[Haskell-cafe] Writing and testing a Storable instance for a record wrapping ForeignPtrs

[Oliver Charles]

You are strict in your definition of `sizeOf`. You probably want:

  sizeOf ~SignedMessage{messageLength} = ...

On Mon, 28 Nov 2022, at 4:04 PM, Hécate wrote:
> Hi everyone,
> 
> 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.
> 
> 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-signing-hs
> 
> 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-undefined_error-txt
> 
> 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-package/sel/src/Sel/Signing.hs
> 
> I'm not sure how to proceed from here. What would be a good angle to 
> approach this?
> 
> Cheers,
> Hécate
> 
> -- 
> Hécate ✨
> 🐦: @TechnoEmpress
> IRC: Hecate
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> 
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