Re: calling void foo(void) C functions from haskell

[Manuel M. T. Chakravarty]

Peter Amstutz <[EMAIL PROTECTED]> wrote,

> I'm experimenting with the haskell FFI, and have run into a odd little
> problem.  For some reason, ghc won't let me import functions with no
> arguments...
> 
[...]
> And my first try (for.hs)
> 
> foreign import ccall "hiworld.so" "hiworld" hiworld :: () -> IO () 
> 
> main = hiworld ()
> 
> and I get:
> $ ghc for.hs hw.so -fglasgow-exts
> for.hs:5:
>     Unacceptable argument type in foreign declaration: ()
>     When checking declaration:
>         foreign import _ccall "hiworld.so" "hiworld" hiworld :: () -> IO
> ()

Try

  foreign import ccall "hiworld.so" "hiworld" hiworld :: IO () 

In other words, a function of no arguments is a constant.
Remember that evaluating a monad expression of `IO ()' does
not yet execute the I/O operation - it merely produces an
``action'' that, once executed, performs the I/O operation.
This is a fine, but very important difference.

Manuel