On Mon, 2022-11-28 at 17:04 +0100, Hécate wrote:
In a fit of madness I have found myself writing Haskell code where I need to implement a Storable instance. However, by virtue of not being a C programmer, I'm fairly lost on some of the details, especially the value of the sizeOf and alignment methods.
Next to the other replies, did you consider using `hsc2hs` to create these bindings? Using the tool, you can use its `#{size ...}`, `#{alignment ...}` and `#{peek ...}`/`#{poke ...}` helpers to implement `Storable` for some C struct, without running a C compiler in your head (e.g., taking padding into account to implement `sizeOf`). See the docs at https://ghc.gitlab.haskell.org/ghc/doc/users_guide/utils.html#writing-haskel.... Note `hsc2hs` is fully integrated in Cabal (use `Foo.hsc` as module filename), no need to reach out to its CLI. You can find a simple example of a Storable instance using it at https://github.com/NicolasT/landlock-hs/blob/b5638684869ad4f85bea53f10a3f0b9.... Nicolas
My Haskell-level record is the following:
data SignedMessage = SignedMessage { messageLength :: CSize , messageForeignPtr :: ForeignPtr CUChar , signatureForeignPtr :: ForeignPtr CUChar }
Here is the code of the Storable instance: https://gist.github.com/Kleidukos/31346d067f309f2a86cbd97a85c0f1e8#file-sign...
And so I used `hedgehog-classes` to test the Storable instance. However, all the tests fail with the same reason: Prelude.undefined:
https://gist.github.com/Kleidukos/31346d067f309f2a86cbd97a85c0f1e8#file-unde...
The main problem (and that's certainly a red herring for me) is that the `undefined` call comes frombase:Foreign.Marshal.Array. Which shouldn't be a problem, as it is not supposed to be evaluated! Yet apparently it is.
If you're interested to see the full code, it's located here: https://github.com/haskell-cryptography/libsodium-bindings/blob/add-sel-pack...
I'm not sure how to proceed from here. What would be a good angle to approach this?
Cheers, Hécate