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[Caml-list] Some/None
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Date: 2002-04-24 (15:43)
From: Samuel Lacas <Samuel.Lacas@t...>
Subject: Re: [Caml-list] Some/None
Oliver Bandel a écrit 1.8K le Wed, Apr 24, 2002 at 04:23:34PM +0200:
# 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"

Naturally. The type ('int option) is not the same as the type "int",
because you have the "None" value.

# Is this really a common way of programming?

I suppose so.

# If I work on a list of integers or a list of
# strings, does it really have advantages to
# use such a type?

Yes: it allows you to manage an error case for a function supposed to
return a value of type a, but which may sometimes return nothing. It's
lighter than exceptions (I think) and much more safe than having a
"null" pointer-style ugly coding (or other twisted way such as saying
"return -1" if not found).

# Pattern match itself is not functional.

Don't know, but without much thinking, no imperative language having a
similar mechanism built-in comes to my mind. Don't trust me, though :)
# 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).

Haskell preludes contains:

   -- Maybe type
   data  Maybe a  =  Nothing | Just a      deriving (Eq, Ord, Read, Show)

which serves exactly the same. Replace Maybe by option, Nothing by
None, and Just by Some. No "unconscious" imperative creeping here :)


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