EnumExample.elm

type Enum a
    = Enum
        { predFn : a -> Maybe a
        , succFn : a -> Maybe a
        }


pred : Enum a -> a -> Maybe a
pred (Enum { predFn }) =
    predFn


succ : Enum a -> a -> Maybe a
succ (Enum { succFn }) =
    succFn


intEnum : Enum Int
intEnum =
    Enum
        { predFn = \x -> Just (x - 1)
        , succFn = \x -> Just (x + 1)
        }


boolEnum : Enum Bool
boolEnum =
    Enum
        { predFn =
            \b ->
                if b then
                    Just False
                else
                    Nothing
        , succFn =
            \b ->
                if not b then
                    Just True
                else
                    Nothing
        }

> pred intEnum 10
Just 9 : Maybe.Maybe Int
> succ intEnum 10
Just 11 : Maybe.Maybe Int
> pred boolEnum True
Just False : Maybe.Maybe Bool
> pred boolEnum False
Nothing : Maybe.Maybe Bool