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[Caml-list] Some/None
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Date: 2002-04-24 (14:55)
From: John Max Skaller <skaller@o...>
Subject: Re: [Caml-list] Some/None
Oliver Bandel wrote:

>Does it make sense to give back None/Some
>out of a function 
Oh yes! I do this all the time, lots and lots.


let lst = [1;2;3;4]
let pos lst x =
  let pos lst n = match lst with
    | h :: t ->
      if h = x
      then Some n
      else pos t (n+1)
    | [] -> None
    pos lst 0

which searches a list for a number x,
and returns its position n as 

  Some n 

if it is found in the list, or


if it isn't: its position in the list is None,
it doesn'tr have a position, or Some position
if it does.

>(e.g. is_regularfile/is_directory/...)

What you want here is:

    type file_type_t = Regular | Directory | Special | Root | NonExistant

   let file_type (s:string): file_type_t =

    (code to find the file type here) ...

This function finds the type of the file and returns it,
I include the case the file is non-existant.

This is exactly a C enumeration. Only you can have
arguments. For example, an extended file function might
use the construction

    type file_data =
      | Regular of int (* file size in megs *)
      | Directory of int (* number of entries *)
      | Special

This is a union of cases, just like an enumeration,
only now some extra data is tacked on to some
of the cases.

John Max Skaller,
snail:10/1 Toxteth Rd, Glebe, NSW 2037, Australia.

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