Haskell syntax for 'or' in case expressions

There is no way of sharing the same right hand side for different patterns. However, you can usually get around this by using guards instead of patterns, for example with elem.

foo x | x `elem` [A, C, G] = ...
      | x `elem` [B, D, E] = ...
      | otherwise          = ...

with guards:

factorial n
    | n < 2 = 1
    | otherwise = n * (factorial (n - 1))

with pattern matching:

factorial 0 = 1
factorial 1 = 1
factorial n = n * (factorial (n - 1))