On Thursday 30 March 2006 14:13, Brian Hulley wrote:
John Meacham wrote:
On Thu, Mar 30, 2006 at 03:50:06AM +0100, Brian Hulley wrote:
where the intention is that the callback will take the width and height of the window and return a RenderM action, the problem is that because the FFI does not allow RenderM to appear in a foreign type.
it should, the types in foreign declarations should "see through" newtypes.
Unfortunately GHC does not seem to support this:
foreign import ccall duma_clear :: Word.Word32 -> RenderM ()
Unacceptable result type in foreign declaration: RenderM () When checking declaration: foreign import ccall safe "static &duma_clear" duma_clear :: GHC.Word.Word32 -> RenderM ()
even though the FFI spec agrees with you (Section 3.2):
The argument types ati produced by fatype must be marshallable foreign types; that is, each ati is either (1) a basic foreign type or (2) a type synonym or renamed datatype of a marshallable foreign type. Moreover, the result type rt produced by frtype must be a marshallable foreign result type; that is, it is either a marshallable foreign type, ...
Note that the addendum continues "...that is, it is either a marshallable foreign type, the type (), or a type matching Prelude.IO t, where t is a marshallable foreign type or ()." Nowhere is it written that 'IO t' (in itself) is a marshallable type whenever 't' is a marshallable type. [I am not sure what it means exactly if a type "matches" 'Prelude.IO t'.] Thus, GHC does nothing wrong, according to the addendum. That doesn't mean allowing IO-equivalent newtypes wouldn't be a good idea. It is just not written in the addendum. Ben