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Re: [Haskell-cafe] FFI, C/C++ and undefined references

Subject: Re: [Haskell-cafe] FFI, C/C++ and undefined references
From: DNM
Date: Wed, 13 Jan 2010 20:07:43 -0800 PST
Bulat,

Some very good suggestions.  I will try to appease Ceiling Cat and
reduce my (perhaps gratuitous) use of unsafePerformIO.  I'm going to
have to use it somewhere, since I want referentially transparent code
(and I try to avoid the IO monad when possible, anyway).

> 2. if your function returns Ptr a - then hold in haskell types this Ptr a.
> no need to convert it back and forth to ForeignPtr
Yes, I thought of doing this, but then thought it was better to use a
so-called
"managed" foreign pointer via newForeignPtr.  I thought this was the best
way to have a foreign pointer that the Haskell garbage collector would eat
up when
it was no longer in use.  I could be wrong.  I convert from a ForeignPtr to
a Ptr, 
because the FFI code wasn't compiling at all (nevermind the missing C/C++
reference 
problem), as apparently a ForeignPtr isn't the sort of thing that an
imported foreign 
function can take as an argument (or so said GHC). I just assumed that the 
back-and-forth between Ptr and ForeignPtr would be compiled away by GHC.  
I could be wrong, though. If performance starts to suffer, I'll manage the
Ptr memory 
in my code directly.

> 4. i don't looked in your code but if C functions defines
> *modifiable* datastructure - you should use it at Haskell side via
> imperatiove functions, i.e. those with return type IO a. using
> unsafePerformIO in this case will lead to Haskell compiler will
> consider this datatype as permanent and reorder operations on the will

Good point.  I would do this if I planned to train or update the language
model 
from within Haskell, but, as it stands, I just want to train it once (at the
command line, 
using the built-in mechanisms of SRILM) and then read in the ARPA-formatted
language 
model file for use in Haskell.

> 3. why c_dlm is FunPtr in your definition? it should be
> 
> foreign import ccall "srilm.h deleteLM"
>    c_dlm :: Ptr Ngram -> IO ()
No reason.  Just because I don't know what I'm doing yet.
Thanks for the correction.

Thanks for the help, Bulat.  Much appreciated.

Best,
Dennis
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