On Fri, 16 Sep 2011 22:20:05 +0100
"Richard W.M. Jones" <[email protected]> wrote:
> On Wed, Sep 14, 2011 at 10:40:11PM +0200, Basile Starynkevitch wrote:
> > And there is a reason why you cannot match (in Ocaml) on the content
> > of strings (or arrays). It won't be easy to implement efficiently
> > (you would need to copy a substring or subarray when matching)
>
> How about just prefix matching? That on its own would be very useful.
>
> For example in web app that was passed arguments foo_1, foo_2, bar_1,
> bar_5 you could parse the arguments like this:
>
> match arg with
> | "foo_" ^ s -> (* ..code_foo.. *)
> | "bar_" ^ s -> (* ..code_bar.. *)
[[NB I edited Richard's examples' comments]]
If the pattern variable s is indeed used in code_foo or code_bar you need to
copy a
substring of arg, don't you?
And we could do that (less efficiently) with something like
match arg with
| ss when string_starts_with ss "foo_" ->
let s = rest_of_string ss "foo_" in (* ..code_foo.. *)
| ss when string_starts_with ss "bar_" ->
let s = rest_of_string ss "bar_" in (* ..code_foo.. *)
[the code of string_starts_with & rest_of_string is obvious]
Actually, a syntactic camlp4 trick could do the above
However, what would be great would be to be able to code
match arg with
| "beef_" ^ s -> beef_case s
| "gee_" ^ t -> gee_case t
and have the generated code factorize the common sub-test, that is that arg.[1]
is 'e' and
arg.[2] is 'e' in both cases.
We don't have that, and we can't do that with syntactic preprocessing.
Cheers.
--
Basile STARYNKEVITCH http://starynkevitch.net/Basile/
email: basile<at>starynkevitch<dot>net mobile: +33 6 8501 2359
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*** opinions {are only mine, sont seulement les miennes} ***
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