[Haskell-cafe] Pattern matching with where free variables can be used more than once

[Wolfgang Jeltsch]

Am Freitag, 17. Juli 2009 20:38 schrieb Stefan Holdermans:
> Christopher,
>
> > Wouldn't it be great if pattern variables could be used more than once
> > in a pattern? Like so:
> >
> >    foo [x,x,_,x] =  "The values are the same!"
> >    foo _      = "They're not the same!"
>
> These are called nonlinear patterns. I think Miranda (a precursor of
> Haskell, sort of) used to have them.

Yes, Miranda had them.

I see the following problem with them: Patterns are about the structure of 
data. So using the same variable twice in the same pattern should mean that 
the values that match the variable must have the same structure. This would 
break data abstraction. For example, matching a pair of sets against the 
pattern (x,x) would succeed if both sets were represented by the same tree 
internally, but not succeed if both sets were equal but represented 
differently.

Best wishes,
Wolfgang