2315,43 @@ disambigGroup default_tys dicts
                -- After this we can't fail
           ; warnDefault dicts default_ty
           ; unifyType default_ty (mkTyVarTy tyvar) }
+
+
+getDefaultTys :: Bool -> Bool -> TcM [Type]
+getDefaultTys extended_deflts ovl_strings
+  = do { mb_defaults <- getDeclaredDefaultTys
+       ; case mb_defaults of
+          Just tys -> return tys       -- User-supplied defaults
+          Nothing  -> do
+
+       -- No use-supplied default
+       -- Use [Integer, Double], plus modifications
+       { integer_ty <- tcMetaTy integerTyConName
+       ; checkWiredInTyCon doubleTyCon
+       ; string_ty <- tcMetaTy stringTyConName
+       ; return (opt_deflt extended_deflts unitTy
+                       -- Note [Default unitTy]
+                       ++
+                 [integer_ty,doubleTy]
+                       ++
+                 opt_deflt ovl_strings string_ty) }}
+  where
+    opt_deflt True  ty = [ty]
+    opt_deflt False ty = []
 \end{code}
 
+Note [Default unitTy]
+~~~~~~~~~~~~~~~~~~~~~
+In interative mode (or with -fextended-default-rules) we add () as the first type we
+try when defaulting.  This has very little real impact, except in the following case.
+Consider: 
+       Text.Printf.printf "hello"
+This has type (forall a. IO a); it prints "hello", and returns 'undefined'.  We don't
+want the GHCi repl loop to try to print that 'undefined'.  The neatest thing is to
+default the 'a' to (), rather than to Integer (which is what would otherwise happen;
+and then GHCi doesn't attempt to print the ().  So in interactive mode, we add
+() to the list of defaulting types.  See Trac #1200.
+
 Note [Avoiding spurious errors]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 When doing the unification for defaulting, we check for skolem