Fwd: [commit: ghc] master: Relax the restriction on using abstract newtypes in FFI declarations. (c6b0fd6)

24 Nov 2011

This deserves a wider audience: if you've encountered failures when 
compiling FFI code with recent GHCs due to the new requirement that 
newtypes be non-abstract, note that this will now be a warning, not an 
error, in 7.4.1.  We're deferring turning it into an error until 7.6.1.

Cheers,
	Simon

-------- Original Message --------
Subject: [commit: ghc] master: Relax the restriction on using abstract 
newtypes in FFI declarations. (c6b0fd6)
Date: Thu, 24 Nov 2011 06:57:21 -0800
From: Simon Marlow 
To: cvs-ghc@haskell.org

Repository : ssh://darcs.haskell.org//srv/darcs/ghc

On branch  : master

http://hackage.haskell.org/trac/ghc/changeset/c6b0fd62fc715aa6c666eb8afe0907...
...
---------------------------------------------------------------
commit c6b0fd62fc715aa6c666eb8afe09073ac7b87a83
Author: Simon Marlow 
Date:   Thu Nov 24 14:04:23 2011 +0000

     Relax the restriction on using abstract newtypes in FFI declarations.

     Given the high impact of this change, we decided to back off and make
     abstract newtypes give a warning for one release, before we make it an
     error in 7.6.1.

     Codec/Compression/Zlib/Stream.hsc:884:1:
         Warning: newtype `CInt' is used in an FFI declaration,
                  but its constructor is not in scope.
                  This will become an error in GHC 7.6.1.
         When checking declaration:
           foreign import ccall unsafe "static zlib.h deflate" c_deflate
             :: StreamState -> CInt -> IO CInt
...
---------------------------------------------------------------
compiler/typecheck/TcForeign.lhs |   22 +++++++++++++++++-----
  1 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/compiler/typecheck/TcForeign.lhs 
b/compiler/typecheck/TcForeign.lhs
index 886b84d..5a4bf77 100644
--- a/compiler/typecheck/TcForeign.lhs
+++ b/compiler/typecheck/TcForeign.lhs
@@ -121,17 +121,29 @@ normaliseFfiType' env ty0 = go [] ty0
                                                    panic 
"normaliseFfiType': Got more GREs than expected"
                                        _ ->
                                            return False
-                  if newtypeOK
-                      then do let nt_co = mkAxInstCo (newTyConCo tc) tys
-                              add_co nt_co rec_nts' nt_rhs
-                      else children_only
+                  when (not newtypeOK) $
+                     -- later: stop_here
+                    addWarnTc (ptext (sLit "newtype") <+> quotes (ppr 
tc) <+>
+                               ptext (sLit "is used in an FFI 
declaration,") $$
+                               ptext (sLit "but its constructor is not 
in scope.") $$
+                               ptext (sLit "This will become an error 
in GHC 7.6.1."))
+
+                  let nt_co = mkAxInstCo (newTyConCo tc) tys
+                  add_co nt_co rec_nts' nt_rhs
+
          | isFamilyTyCon tc              -- Expand open tycons
          , (co, ty) <- normaliseTcApp env tc tys
          , not (isReflCo co)
          = add_co co rec_nts ty
+
          | otherwise
-        = children_only
+        = return (mkReflCo ty, ty)
+            -- If we have reached an ordinary (non-newtype) type 
constructor,
+            -- we are done.  Note that we don't need to normalise the 
arguments,
+            -- because whether an FFI type is legal or not depends only on
+            -- the top-level type constructor (e.g. "Ptr a" is valid 
for all a).
          where
+
            children_only = do xs <- mapM (go rec_nts) tys
                               let (cos, tys') = unzip xs
                               return (mkTyConAppCo tc cos, mkTyConApp 
tc tys')



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