Re: [MLton] Changing MLton.Pointer.word

On Mon, Jun 23, 2014 at 10:24 AM, Christopher Cramer
<[email protected]> wrote:
> On Mon, Jun 23, 2014 at 08:03:23AM +0200, Florian Weimer wrote:
>> I would like to change MLton.Pointer.word to Word32.word on 32-bit
>> architectures, and Word64.word on 64-bit architectures.
>
> If you need a word the size of a pointer, it's actually already available
> in the C_Pointer structure, made available in $(SML_LIB)/basis/c-types.mlb.
> See http://mlton.org/ForeignFunctionInterfaceTypes.

There are also C_Size, C_Ptrdiff, C_Intptr, and C_UIntptr, all of
which should correspond to an integer or word of the appropriate,
target-dependent bit size.  (Technically, the C standard does not
require all of these types to have the same number of bits, though in
practice I suspect that they almost always do.)

>> What's the best way to implement this?  Compiler hacking with primitives?
>> Changing MLB files?  Some form of preprocessing?
>
> The easiest way to do it would be to just include that c-types.mlb
> in basis-library/build/sources.mlb, and put "type word = C_Pointer.word"
> in basis-library/mlton/pointer.sml.
>
> Although now that I think about it, just having the type is kind of
> useless, since you can't do anything with it without the functions. You
> probably want to include the whole C_Pointer structure somehow.

Well, if you propagate the type equality of MLton.Pointer.t and
C_Pointer.word, then you can simply use the C_Pointer.* operations on
MLton.Pointer.t values.  There isn't a need to include the WORD
signature in the MLTON_POINTER signature.

But, I'm not sure that that would work with the current MLton
implementation.  The various MLton.Pointer.* operations are primitives
and expect that MLton.Pointer.t is a distinct abstract type.

A better approach might be to simply expose the MLton.Pointer.toWord
and MLton.Pointer.fromWord operations that are provided as primitives,
but not exported.  These convert to and from C_Size.word values.
Similarly, the MLton.Pointer.{add, sub} operations are internally
implemented by primitives that use C_Ptrdiff.int values, but for
convenience, are exposed and wrapped to use Word.word values.

It might also help if you could describe in a little more detail
how/why you want to use MLton.Pointer.t values as Word<N>.word values.

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