[Haskell-cafe] Why Maybe exists if there is Either?

[Artyom Kazak]

Besides, even defining

type Maybe a = Either () a

in standard library wouldn’t be the same as

data Maybe a = Nothing | Just a

since in Haskell 98 type synonyms aren’t allowed in instance 
declarations, which means that programmers would still have to remember 
that `Maybe` is actually `Either ()` under-the-hood every time when 
writing an instance.