Export constructors for IntPtr and WordPtr

This finishes what #5529 started by exporting the constructors for
`IntPtr` and `WordPtr` from `Foreign.Ptr`, allowing them to be used in
`foreign` declarations.

Fixes #11983.

Test Plan: `make TEST=T11983`

Reviewers: simonpj, hvr, bgamari, austin

Reviewed By: simonpj

Subscribers: simonpj, thomie

Differential Revision: https://phabricator.haskell.org/D2142

GHC Trac Issues: #11983

8 files changed, 67 insertions, 5 deletions

diff --git a/libraries/base/Foreign/C/Types.hs b/libraries/base/Foreign/C/Types.hs
index fef8e4a3b6..b725a4a11b 100644
--- a/libraries/base/Foreign/C/Types.hs
+++ b/libraries/base/Foreign/C/Types.hs
@@ -10,7 +10,7 @@
 -- Module      :  Foreign.C.Types
 -- Copyright   :  (c) The FFI task force 2001
 -- License     :  BSD-style (see the file libraries/base/LICENSE)
--- 
+--
 -- Maintainer  :  ffi@haskell.org
 -- Stability   :  provisional
 -- Portability :  portable
@@ -63,6 +63,10 @@ module Foreign.C.Types
         -- XXX GHC doesn't support CLDouble yet
         -- , CLDouble(..)
 
+          -- See Note [Exporting constructors of marshallable foreign types]
+          -- in Foreign.Ptr for why the constructors for these newtypes are
+          -- exported.
+
           -- ** Other types
 
           -- Instances of: Eq and Storable
diff --git a/libraries/base/Foreign/Ptr.hs b/libraries/base/Foreign/Ptr.hs
index efe580a70d..5e6bccf7ee 100644
--- a/libraries/base/Foreign/Ptr.hs
+++ b/libraries/base/Foreign/Ptr.hs
@@ -7,7 +7,7 @@
 -- Module      :  Foreign.Ptr
 -- Copyright   :  (c) The FFI task force 2001
 -- License     :  BSD-style (see the file libraries/base/LICENSE)
--- 
+--
 -- Maintainer  :  ffi@haskell.org
 -- Stability   :  provisional
 -- Portability :  portable
@@ -41,12 +41,15 @@ module Foreign.Ptr (
     -- Free the function pointer created by foreign export dynamic.
 
     -- * Integral types with lossless conversion to and from pointers
-    IntPtr,
+    IntPtr(..),
     ptrToIntPtr,
     intPtrToPtr,
-    WordPtr,
+    WordPtr(..),
     ptrToWordPtr,
     wordPtrToPtr
+
+    -- See Note [Exporting constructors of marshallable foreign types]
+    -- for why the constructors for IntPtr and WordPtr are exported.
  ) where
 
 import GHC.Ptr
@@ -97,3 +100,21 @@ ptrToIntPtr (Ptr a#) = IntPtr (I# (addr2Int# a#))
 -- | casts an @IntPtr@ to a @Ptr@
 intPtrToPtr :: IntPtr -> Ptr a
 intPtrToPtr (IntPtr (I# i#)) = Ptr (int2Addr# i#)
+
+{-
+Note [Exporting constructors of marshallable foreign types]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+One might expect that IntPtr, WordPtr, and the other newtypes in the
+Foreign.C.Types and System.Posix.Types modules to be abstract, but this is not
+the case in GHC (see Trac #5229 and #11983). In fact, we deliberately export
+the constructors for these datatypes in order to satisfy a requirement of the
+Haskell 2010 Report (§ 8.4.2) that if a newtype is used in a foreign
+declaration, then its constructor must be visible.
+
+This requirement was motivated by the fact that using a type in a foreign
+declaration necessarily exposes some information about the type to the user,
+so being able to use abstract types in a foreign declaration breaks their
+abstraction (see Trac #3008). As a result, the constructors of all FFI-related
+newtypes in base must be exported in order to be useful for FFI programming,
+even at the cost of exposing their underlying, architecture-dependent types.
+-}
diff --git a/libraries/base/System/Posix/Types.hs b/libraries/base/System/Posix/Types.hs
index c2ac65e7e0..5389e6eeed 100644
--- a/libraries/base/System/Posix/Types.hs
+++ b/libraries/base/System/Posix/Types.hs
@@ -11,7 +11,7 @@
 -- Module      :  System.Posix.Types
 -- Copyright   :  (c) The University of Glasgow 2002
 -- License     :  BSD-style (see the file libraries/base/LICENSE)
--- 
+--
 -- Maintainer  :  libraries@haskell.org
 -- Stability   :  provisional
 -- Portability :  non-portable (requires POSIX)
@@ -68,6 +68,10 @@ module System.Posix.Types (
 
