Hi all, I've been trying to call a foreign function (i.e. via the FFI) inside a splice, which doesn't seem to work. As an example, I'll attach a small program which does work, and then modify it so it doesn't work. So, here's a program which does work: Splicer.hs:

module Main where

import Templates

main :: IO () main = do putStrLn (show ($(doubleWrapper) 5))

Templates.hs:

module Templates where

import Language.Haskell.THSyntax

doubleWrapper :: Expr doubleWrapper = [| \x -> myDouble x |]

myDouble :: Int -> Int myDouble x = x * 2

Pretty simple: the program splices in an Expr named doubleWrapper; doubleWrapper in turn calls myDouble, which is a simple function of type Int -> Int. Execution:

20:10 ~/th/ffi-bug % ghc -fglasgow-exts --make Splicer.hs ... 20:10 ~/th/ffi-bug % ./a.out 10

So that works. Here's a version of the program which modifies the 'myDouble' function so that it's now a foreign C function rather than a Haskell function. (Note that the main function and doubleWrapper functions are exactly the same): SplicerFFI.hs:

module Main where

import TemplatesFFI

main :: IO () main = do putStrLn (show ($(doubleWrapper) 5))

TemplatesFFI.hs:

module TemplatesFFI where

import Language.Haskell.THSyntax

doubleWrapper :: Expr doubleWrapper = [| \x -> myDouble x |]

foreign import ccall safe "c_double.h myDouble" myDouble :: Int -> Int

c_double.h:

int myDouble (int x);

c_double.c:

int myDouble (int x) { return x * 2; }

Compiling the FFI version of this small program doesn't work:

20:20 ~/th/ffi-bug % gcc -c c_double.c 20:20 ~/th/ffi-bug % ghc -fglasgow-exts --make c_double.o SplicerFFI.hs c_double.o Chasing modules from: SplicerFFI.hs Compiling TemplatesFFI ( TemplatesFFI.hs, ./TemplatesFFI.o ) Compiling Main ( SplicerFFI.hs, ./SplicerFFI.o ) Loading package base ... linking ... done. Loading package haskell98 ... linking ... done. Loading package haskell-src ... linking ... done.

TemplatesFFI.o: unknown symbol `_myDouble'

SplicerFFI.hs:7: Exception when trying to run compile-time code: Code: doubleWrapper Exn: In the first argument of `show', namely `($[splice]doubleWrapper 5)' In the first argument of `putStrLn', namely `(show ($[splice]doubleWrapper 5))' In the result of a 'do' expression: putStrLn (show ($[splice]doubleWrapper 5))

I suspect the compilation isn't working because the 'myDouble' symbol in c_double.o isn't being loaded _at splice time_ into the list of symbols that GHC knows about -- which is also why the exception occurs. I've tried all sorts of kludges to convince GHC to load the symbol from the foreign object file, like doing "ar rcs libcdouble.a c_double.o" and then putting -L. -lcdouble on the commandline, passing additional linker/compiler flags with -optl and -optc, etc, and none of them have worked so far. This was tested with GHC on Mac OS X built from CVS 9 March 2003; my apologies for a fairly out-of-date GHC build, but I'm having trouble building the recent GHCs on Mac OS X. Can somebody verify that this also occurs on Linux with a recent build of GHC, and hopefully fix it or give me a clue on where to start looking for the bug? Thanks very much! -- % Andre Pang : just.your.average.bounty.hunter