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Re: [Caml-list] "Or" patterns when both matchings
- Luc Maranget
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Date: | 2001-10-29 (10:38) |
From: | Luc Maranget <luc.maranget@i...> |
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 ------------------- Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr