[Haskell-beginners] FFI export lazy list of string

[Alexander.Vladislav.Popov]

Hello Edward,

Would you help me to map this:

-- genexlib.hs
{-# LANGUAGE BangPatterns, ForeignFunctionInterface #-}

module GenexLib where

import Regex.Genex
import System.IO
import System.Environment

data CLazyList a = Empty | CLL !a [a]

instance (Show a) => Show (CLazyList a) where
  show Empty = "Empty"
  show (CLL x xs) = show x ++ ":.."

next (CLL _ [])     = Empty
next (CLL _ (x:xs)) = CLL x xs

fromList []     = Empty
fromList (x:xs) = CLL x xs

open = fromList . genexPure
-- end genexlib.hs

to

-- genex.c
typedef struct CLL {
  unsigned char empty;
  char          *current;
  struct CLL    *next;
} CLL;

CLL *open(const char *regex);
CLL *next(const CLL  *cll);

// void printf(const char *s, ...);

void usage() {
  CLL *genex = open("\\d+");
  while(!genex->empty) {
    printf(genex->current);
    genex = next(genex->next);
  }
}
-- end genex.c

Alexander Popov


2011/12/6 Edward Z. Yang <ezyang at mit.edu>

> Hello Alexander,
>
> Your best bet is to make a Haskell functions which gives access to
> the head and tail of a list (the head should be converted into a C-friendly
> form) and then export them as functions which are callable from C.
>
> http://www.haskell.org/haskellwiki/GHC/Using_the_FFI#Calling_Haskell_from_C
>
> Edward
>
> Excerpts from Alexander.Vladislav.Popov's message of Mon Dec 05 03:08:59
> -0500 2011:
> > Hi, Haskellers.
> >
> > Advise me please, how I can export lazy and potentially infinite list of
> > string from Haskell program. I think I must call it iteratively: the
> first
> > call initiate some structure and other calls iterate over it, something
> > like pair of function `find_first' and `find_next'. And how to marshall
> > this structure between programs. Or think in a wrong way? Does any
> example
> > exist how I can make it?
> >
> > Alexander Popov
>
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