Bogus unacceptable type in foreign declaration.
I'm trying 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.