Op 31-10-2015 om 13:44 schreef Darren Grant:
You're very close. Take a look at the even clause: Let (k :: Integer), and consider how the type of the expression producing the error,
Just (f2Maybe k)
differs from the type of the expression,
(f2Maybe k)
Cheers, Darren
Just (f2Maybe k) is type Maybe (Maybe Integer) f2Maybe k is type Maybe Integer But when I change the code to this : f2Maybe :: Integer -> Maybe Integer f2Maybe n | n > 0 = Nothing | n == 0 = Just 1 | even n = f2Maybe ( n `div` 2) ^ 2 | odd n = (f2Maybe ( n `div` 2) ^ 2) * 2 Then I see this error message : No instance for (Num (Maybe Integer)) arising from a use of ‘^’ In the expression: f2Maybe (n `div` 2) ^ 2 In an equation for ‘f2Maybe’: f2Maybe n | n > 0 = Nothing | n == 0 = Just 1 | even n = f2Maybe (n `div` 2) ^ 2 | odd n = (f2Maybe (n `div` 2) ^ 2) * 2 Failed, modules loaded: none. No instance for (Num (Maybe Integer)) arising from a use of ‘^’ In the expression: f2Maybe (n `div` 2) ^ 2 In an equation for ‘f2Maybe’: f2Maybe n | n > 0 = Nothing | n == 0 = Just 1 | even n = f2Maybe (n `div` 2) ^ 2 | odd n = (f2Maybe (n `div` 2) ^ 2) * 2 Failed, modules loaded: none. No instance for (Num (Maybe Integer)) arising from a use of ‘^’ In the expression: f2Maybe (n `div` 2) ^ 2 In an equation for ‘f2Maybe’: f2Maybe n | n > 0 = Nothing | n == 0 = Just 1 | even n = f2Maybe (n `div` 2) ^ 2 | odd n = (f2Maybe (n `div` 2) ^ 2) * 2 Failed, modules loaded: none.