[Haskell-beginners] No ghci return for homemade types

3 Jan 2021

      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])

Similarly,

data Color a = Blue a | Green a | Red a

myFavoriteColor :: Color Int
myFavoriteColor = Green 50

but now I'm stuck. How can I access the "Green 50" inside myFavoriteColor?

LB

[Haskell-beginners] No ghci return for homemade types

Lawrence Bottorff