[GHC] #14125: Bogus unacceptable type in foreign declaration.

16 Aug 2017

      #14125: Bogus unacceptable type in foreign declaration.
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           Reporter:  winter         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I'm try to abstract FFI return type so that different errno scheme can
 work together, namely i create such a class:

 {{{
 class Integral (IOErrno r) => IOReturn r where
     -- | The errno type associated with 'r'
     data IOErrno r :: *
     -- | Is this return value indicate an error?
     isError   :: Integral a => r a -> Bool
     -- | How can i get a errno then?
     getErrno  :: Integral a => r a -> IO (IOErrno r)
     -- | Is this errno indicate interrupted blocking call?
     isInterrupt :: IOErrno r -> Bool
     -- | Is this errno indicate no data on a non-blocking device?
     isBlock   :: IOErrno r -> Bool
     -- | OK, i want my return value if no errno.
     getReturn :: Integral a => r a -> a
     -- | Read the errno name for me.
     nameErrno :: IOErrno r -> IO String
     -- | Read the errno description for me.
     descErrno :: IOErrno r -> IO String
 }}}

 But when i'm try to defining instance for it, such as a standard unix
 like:

 {{{
 newtype UnixReturn a = UnixReturn a
     deriving (Bounded, Enum, Eq, Integral, Num, Ord, Read, Real, Show,
 FiniteBits, Bits, Storable)

 instance IOReturn UnixReturn where
     newtype IOErrno UnixReturn = UnixErrno CInt
         deriving (Bounded, Enum, Eq, Integral, Num, Ord, Read, Real, Show,
 FiniteBits, Bits, Storable)
     isError (UnixReturn r) = r == (-1)
     isInterrupt e = e == eINTR
     isBlock e = e == eAGAIN || e == eWOULDBLOCK
     getErrno _ = get_errno
     getReturn (UnixReturn r) = r
     nameErrno = return . nameUnixErrno
     descErrno e = strerror e >>= peekCString

 eINTR           = UnixErrno (CONST_EINTR)
 eWOULDBLOCK     = UnixErrno (CONST_EWOULDBLOCK)
 ...

 foreign import ccall unsafe "string.h" strerror :: IOErrno UnixReturn ->
 IO CString
 foreign import ccall unsafe "HsBase.h __hscore_get_errno" get_errno :: IO
 (IOErrno UnixReturn)
 }}}

 I got a error on GHC 8.2:

 {{{
 System/IO/Exception.hs:282:1: error:
     • Unacceptable argument type in foreign declaration:
         ‘IOErrno UnixReturn’ cannot be marshalled in a foreign call
     • When checking declaration:
         foreign import ccall unsafe "string.h" strerror
           :: IOErrno UnixReturn -> IO CString
     |
 282 | foreign import ccall unsafe "string.h" strerror :: IOErrno
 UnixReturn -> IO CString
     |
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 System/IO/Exception.hs:283:1: error:
     • Unacceptable result type in foreign declaration:
         ‘IOErrno UnixReturn’ cannot be marshalled in a foreign call
     • When checking declaration:
         foreign import ccall unsafe "HsBase.h __hscore_get_errno"
 get_errno
           :: IO (IOErrno UnixReturn)
     |
 283 | foreign import ccall unsafe "HsBase.h __hscore_get_errno" get_errno
 :: IO (IOErrno UnixReturn)
     |
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 And at every place i want mark my FFI return got this error too. e.g.
 following code compiles without problem on GHC < 8.2 (Note that the type
 and constructor are both available):

 {{{

 foreign import CALLCONV unsafe "HsNet.h recv"
     c_recv :: CInt -> Ptr Word8 -> CSize -> CInt -> IO (UnixReturn CSsize)
 }}}

 But on GHC 8.2 a `Unacceptable result type in foreign declaration` is
 emitted. And I think it's a regression.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14125
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