[project @ 2003-04-08 13:06:48 by simonpj]

----------------------------------
   Try swapping order of inference for applications
	----------------------------------

For a long time we've inferred the argument types of a function
call before using information from the expected type passed in.
This commit swaps the order.  See comments with tcApp for an
example where this makes a real difference.

I don't think any error messages get signficantly worse, but yell
if you find any.

diff --git a/ghc/compiler/typecheck/TcExpr.lhs b/ghc/compiler/typecheck/TcExpr.lhs
index 6cfd445..8230d2e 100644 (file)
--- a/ghc/compiler/typecheck/TcExpr.lhs
+++ b/ghc/compiler/typecheck/TcExpr.lhs
@@ -658,17 +658,33 @@ tcApp fun args res_ty
        split_fun_ty fun_ty (length args)
     )                                          `thenM` \ (expected_arg_tys, actual_result_ty) ->
 
-       -- Now typecheck the args
-    mappM (tcArg fun)
-         (zip3 args expected_arg_tys [1..])    `thenM` \ args' ->
-
-       -- Unify with expected result after type-checking the args
-       -- so that the info from args percolates to actual_result_ty.
+       -- Unify with expected result before (was: after) type-checking the args
+       -- so that the info from res_ty (was: args) percolates to args (was actual_result_ty).
        -- This is when we might detect a too-few args situation.
        -- (One can think of cases when the opposite order would give
        -- a better error message.)
+       -- [March 2003: I'm experimenting with putting this first.  Here's an 
+       --              example where it actually makes a real difference
+       --    class C t a b | t a -> b
+       --    instance C Char a Bool
+       --
+       --    data P t a = forall b. (C t a b) => MkP b
+       --    data Q t   = MkQ (forall a. P t a)
+    
+       --    f1, f2 :: Q Char;
+       --    f1 = MkQ (MkP True)
+       --    f2 = MkQ (MkP True :: forall a. P Char a)
+       --
+       -- With the change, f1 will type-check, because the 'Char' info from
+       -- the signature is propagated into MkQ's argument. With the check
+       -- in the other order, the extra signature in f2 is reqd.]
+
     addErrCtxtM (checkArgsCtxt fun args res_ty actual_result_ty)
-                 (tcSubExp res_ty actual_result_ty)    `thenM` \ co_fn ->
+               (tcSubExp res_ty actual_result_ty)      `thenM` \ co_fn ->
+
+       -- Now typecheck the args
+    mappM (tcArg fun)
+         (zip3 args expected_arg_tys [1..])    `thenM` \ args' ->
 
     returnM (co_fn <$> foldl HsApp fun' args')