[Haskell-cafe] Accessing to a Haskell type in C

Antoine Latter aslatter at gmail.com
Thu Mar 31 00:44:40 CEST 2011

On Wed, Mar 30, 2011 at 5:27 PM, Yves Parès <limestrael at gmail.com> wrote:
> Hello,
> To access a haskell type from C code, do we have to make this type instance
> of Storable (and so to also define an homomorph structure on the C side), or
> does FFI specifies some C function to enable C code to access the fields, or
> even some GHC-specific functions? (Even if I'd prefer a portable way)
> I mean, if I have the type:
> data Character = Character {
>     pos :: (Float, Float),
>     size :: Float,
>     sprite :: Image
> }
> Do I compulsorily have to double its definition on C side:
> stuct CCharacter {
>     float x;
>     float y;
>     float size;
>     Image sprite;
> }
> and then make Character instance of Storable, which will make the program
> *copy* Characters to CCharacters each time that I must call a C function to,
> say, draw a character?

I haven't needed to access Haskell data from C, but the way I see it
you have two choices:

* Write a Storable instance for your Haskell type, as you said. Be
sure to use the Haskell CTypes as intermediaries so you don't go crazy
on the other side.

* StablePtr magic and foreign-exported wrapper functions to do all of
your data-manipulation in Haskell. There's less duplication this way,
but perhaps more boiler-plate. Plus you need to manage the lifetime of
the StablePtr.

I don't know of anything else off-hand. Most people focus on going the
other way (using C types in Haskell).


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