Sie benötigen 4 Fälle, aber Sie müssen nicht zwei Singleton-Listen als separaten Fall behandeln.
longer :: [a] -> [a] -> Bool
-- 1) Two empty lists
longer [] [] = False
-- 2) An non-empty list and an empty list
longer _ [] = True
-- 3) An empty list and a non-empty list
longer [] _ = ???
-- 4) Two non-empty lists
longer (_:xs) (_:ys) = longer xs ys
Eigentlich müssen Sie nur 3 Fälle in der richtigen Reihenfolge, je nachdem, was longer [] _
sein sollte.
-- First case: if longer [] _ is suppose to be True
longer :: [a] -> [a] -> Bool
longer [] [] = True
longer (_:xs) (_:ys) = longer xs ys
-- We get this far if one is empty and the other is not,
-- but we don't care which one is which.
longer _ _ = False
-- Second case: if longer [] _ is supposed to be False
longer :: [a] -> [a] -> Bool
longer (_:xs) (_:ys) = longer xs ys
longer _ [] = True
longer [] _ = False -- This covers longer [] [] as well.
Verwenden: 'ghc -Wall prog.hs 'und ghc sollten Ihnen die Muster erklären, die nicht berücksichtigt werden. – ErikR