Re: [Caml-list] Pattern matching and strings

[Sven LUTHER <luther@dpt-info.u-strasbg.fr>]

On Fri, Oct 04, 2002 at 02:00:37PM +0200, Andreas Rossberg wrote:
> 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.

Well, like i said, it was just a quick implementation, but i am sure you
can do this more or less optimaly with a hand coded String.get using
comparison function. I don't know if you really can escape the use of
the final copying of the string though.

Friendly,

Sven Luther
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