Question: Deriving boilerplate matching function.

gistfile1.hs

data A = A | B () | C () () 

isA, isB, isC :: A -> Bool
isA A  = True
isA _  = False

isB (B _)  = True
isB _      = False

isC (C _ _) = True
isC _       = False

extractA :: A -> Maybe ()
extractA A = Just ()
extractA _ = Nothing

extractB :: A -> Maybe ()
extractB (B a) = Just a
extractB   _   = Nothing

extractC :: A -> Maybe ((), ())
extractC (C a b) = Just (a, b)
extractC   _     = Nothing