[Haskell-cafe] Why does Haskell have both `Maybe a` and `Either a ()`?

[leesteken at pm.me]

On Friday, May 29, 2020 12:24 PM, Wiebe-Marten Wijnja <w-m at wmcode.nl> wrote:

> Greetings, everyone!
>
> Recently I was involved in a discussion on the new ML-style language
> 'gleam'.
>
> Gleam has for quite a while now only had an `Either a b` type,
> with all functions that in Haskell one would use a `Maybe a` for,
> working on an `Either a ()` instead.
>
> In the discussion(https://github.com/gleam-lang/gleam/issues/591), the
> language designers were asking the community whether it would make sense
> to add `Maybe` to the language as well,
> or keep using only `Either a ()`.
>
> My question: Is the difference between `Maybe a` and `Either a ()` only
> semantic and are they functionally equivalent,
> or are there differences in functionality as well?

One can define instance Functor Maybe, and instance Functor (Either ()),
but not something like instance Functor (\a -> Either a ()).
Therefore, Either () a would be more like Maybe a than Either a (), in Haskell/GHC.

kind regards, Arjen