Hello, Richard. As a result I did with "...where f c = ..." :) A way with "which" is interesting but duplicates unique constructors (A -> A', B -> B'). Interesting, is F# can solve this problem with active pattern? Pattern matching in Haskell does not seem enought flexible, for example, if I have `data D = D a b c d`, how can I match it without placeholders for `D` args? Something like `D...`? I need to write `D _ _ _ _` if I want to match without to bind args, right? I want to say, that may be (I'm not sure, I'm newbie in Haskell) active patterns can solve this and many other problems, may be active-pattern + reflection. For last example, I create pattern `IsD` in place where `D` is defined and use it - to be more independent on D args (D can be record and I can use getters/lens only to access its args, so I need a way to be similar independent from its args in pattern-matching too). But again, I'm not sure about right approaches - I'm newbie yet. === Best regards, Paul
There is another elementary alternative. If you need to treat C and D the same in just one place, you don't really have a problem. If you need to treat them the same in several places, do this:
data T a b c = A a | B b | C c | D c -- existing type
data Which a b c = A' a | B' b | CD Bool c
which :: T a b c -> Which a b c which (A a) = A' a which (B b) = B' b which (C c) = CD False c which (D c) = CD True c
then case which $ x of A' a -> B' b -> CD _ c -> ...
If you want to merge the C and D cases often, I like this approach, otherwise the C c -> f c D c -> f c where f c = ... approach is better.
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