From: Marcin 'Qrczak' Kowalczyk <qrczak@...>
Date: 2002-08-26T19:11:44+09:00
Subject: Re: A Repeat: New Language After Ruby?

Fri, 23 Aug 2002 05:57:05 +0900, William Djaja Tjokroaminata <billtj@y.glue.umd.edu> pisze:

> That's why I really hesitate to start learning Haskell. It seems
> Haskell can only be interfaced with C through some intermediary
> called "Greed Card" and not directly as between C and Ruby.

You don't have to use GreenCard (which is kind of obsolete). You can
write glue code by hand; most of glue is on the Haskell side. The
language offers the ability to call C functions directly from Haskell,
provided that you have arguments ready in Haskell types corresponding
to C types:

foreign import "fnmatch" unsafe
   fnmatchC :: Ptr CChar -> Ptr CChar -> CInt -> IO CInt

and offers a library for converting basic types and managing C memory.
Here is how fnmatch might be wrapped in Haskell: pattern and string
are Haskell strings converted to Ptr CChar values pointing to temporary
buffers, and flags is a list of values instead of a bitmask:

fnmatch :: String -> String -> [MatchFlag] -> Bool
fnmatch pat str flags = (
    unsafePerformIO $
    withCString pat $ \patPtr ->
    withCString str $ \strPtr ->
    fnmatchC patPtr strPtr flagsInt) /= 0
  where
    flagsInt = (flag FnmFileName   (#const FNM_FILE_NAME))   .|.
               (flag FnmPeriod     (#const FNM_PERIOD))      .|.
               (flag FnmNoEscape   (#const FNM_NOESCAPE))    .|.
               (flag FnmLeadingDir (#const FNM_LEADING_DIR)) .|.
               (flag FnmCaseFold   (#const FNM_CASEFOLD))
    flag cons bit = if cons `elem` flags then bit else 0

The #const construct is not real Haskell and this file must be run
through hsc2hs preprocessor which replaces them with integer literals
corresponding to these C constants.

You can convert Haskell functions to C function pointers (requires
nonportable magic from the Haskell compiler to build code on the
heap) and directly access C variables (i.e. obtain the pointer to
the variable from Haskell and use Haskell functions to read and write
through it).

-- 
  __("<      Marcin Kowalczyk
  \__/     qrczak@knm.org.pl
   ^^    http://qrnik.knm.org.pl/~qrczak/