CPP: Macros in Haskell

Question

I'm using bindings-DSL to help deal with some boilerplate in FFI declarations. But I find myself declaring groups of related functions that differ by only a couple of textual elements, and I'd really rather declare these with a macro. CPP or CPPHS seems the ideal choice for this, but I can't find any examples to its usage in the context of Haskell.

I've put this into what I approximately expect to work from my knowledge of C macros:

#define declare_vector_funcs (t, tn, ct) \ 
    #opaque_t vector_##t \
    #ccall create_std_vector##tn , IO (Ptr <vector_##t##>) \
    #ccall carray_to_std_vector##tn , Ptr ct -> CSize -> IO (Ptr <vector_##t##>) \
    #ccall std_vector##tn##_to_carray , Ptr <vector_##t##> -> IO (Ptr ct) \
    #ccall std_vector##tn##_length , Ptr <vector_##t##> -> IO CSize

Essentially, I'd like to define a foreign (opaque) type and 4 foreign functions upon expanding this macro. However, this does not work as it reads everything subsequent to the argument list as GHC pragmas, and fails.

I've already tried a couple different iterations of this, such as messing with spacing and putting everything on one line (wrapped in parentheses to distinguish separate macro calls).

How can I fix this to work? Answers that drop the bindings-DSL usage in favor of a direct translation are fine, but I definitely don't want to write all this out by hand.

I would also very much appreciate a few examples of this kind of CPP usage.

Here's the error message I get if I remove the space between the macro name and argument list:

CPP.hsc:13:39: error: '#' is not followed by a macro parameter
compiling dist/build/Foreign/CPP_hsc_make.c failed (exit code 1)
command was: /usr/bin/g++ -c dist/build/Foreign/CPP_hsc_make.c -o dist/build/Foreign/CPP_hsc_make.o -fno-stack-protector -D__GLASGOW_HASKELL__=708 -Dlinux_BUILD_OS=1 -Dx86_64_BUILD_ARCH=1 -Dlinux_HOST_OS=1 -Dx86_64_HOST_ARCH=1 -Iinclude/ -fpermissive -std=c++11 -fPIC -Idist/build/autogen -include dist/build/autogen/cabal_macros.h -I/usr/local/lib/x86_64-linux-ghc-7.8.2/bindings-DSL-1.0.21/include -I/usr/lib/ghc-7.8.2/base-4.7.0.0/include -I/usr/lib/ghc-7.8.2/integer-gmp-0.5.1.0/include -I/usr/lib/ghc-7.8.2/include -I/usr/lib/ghc-7.8.2/include/

With the space, I get a much longer error message:

dist/build/Foreign/CPP.hs:1:16:
    unknown flag in  {-# OPTIONS_GHC #-} pragma: tn,
dist/build/Foreign/CPP.hs:1:16:
    unknown flag in  {-# OPTIONS_GHC #-} pragma: ct)
dist/build/Foreign/CPP.hs:1:16:
    unknown flag in  {-# OPTIONS_GHC #-} pragma: #opaque_t
dist/build/Foreign/CPP.hs:1:16:
    unknown flag in  {-# OPTIONS_GHC #-} pragma: vector_##t

And this continues for every token. I'm fairly certain this just means the space shouldn't be included, but I'm not really sure what's going on.

---

EDIT:

New info.

I swapped methodology and I'm trying to generate the final foreign imports directly. The macro (I'll paste in a sec) passes the preprocessor with a couple of warnings, but actually trying to use the macro doesn't yet work:

#define declare_vector_funcs(t, tn ,ct) \
    data C'vector_##t = C'vector_##t \
    foreign import ccall "create_std_vector##tn" c'create_std_vector##tn :: IO (Ptr C'vector_##t) \
    foreign import ccall "carray_to_std_vector##tn" c'carray_to_std_vector##tn :: Ptr ct -> CSize -> IO (Ptr vector_##t) \
    foreign import ccall "std_vector##tn##_to_carray" c'std_vector##tn##_to_carray :: Ptr vector_##t -> IO (Ptr ct) \
    foreign import ccall "std_vector##tn##_length" c'std_vector :: Ptr vector_##t -> IO CSize

At the usage site, I'm trying to use it like:

#declare_vector_funcs int , i , CInt

To match the equivalent declaration on the C side. I expect it to generate a block that looks like:

data C'vector_int = C'vector_int 
foreign import ccall "create_std_vectori" c'create_std_vectori :: IO (Ptr C'vector_int) 
foreign import ccall "carray_to_std_vectori" c'carray_to_std_vectori :: Ptr CInt -> CSize -> IO (Ptr vector_int) 
foreign import ccall "std_vectori_to_carray" c'std_vectori_to_carray :: Ptr vector_int -> IO (Ptr CInt) 
foreign import ccall "std_vectori_length" c'std_vector :: Ptr vector_int -> IO CSize

But instead I get an error:

CPP.hsc: In function ‘int main(int, char**)’:
CPP.hsc:22:31: error: expected primary-expression before ‘int’
CPP.hsc:22:37: error: ‘i’ was not declared in this scope
CPP.hsc:22:41: error: ‘CInt’ was not declared in this scope
CPP.hsc:22:45: error: ‘hsc_declare_vector_funcs’ was not declared in this scope
CPP.hsc:23:31: error: expected primary-expression before ‘float’
CPP.hsc:23:39: error: ‘f’ was not declared in this scope
CPP.hsc:23:43: error: ‘CFloat’ was not declared in this scope
CPP.hsc:24:31: error: expected primary-expression before ‘double’
CPP.hsc:24:40: error: ‘d’ was not declared in this scope
CPP.hsc:24:44: error: ‘CDouble’ was not declared in this scope

So I obviously need to add import Foreign.C to the top, but even so, there's a deeper issue -- I can't treat these tokens as meaningless like I'd like to, something is trying to actually interpret them. Anyone have any ideas?

