[Haskell-beginners] No ghci return for homemade types

[Francesco Ariis]

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.