Re: [Caml-list] pattern matching on strings

[Philippe Veber <philippe.veber@gmail.com>]

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Hi Walter,

Contrary to Prolog or Haskell, strings in ocaml are not represented as char
lists. They are exactly like char array, but have their own type, operations
and syntax : strings are created with String.make (similar to Array.make),
their length is given by String.length (sim. to Array.length) and the chars
are accessed with the notation s.[i] (similar to t.(i)). Actually I don't
know why they are not defined like char arrays (anyone on this ?). Long
story short, recursive formulations on strings (likewise for array) will
often rely on indices (and thus, not much on pattern matching). Note that
you can use optional arguments to hide indices :

let rec iter f ?(k = 0) s =
  if k < String.length s then (
    f s.[k] ;
    iter f ~k:(k + 1) s
  )

let _ = iter print_char "abc";;


The closest to your request I see can be achieved using ocaml batteries (*),
by transforming your string into a list:

let rec iter_aux f = function
    [] -> ()
  | c :: s1 -> f c ; iter_aux f s1
let iter f s = iter_aux f (String.explode s)

But this won't be very efficient !

You won't find advanced string pattern matching in core ocaml. But micmatch
seems a nice way to go if that's what you're looking for.

cheers,
Philippe.

(*) http://batteries.forge.ocamlcore.org/


2011/9/14 Walter Cazzola <cazzola@dico.unimi.it>

> Hi all,
> I'm just trying to write a recursive function that iterates¹ on a string
> and I'd like to use pattern matching as in:
>
> let rec iter f s =
>  match s with
>     | ""  -> unit;
>     | c^s1 -> f c; iter f s1;;
>
> but the ^ concatenates 2 strings and not a char with a string and above
> all seems to be inadmissible in the patterns.
>
> Does this mean that I can't write a function on strings by pattern
> matching or is there something I don't know?²
>
> Thanks for the help
> Walter
>
> ¹ I know that exists String.iter but I'd like to improve my skill in
>  writing functions by using pattern matching
> ² I read about micmatch but I'd like to avoid non standard packages.
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