Hello,

On Wed, 24 Apr 2002, Michal Moskal wrote:

> 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.

OK, In had typed it into the toplevel and as
expected, it does not give back a pure type
(like int's or strings).
It gives back
 None    
     or
 Some 8
     or
 Some "hello"

If aplicated to an empty list, a list of
integers or a list of strings.

Is this really a commeon way of programming?
If I work on a list of integers or a list of
strings, does it really have advantages to
use such a type?

> 
> I personally use mutable fields in records of option type. They are
> initially None, and are filled at some step of computations.


Hmhh, ok. I now see the similarity to NULL in C.

> However
> this is non-functional/ugly/etc ;)

Pattern match itself is not functional.

How can such things be expressed in a functional
way? (Maybe I have to try it in Haskell, it's
constraints to be functional are much stronger;
when using Ocaml it seems to be that very often
the imperative style creeps in - even unconsciously).

Ciao,
   Oliver

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