[Haskell-cafe] FFI for a beginner

[Simon Peyton-Jones]

FIXED IT!!!  The problem with charm.hs and its FFI dependency charm.c is that both want to produce an intermediary charm.o file. Solution: rename charm.hs to hscharm.hs.

Would you (or anyone else) care to update the tutorials on the Haskell Wiki?  As someone wrote below:
While the tutorials at HaskellWiki<http://www.haskell.org/haskellwiki/FFI_complete_examples> are helpful, they're outdated. Argh!

It would be great if someone was public-spirited enough to fix this. The root page is here
                http://haskell.org/haskellwiki/GHC/Using_the_FFI
which in turn links to the "complete examples" page mentioned above.

The docs<http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi-ghc.html#glasgow-foreign-headers> say that -#include pragmas no longer work, but fail to explain how to load code without them. Suffice to say I have no recourse but trial and error."

Ah, now that is a GHC documentation question.  Can you tell us what should the docs should say instead?  Then we can fix the docs.

Simon



From: haskell-cafe-bounces at haskell.org [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of Andrew Pennebaker
Sent: 14 July 2011 07:57
To: Jason Dagit
Cc: Haskell Cafe
Subject: Re: [Haskell-cafe] FFI for a beginner

FIXED IT!!!

The problem with charm.hs and its FFI dependency charm.c is that both want to produce an intermediary charm.o file.

Solution: rename charm.hs to hscharm.hs.

Cheers,

Andrew Pennebaker
www.yellosoft.us<http://www.yellosoft.us>


On Tue, Apr 12, 2011 at 5:55 PM, Andrew Pennebaker <andrew.pennebaker at gmail.com<mailto:andrew.pennebaker at gmail.com>> wrote:
hsc2hs and c2hs are good suggestions, and some of the tutorials I'm following use them.

But 1) Many Haskell FFI tutorials don't require them, so they only seem to help, or only help in older versions of GHC.

And 2) When I did compile using c2hs, it just produced the same file, but with filler comments like {#- LINE 13 #-}. And they still refused to compile for the same reason:

ld: duplicate symbol _Charm_getWidth_info in charm.o and charm.o

Cheers,

Andrew Pennebaker
www.yellosoft.us<http://www.yellosoft.us>

On Sat, Apr 9, 2011 at 9:50 PM, Jason Dagit <dagitj at gmail.com<mailto:dagitj at gmail.com>> wrote:

I don't know how to make ghc load them without using either hsc2hs or c2hs. I've had better experiences with hsc, but your mileage may vary.
On Apr 8, 2011 3:34 AM, "Andrew Pennebaker" <andrew.pennebaker at gmail.com<mailto:andrew.pennebaker at gmail.com>> wrote:

ncurses is proving too difficult to setup, so I'm working on a new library called charm. The C code works by itself, but I can't compile a Haskell wrapper for it. While the tutorials at HaskellWiki<http://www.haskell.org/haskellwiki/FFI_complete_examples> are helpful, they're outdated. Argh! The docs<http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi-ghc.html#glasgow-foreign-headers> say that -#include pragmas no longer work, but fail to explain how to load code without them. Suffice to say I have no recourse but trial and error.

GitHub: charm<https://github.com/mcandre/charm> and hscharm<https://github.com/mcandre/hscharm>

$ make
cp /usr/include/charm.c .
ghc --make -fforce-recomp -o hellocharm hellocharm.hs charm.hs charm.c -I/usr/include -dylib-install-name /usr/lib/libcharm.dynlib
[1 of 2] Compiling Charm            ( charm.hs, charm.o )
[2 of 2] Compiling Main             ( hellocharm.hs, hellocharm.o )
Linking hellocharm ...
ld: duplicate symbol _Charm_getWidth_info in charm.o and charm.o
collect2: ld returned 1 exit status
make: *** [hellocharm] Error 1

Cheers,

Andrew Pennebaker
www.yellosoft.us<http://www.yellosoft.us>

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