No default marshallers for Char

[RyanGlScott]

Currently, C's char types can be marshalled to and from Integral types via the default marshaller fromIntegral. Unfortunately, this makes it difficult to use Haskell's own Char type alongside C's, since Char is not an instance of Integral. For example,

/* foo.h */
#ifndef FOO_H
#define FOO_H

char identichar(char c);
unsigned char identiuchar(unsigned char c);
signed char identischar(signed char c);

#endif

/* foo.c */
#include "foo.h"

char identichar(char c) {
    return c;
}

unsigned char identiuchar(unsigned char c) {
    return c;
}

signed char identischar(signed char c) {
    return c;
}

-- Main.chs
{-# LANGUAGE ForeignFunctionInterface #-}
module Main where

import Foreign.C.String

#include "foo.h"

{#fun pure identichar  as ^ { `Char' } -> `Char' #}
{#fun pure identiuchar as ^ { `Char' } -> `Char' #}
{#fun pure identischar as ^ { `Char' } -> `Char' #}

main :: IO ()
main = print $ map ($ 'A') [identichar, identiuchar, identischar]

Preprocessing this with c2hs reveals this:

c2hs: Errors during expansion of binding hooks:

./Main.chs:9: (column 43) [ERROR]  >>> Missing "out" marshaller!
  There is no default marshaller for this combination of Haskell and C type:
  Haskell type: Char
  C type      : CSChar
./Main.chs:8: (column 43) [ERROR]  >>> Missing "out" marshaller!
  There is no default marshaller for this combination of Haskell and C type:
  Haskell type: Char
  C type      : CUChar
./Main.chs:7: (column 43) [ERROR]  >>> Missing "out" marshaller!
  There is no default marshaller for this combination of Haskell and C type:
  Haskell type: Char
  C type      : CChar

So one must manually provide out marshallers:

import Foreign.C.String
import Foreign.C.Types

{#fun pure identichar  as ^ { `Char' } -> `Char' castCCharToChar  #}
{#fun pure identiuchar as ^ { `Char' } -> `Char' castCUCharToChar #}
{#fun pure identischar as ^ { `Char' } -> `Char' castCSCharToChar #}

This, in turn, reveals the lack of in marshallers for Char:

c2hs: Errors during expansion of binding hooks:

./Main.chs:11: (column 31) [ERROR]  >>> Missing "in" marshaller!
  There is no default marshaller for this combination of Haskell and C type:
  Haskell type: Char
  C type      : CSChar
./Main.chs:10: (column 31) [ERROR]  >>> Missing "in" marshaller!
  There is no default marshaller for this combination of Haskell and C type:
  Haskell type: Char
  C type      : CUChar
./Main.chs:9: (column 31) [ERROR]  >>> Missing "in" marshaller!
  There is no default marshaller for this combination of Haskell and C type:
  Haskell type: Char
  C type      : CChar

So the final workaround ends up being pretty verbose:

{#fun pure identichar  as ^ { castCharToCChar  `Char' } -> `Char' castCCharToChar  #}
{#fun pure identiuchar as ^ { castCharToCUChar `Char' } -> `Char' castCUCharToChar #}
{#fun pure identischar as ^ { castCharToCSChar `Char' } -> `Char' castCSCharToChar #}

Is it possible to have these marshallers be the defaults for Haskell's Char?

[RyanGlScott]

I didn't see it earlier, but it looks like this is closely related to issues #20, #25, and #48 (i.e., map Haskell type X to C type x_t).

[ian-ross]

Oh, these are nasty. I have them filed under "deferred for the moment as too hard to fix", but maybe I should go back and think about them a bit more.

[RyanGlScott]

Thank you for fixing this—this should save me from typing out quite a bit of boilerplate!

By the way, can you update this wiki article to reflect that Char has new default marshallers?