T2310.hs:5:22:
    Illegal result type signature ‘a’
      Result signatures are no longer supported in pattern matches
    In a lambda abstraction: \ x :: a -> (x :: a)

T2310.hs:5:39:
    Not in scope: ‘co’
    Perhaps you meant one of these:
      ‘c’ (line 5), ‘cos’ (imported from Prelude)