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