Answer 1

What you are trying to do isn't quite possible, I don't think, when you define a macro in a hsc file it will get inlined into your generated .hs file, and if that macro calls .hsc macros.. well those won't be available anymore. The best you can do:

module Test where

import Foreign.Ptr
import Foreign.C.Types

data T
data Tn
data Ct

#opaque_t vector_T
#ccall create_std_vectorTn , IO (Ptr <vector_T>)
#ccall carray_to_std_vectorTn , Ptr Ct -> CSize -> IO (Ptr <vector_T>) 
#ccall std_vectorTn_to_carray , Ptr <vector_T> -> IO (Ptr Ct) 
#ccall std_vectorTn_length , Ptr <vector_T> -> IO CSize

If your C compiler knows where to find bindings.dsl.h then you should include it at the top; mine doesn't so I compile with hsc2hs --include=<path-to-bindings.dsl.h> test.hsc. This produces the following file:

{-# LINE 1 "test.hsc" #-}
module Test where
{-# LINE 2 "test.hsc" #-}

import Foreign.Ptr
import Foreign.C.Types

data T
data Tn
data Ct

data C'vector_T = C'vector_T

{-# LINE 11 "test.hsc" #-}
foreign import ccall "create_std_vectorTn" c'create_std_vectorTn
  :: IO (Ptr C'vector_T)
foreign import ccall "&create_std_vectorTn" p'create_std_vectorTn
  :: FunPtr (IO (Ptr C'vector_T))

{-# LINE 12 "test.hsc" #-}
foreign import ccall "carray_to_std_vectorTn" c'carray_to_std_vectorTn
  :: Ptr Ct -> CSize -> IO (Ptr C'vector_T)
foreign import ccall "&carray_to_std_vectorTn" p'carray_to_std_vectorTn
  :: FunPtr (Ptr Ct -> CSize -> IO (Ptr C'vector_T))

{-# LINE 13 "test.hsc" #-}
foreign import ccall "std_vectorTn_to_carray" c'std_vectorTn_to_carray
  :: Ptr C'vector_T -> IO (Ptr Ct)
foreign import ccall "&std_vectorTn_to_carray" p'std_vectorTn_to_carray
  :: FunPtr (Ptr C'vector_T -> IO (Ptr Ct))

{-# LINE 14 "test.hsc" #-}
foreign import ccall "std_vectorTn_length" c'std_vectorTn_length
  :: Ptr C'vector_T -> IO CSize
foreign import ccall "&std_vectorTn_length" p'std_vectorTn_length
  :: FunPtr (Ptr C'vector_T -> IO CSize)

{-# LINE 15 "test.hsc" #-}

---

So looks like defining your own macro is the way to go:

mymacro.h

#define hsc_declare_vector_funcs(t,tn,ct)\
  hsc_opaque_t(vector_##t)\
  hsc_ccall(create_std_vector##tn ,IO (Ptr <vector_##t>))\
  hsc_ccall(carray_to_std_vector##tn , Ptr ct -> CSize -> IO (Ptr <vector_##t>)) \
  hsc_ccall(std_vector##tn##_to_carray , Ptr <vector_##t> -> IO (Ptr ct)) \
  hsc_ccall(std_vector##tn##_length , Ptr <vector_##t> -> IO CSize)

test.hsc

module Test where

#include "mymacro.h"

import Foreign.Ptr
import Foreign.C.Types

#declare_vector_funcs int, i, CInt

test.hs

module Test where

import Foreign.Ptr
import Foreign.C.Types

data C'vector_int = C'vector_int
foreign import ccall "create_std_vectori" c'create_std_vectori
  :: IO (Ptr C'vector_int)
foreign import ccall "&create_std_vectori" p'create_std_vectori
  :: FunPtr (IO (Ptr C'vector_int))
foreign import ccall "carray_to_std_vectori" c'carray_to_std_vectori
  :: Ptr CInt -> CSize -> IO (Ptr C'vector_int)
foreign import ccall "&carray_to_std_vectori" p'carray_to_std_vectori
  :: FunPtr (Ptr CInt -> CSize -> IO (Ptr C'vector_int))
foreign import ccall "std_vectori_to_carray" c'std_vectori_to_carray
  :: Ptr C'vector_int -> IO (Ptr CInt)
foreign import ccall "&std_vectori_to_carray" p'std_vectori_to_carray
  :: FunPtr (Ptr C'vector_int -> IO (Ptr CInt))
foreign import ccall "std_vectori_length" c'std_vectori_length
  :: Ptr C'vector_int -> IO CSize
foreign import ccall "&std_vectori_length" p'std_vectori_length
  :: FunPtr (Ptr C'vector_int -> IO CSize)

Answer 2

The following example compiles:

{-# LANGUAGE CPP #-}

#define MYMACRO 3
#define MAX(a,b) (if ((a) < (b)) \
              then (b) \
              else (a))

main = print $ show (MAX(MYMACRO,4) :: Int)

I need the parens around the code macro so that the following will (also) work:

main = print $ show MAX(MYMACRO,4)