Re: [Caml-list] Pattern matching and strings

[Andreas Rossberg <rossberg@ps.uni-sb.de>]

Sven Luther wrote:
> 
> On Wed, Oct 02, 2002 at 04:12:34PM +0200, Alessandro Baretta wrote:
> > I have to do a little bit of pattern matching on strings. My
> > first instict was to write something like the following.
> >
> > let foo x = ...
> > let bar x = ...
> > ... = function
> > | "foo" ^ rest -> foo rest
> > | "bar" ^ rest -> bar rest
> > | _ -> raise Unrecognized
> 
> What about :
> 
> ... = function
>   | str when String.sub str 0 3 = "foo" -> foo (String.sub str 2 (String.length str - 3))
>   | str when String.sub str 0 3 = "bar" -> bar (String.sub str 2 (String.length str - 3))
>   | _ -> raise Unrecognized
> 
> Sure, this code is not very optimal, i guess you could write a nicer
> function which will test the string incrementally using just String.get
> or something such, but i suppose it will do the thing you want.

The SML basis library provides the nice but often overlooked concept of
substrings (or more generally, vector slices,
http://SML.sourceforge.net/Basis/substring.html). Ported to Caml the
Substring module would enable

> ... = function
>   | s when Substring.is_prefix "foo" s -> foo (Substring.triml 3 s)
>   | s when Substring.is_prefix "bar" s -> bar (Substring.triml 3 s)
>   | _ -> raise Unrecognized

where your parsing functions operate on substrings instead of strings.
Such an approach seems like a good compromise between use of a heavy
regexp lib and inefficient repeated copying of parts of strings.

-- 
Andreas Rossberg, rossberg@ps.uni-sb.de

"Computer games don't affect kids; I mean if Pac Man affected us
 as kids, we would all be running around in darkened rooms, munching
 magic pills, and listening to repetitive electronic music."
 - Kristian Wilson, Nintendo Inc.
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