Re: [Caml-list] Pattern matching

[Luc Maranget <luc.maranget@inria.fr>]

About pattern matching.
> 
> "Diego Olivier Fernandez Pons" <FernandezPons@iFrance.com> writes:
is right to stress on the fact that variables in patterns are
binding variables, some variable is bound to something.
when you write

let x = ex in
let x = ex' in

You certainly do not mean that ex and ex' are equal (whatever it
means).



On linearity :

> 
> > let egalite = function
> >    | (x,x) -> true
> >    | (x,y) -> false
> > ;;
> > # This variable is bound several times in this matching
> 
> i wonder why this one is not allowed since it can't have any other meaning
> than the one it has in prolog. What's the reason for not having it rewritten
> to:
> 

Does not mean :

> > let egalite = function
> >    | (x,x') when x=x' -> true
> >    | (x,y) -> false
> 

Well, it could be that way. But it is not.

This can be argueed as follows.

  As already said the meaning of (x,x) is not clear, should we use
  ``='' (and then pattern matching may last for ever, or may raise
  an exception (for functions)) or ``=='' (and this is not very useful).

  Futhermore the theory of non-left-linear rewriting is more
  complicated than the one of left-linear rewriting and this theory
  somehow applies to ML pattern matching.


So my opinion is that non-linear pattern brings little benefits to
users, complicates much the life of the compiler writer and may even
introduce subtle bugs in users programs.


--Luc

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