[MLton-user] NLFFI: What is the correct way to convert C values to ML?

[Ivan Tomac]

Hi Matthew,

On Wed, Mar 2, 2011 at 2:26 AM, Matthew Fluet <matthew.fluet at gmail.com>wrote:

> It would help to know a little more about exactly where your C int is
> coming from to know the best way to convert it to an ML int.
>
> The differences between the MLRep structures in MLton and SML/NJ are a
> documented design decision:
>  http://mlton.org/MLNLFFIImplementation
>  http://mlton.org/pipermail/mlton-user/2007-February/001025.html
> I believe that the fine-grained C types better document an interface,
> even if the underlying SML implementation needs to coerce small sized
> values to larger sized values.
>
>
Thanks for the links. I remember having read the page at the first link a
while ago but could not remember where it was exactly I read that.


> You are meant to use MLRep.Int.Signed.toInt and
> MLRep.Int.Signed.fromInt to convert a C int to and from an SML Int.int
> (when using NLFFI generated code).  Of course, those conversions could
> fail due to Overflow if the C int size and the SML Int.int size
> differ.  But, note that the
> MLRep.{Char,Short,Int,Long,LongLong}.Signed structures match the
> INTEGER signature and the
> MLRep.{Char,Short,Int,Long,LongLong}.Unsigned structures match the
> WORD signature, so, often if one is doing simple arithmetic on these
> values, there isn't a need to explicitly convert it to an SML Int.int.
>
> If you are using NLFFI, my best suggestion is to introduce a
> compatibility structure.  For your MLton code, you just need the
> following:
>
> mlton-mlrep.sml:
> structure ZMLRep = MLRep
>
> For your SML/NJ code, you just need the following:
>
> smlnj-mlrep.sml:
> structure ZMLRep = struct
>  structure Char = struct
>    structure Signed = MLRep.Signed
>    structure Unsigned = MLRep.Unsigned
>    structure SignedBitops = MLRep.SignedBitops
>  end
>  structure Short = struct
>    structure Signed = MLRep.Signed
>    structure Unsigned = MLRep.Unsigned
>    structure SignedBitops = MLRep.SignedBitops
>  end
>  structure Int = struct
>    structure Signed = MLRep.Signed
>    structure Unsigned = MLRep.Unsigned
>    structure SignedBitops = MLRep.SignedBitops
>  end
>  structure Long = struct
>    structure Signed = MLRep.Signed
>    structure Unsigned = MLRep.Unsigned
>    structure SignedBitops = MLRep.SignedBitops
>  end
>  structure LongLong = struct
>    structure Signed = MLRep.LongLongSigned
>    structure Unsigned = MLRep.LongLongUnsigned
>    (* structure SignedBitops = MLRep.LongLongSignedBitops *)
>  end
>  structure Float = MLRep.Real
>  structure Double = MLRep.Real
> end
>
> Now, if you use ZMLRep structures to manipulate the C data, you should
> have code that works with both SML/NJ and MLton.
>
> Let me know if the above works.  I believe that it should, and it is
> an innocuous enough extension of SML/NJ's existing MLRep structure
> that we could probably get it added to the SML/NJ version of the NLFFI
> Library.
>

That is exactly what I've been using prior to posting the question. I just
wanted to check if there was a better, more portable way.
It would be good if both MLton and SML/NJ used the same structure.
I agree that MLton version of MLRep is better than the one in SML/NJ. I'll
open a feature request to update SML/NJ's MLRep structure and look into
submitting a patch.

Thanks,

Ivan
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