On Fri, 29 May 2020, Wiebe-Marten Wijnja 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 Haskell `Maybe a` is more similar to `Either () a` than `Either a ()`. Either has one more redirection on the Left case. You can have both `Left undefined` and `Left ()` whereas Maybe can only have `Nothing`. I hardly think that people actually make use of this difference, though. Btw. from a software engineering point I'd prefer not to use Either for both exception handling with an according Monad and for cases where you just want to handle values of two possible types. I'd define an Except type for the exception usage. Could we remove Maybe in favor of Either? It would make some instances non-Haskell-98. E.g. instance C Maybe where is Haskell 98, but instance C (Either ()) where needs FlexibleInstances and instance (a ~ ()) => C (Either a) where needs TypeFamilies. Unless you find out that you can define a more general instance like instance (Super a) => C (Either a) where .