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Comments: In-reply-to rixed@happyleptic.org
   message dated "Mon, 05 Sep 2011 09:25:53 +0200."
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Date: Mon, 05 Sep 2011 11:16:33 -0400
From: John Carr <jfc@mit.edu>
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Subject: Re: [Caml-list] Conditionally boxed 32 bit integers?


rixed@happyleptic.org wrote:

> I suppose you don't want to use any kind of preprocessor, do you ?

I'd rather not preprocess (or compile alternate files).  Both the
netnumber and core implementations use a preprocessor.

Preprocessor-based solutions have two problems.  One is the extra
compilation step.  Another is bitrot in the unused case.

I think it was Plan 9 where the documentation said don't use the
C preprocessor, our compiler does the right thing with if (false).
In ocaml what I'd like to say is

	module Int : sig
	  type t
	  val add : t -> t -> t
	  ...
	end;
	module Intfast : Int = struct type t = int ... end
	module Int = if sixtyfour then Int end else Int32

with the understanding that the expression sixtyfour is evaluated
at compile time.

Here is some similar code using 3.12 syntax:

	module type M = sig type t val add : t -> t -> t val of_int : int -> t end;;
	module Int : M = struct type t = int let add = (+) let of_int x = x end;;
	module M = (val (if Sys.word_size = 64 then (module Int : M) else (module Int32 : M)) : M);;

	let f x = M.add (M.of_int x) (M.of_int 1)

If the test (Sys.word_size = 64) is replaced by (true) this does what
I want.  The compiler looks through the packing and unpacking and
sees the module Int.  The body of function f compiles to a machine add
instruction.

Ocamlopt does not evaluate (Sys.word_size = 64) at compile time
because Sys.word_size is initialized at runtime.  See sys.ml line 26
and run objinfo on sys.cmx.

INRIA developers, is it easy to add an intrinsic so we can write

	extern word_size : int = "%caml_word_size"

in sys.ml?  If sys.cmx has a constant definition, ocamlopt should do
constant folding on conditional expressions testing Sys.word_size.

If I want to do conditional compilation, can ocamlbuild be taught
"compile file a.ml as module X on 32 bit systems and module b.ml as
module X on 64 bit systems"?

    --John Carr (jfc@mit.edu)