Am 08.12.2011 um 11:13 schrieb Asger Feldthaus:
Haskell doesn't seem to support disjunctive patterns, and I'm having a difficult time writing good Haskell code in situations that would otherwise call for that type of pattern.
In Haskell I can't find any equivalent to the disjunctive pattern. If expanded naively, my Haskell version would look like this:
foo :: T -> T -> Int foo x y = case (x,y) of (Foo a, Foo b) -> a + b (Foo a, Bar b) -> a + b (Bar a, Foo b) -> a + b (Bar a, Bar b) -> a + b (Baz, Foo a) -> -a (Bar a, Baz) -> -a (Baz, Bar a) -> a (Foo a, Baz) -> a _ -> 0
While my example is still managable in size, this quickly goes out of hand in practice. I've tried using pattern guards but with limited success.
value :: T -> Int value (Foo a) = a value (Bar a) = a value Baz = 0 foo :: T -> T -> Int foo Baz (Foo a) = -a foo (Bar a) Baz = -a foo x y = value x + value y