Il 03 gennaio 2021 alle 16:22 Lawrence Bottorff ha scritto:

I've tried this

data MyMaybe a = Just a | Nothing

maybeHead :: [t] -> MyMaybe t maybeHead [] = Nothing maybeHead (x:_) = Just x

and it evaluates fine. But this

maybeHead [1,2,3]

produces error: : • No instance for (Show (MyMaybe Integer)) : arising from a use of ‘show’ : • In the expression: show (maybeHead [1, 2, 3]) : In an equation for ‘it’: it = show (maybeHead [1, 2, 3])

data MyMaybe a = Jus a | Not deriving (Show, Eq) Your text should have introduced to/warned you about deriving and typeclasses.