Re: [Caml-list] infix functions

[Richard Jones <rich@annexia.org>]

On Fri, May 27, 2005 at 10:06:10PM +1200, Jonathan Roewen wrote:
> Hi,
> 
> I see some pervasive functions are infix, and I'm wondering if there's
> any plan to support making any arbitrary infix functions?
> 
> For instance, the Int32 (etc) modules are horrible to use cause of the
> prefix functions. These are perfect candidates for being infix. And
> being an OS project, there are a lot of instances where we need the
> extra precision, and having to do things like add some_int32
> another_int32 complex. Especially when you have to throw in
> bitshifting, AND and OR, and other magic.

You can create infix operators in the basic language.  You have to use
the right first character in the operator - the scanner appears to use
the first character to decide whether the operator is infix or prefix.
This is rather obliquely documented here:

http://caml.inria.fr/pub/docs/manual-ocaml/manual009.html

(Look for the section "Prefix and infix symbols").

So:

	$ ocaml    
	        Objective Caml version 3.08.2

	# #load "nums.cma";;
	# let (+^) = Int32.add;;
	val ( +^ ) : int32 -> int32 -> int32 = <fun>
	# 2000000000l +^ 1l;;
	- : int32 = 2000000001l

It's also possible to create infix functions; however you have to use
the camlp4 preprocessor and your functions become reserved words in
the language.  Here is an example of an infix function which should
get you started:

	open Pcaml
	
	EXTEND
	   expr: AFTER "apply"
	   [ LEFTA
	       [ e1 = expr; "map_with"; e2 = expr ->
	           <:expr< List.map $e2$ $e1$ >>
	       ]
	   ];
	END

So using that extension you could write code like:

	list map_with (fun elem -> ...)

Use the following Makefile rule to compile the extension:

operators.cmo: operators.ml4
        $(OCAMLC) -c -pp "camlp4o pa_extend.cmo q_MLast.cmo -impl" -I +camlp4 \
          -impl $<

and the following rule to compile code using this extension:

OCAMLPP := -pp "camlp4o ./operators.cmo"
OCAMLC := ocamlc.opt
OCAMLCFLAGS := -w s -g $(OCAMLPP)

.ml.cmo:
        $(OCAMLC) $(OCAMLCFLAGS) $(OCAMLCINCS) -c $<

Rich.

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