[SOLVED] inline-c : "`Type` cannot be marshalled in a foreign call"

inline-c : "`Type` cannot be marshalled in a foreign call"

Setting

A C enum type rendered into Haskell by c2hs, complete with Storable instance which compiles correctly (TypesC2Hs.chs). I import this unqualified into the module I have assigned for the inline-c context (Internal.hs). Both the .hs module generated by c2hs and Internal.hs are imported by InlineC.hs, the other inline-c module that holds the quasiquotes wrapping the C calls.

TypesC2Hs.hs ------------- 
    |                    |
    V                    V
Internal.hs -------> InlineC.hs

Question

InlineC.hs complains that this type cannot be marshalled: "Unacceptable argument type in foreign declaration: ‘DMBoundaryType’ cannot be marshalled in a foreign call When checking declaration:"

What is going on? This is the first time inline-c gives me type of this error.

I should note that other types that do not need to be dereferenced directly, e.g. newtype DM = DM (Ptr DM) deriving Storable, work fine with the above approach.

Thanks in advance

TypesC2Hs.chs

{# enum DMBoundaryType as DMBoundaryType {underscoreToCase} deriving (Eq, Show) #}

instance Storable DMBoundaryType where
  sizeOf _ = {# sizeof DMBoundaryType #}
  alignment _ = {# alignof DMBoundaryType #}
  peek = peek
  poke = poke

Internal.hs

{-# LANGUAGE QuasiQuotes, TemplateHaskell ,GeneralizedNewtypeDeriving, StandaloneDeriving ,DeriveDataTypeable, DataKinds, OverloadedStrings #-}

module Internal where

import TypesC2Hs

import qualified Language.C.Inline         as C
import qualified Language.C.Types          as CT
import           Language.C.Inline.Context

import qualified Language.Haskell.TH       as TH

import           Data.Monoid               ((<>), mempty)
import qualified Data.Map                  as Map


ctx :: Context
ctx = baseCtx <> funCtx <> vecCtx <> bsCtx <> pctx where
  pctx = mempty {ctxTypesTable = typesTable}


typesTable :: Map.Map CT.TypeSpecifier TH.TypeQ  
typesTable = Map.fromList
              [ (CT.TypeName "DMBoundaryType", [t| DMBoundaryType |])  ]

InlineC.hs

dmdaCreate1d0' cc bx m dof s =
   withPtr ( \ dm -> [C.exp|int{DMDACreate1d($(int c),
                                              $(DMBoundaryType bx),
                                              $(PetscInt m),
                                              $(PetscInt dof),
                                              $(PetscInt s),
                                              NULL,
                                              $(DM* dm))}|]  )
  where c = unComm cc

Solution

C enum is not marshallable foreign type, that is what compiler tries to tell you. To work around it, pass it as a CInt using fromEnum (looks like c2hs now supports it via hooks, but I never tried it.)