RE: [Caml-list] "Or" patterns when both matchings

["Manuel Fahndrich" <maf@microsoft.com>]

Hmm, I must side with Pixel here. Ease of compilation is rarely a good
design principle for a programming language. The use of or patterns
allows one to factor right hand sides as in the example shown below:

	| Foo(a)
             | a -> <complicated expression involving a>

If Or-patterns do not follow the first-to-last matching order, then
producing correct code and reading it becomes more difficult. I wasn't
aware of the Or-compilation strategy and I'm sure I made this mistake in
the past as well.

-Maf


-----Original Message-----
From: Luc Maranget [mailto:luc.maranget@inria.fr] 
Sent: Monday, October 29, 2001 2:38 AM
To: Pixel
Cc: caml-list@pauillac.inria.fr
Subject: Re: [Caml-list] "Or" patterns when both matchings

> 
> 
> from the documentation:
>   The pattern pattern1 | pattern2 represents the logical ``or'' of the
two
>   patterns pattern1 and pattern2. [...] If both matchings succeed, it
is
>   undefined which set of bindings is selected.
> 
> is there a reason for not using the classical pattern matching rule,
to make
> the ordering matters? (i've been nastily beat by this :-/)
> 
> eg:
> 
> 
> type foo = Bar | Foo of foo
> 
> let f1 = function
>   | Foo(a) 
>   | a -> a
> 
> let f2 = function
>   | Foo(a) -> a
>   | a -> a
> 
> let e1 = f1 (Foo Bar)  (*=> Foo Bar *)
> let e2 = f2 (Foo Bar)  (*=> Bar *)
> 
> 
> thanks
> --
> Pixel

Yes there are two reasons
 1. ease of compilation.
    As you have experienced yourself. In case one of the patterns in
    the or-pattern is a variable, then the or-pattern is reduced to a
    variable. Otherwise, compilation would be a bit more complicated.

 2. Ideology. I consider that priority in or-patterns is something
    obscure, and would discourage relying on it.
    However the current (unspecified) semantics makes the idea
    of a ``partially useless'' matching clause a bit random, and this
    semantics may become more precise in the future.

Cheers,

--Luc
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