   Fd(..),
 
+  -- See Note [Exporting constructors of marshallable foreign types]
+  -- in Foreign.Ptr for why the constructors for these newtypes are
+  -- exported.
+
 #if defined(HTYPE_NLINK_T)
   LinkCount,
 #endif
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index f935c59bda..dd386ed477 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -1,5 +1,11 @@
 # Changelog for [`base` package](http://hackage.haskell.org/package/base)
 
+## next *TBA*
+  * Bundled with GHC *TBA*
+
+  * `Foreign.Ptr` now exports the constructors for `IntPtr` and `WordPtr`
+    (#11983)
+
 ## 4.9.0.0  *TBA*
 
   * Bundled with GHC 8.0
diff --git a/testsuite/tests/ffi/should_compile/T11983.c b/testsuite/tests/ffi/should_compile/T11983.c
new file mode 100644
index 0000000000..2b475fb0d8
--- /dev/null
+++ b/testsuite/tests/ffi/should_compile/T11983.c
@@ -0,0 +1,5 @@
+#include <stdint.h>
+#include "T11983.h"
+
+void intptr_example(intptr_t i)   {}
+void uintptr_example(uintptr_t u) {}
diff --git a/testsuite/tests/ffi/should_compile/T11983.h b/testsuite/tests/ffi/should_compile/T11983.h
new file mode 100644
index 0000000000..33c78cb9a4
--- /dev/null
+++ b/testsuite/tests/ffi/should_compile/T11983.h
@@ -0,0 +1,9 @@
+#ifndef T11983_H
+#define T11983_H
+
+#include <stdint.h>
+
+void intptr_example(intptr_t);
+void uintptr_example(uintptr_t);
+
+#endif // T11983_H
diff --git a/testsuite/tests/ffi/should_compile/T11983.hs b/testsuite/tests/ffi/should_compile/T11983.hs
new file mode 100644
index 0000000000..162d2411fc
--- /dev/null
+++ b/testsuite/tests/ffi/should_compile/T11983.hs
@@ -0,0 +1,11 @@
+{-# LANGUAGE ForeignFunctionInterface #-}
+module T11983 where
+
+{-# INCLUDE T11983.h #-}
+
+import Foreign.Ptr
+
+foreign import ccall "intptr_example"
+    intPtrExample :: IntPtr -> IO ()
+foreign import ccall "uintptr_example"
+    wordPtrExample :: WordPtr -> IO ()
diff --git a/testsuite/tests/ffi/should_compile/all.T b/testsuite/tests/ffi/should_compile/all.T
index ec6326b610..dac31aeec6 100644
--- a/testsuite/tests/ffi/should_compile/all.T
+++ b/testsuite/tests/ffi/should_compile/all.T
@@ -30,3 +30,5 @@ test('T3742', normal, compile, [''])
 test('cc015', normal, compile, [''])
 test('cc016', normal, compile, [''])
 test('T10460', normal, compile, [''])
+test('T11983', [ omit_ways(['ghci']), extra_clean(['T11983.o']) ],
+               compile, ['T11983.c'])