Let's take the ABCD example. data ABCD a b c = A a | B b | C c | D c data ABC a b c = A' A | B' b | C c Bool
You mean data ABC a b c = A' a | B' b | C' c Bool It is very pleasant when a person has such a wide horizon: you compared F#, SML, Erlang and Haskell, IMHO modern FP developer should have basic knowledge about these languages/cultures and to know about their trends/current researches. Example with SML 'OR'-pattern, am I right that this terrible compile time is due search on all permutations of patterns (func arguments)? But what is not obvious to me is what is compiler try to match: P1 P2 ... Pn (0,0) => True _ _ ... _ _ => False (1,1) (2,2) ... (n,n) (1,1) => False is not enought to do depth-first search and to go from P1 to last (0,0)? Or is it try to find all matches, not only one, first successful? May be I miss what is 'Pn', how it's defined. Your example with the introduction of a 'ABC' has good understandable semantic, if they can be united, there is a reason to combine them in new type, semantically it makes sense. And view pattern in your example "unwraps" underlying datatype value to be matched and bound. Good explanation, thanks for it! And from SML example I can understand that obvious solution to the forehead of "OR"-pattern matching can be problematic in practice... === Thanks again, best regards, Paul