Re: [Caml-list] Some/None

[Michal Moskal <malekith@pld-linux.org>]

On Wed, Apr 24, 2002 at 02:48:41PM +0200, Oliver Bandel wrote:
> How are Some/None defined, what are they good for?

This is similar in idea to NULL pointer in C. E.g. struct foo *ptr
is NULL iff ptr cannot be assigned any reasonable value of type foo.
You can use 'foo option' in ML the same way -- x : foo option is
None if no foo is the right value for x.

> A small, simple example will help a lot.

let head x =
  match x with
  | h::_ -> Some h
  | [] -> None

Here if no reasoable value of the head of the list can be returned
'None' is used.

I personally use mutable fields in records of option type. They are
initially None, and are filled at some step of computations. However
this is non-functional/ugly/etc ;)

-- 
: Michal Moskal :::::::: malekith/at/pld.org.pl :  GCS {C,UL}++++$ a? !tv
: PLD Linux ::::::: Wroclaw University, CS Dept :  {E-,w}-- {b++,e}>+++ h
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