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

Richard O'Keefe raoknz at gmail.com
Sat May 30 02:21:28 UTC 2020

There is a subtle difference between Haskell Either and Gleam Result.
  data Either a b = Left a | Right b ...
  pub type Result(a, e) {
The computer doesn't care, but it's important for human thinking:
** Nothing is *not* an Error.
Suppose for example I have a function
  next_smaller_prime :: Int -> Maybe Int
where next_smaller_prime 10 -> Just 7
  and next_smaller_prime 2 -> Nothing
The second case is not an error.  You get the answer Nothing
because the function *worked*, not because it didn't.

To return Error Nil is to commit the YouTube (social) offence:
"You are an evil-doer who has done something wrong.
 I refuse to tell you WHAT you did wrong,
 so you can't fix it, you wrong-thinking PEASANT."
Seriously, if you have decided to return Error(x),
x had BETTER be a 'reason' (as Erlang calls it) for WHY it is
an error.  The pattern in Erlang is, after all,
{ok,Result} | {error,Reason}.

sans_reason (Left _)  = Nothing
sans_reason (Right x) = Just x

with_reason (Nothing) s = Left s
with_reason (Just x)  _ = Right x

are trivial conversion functions.  As I said, the computer does not care.

There are (at least) three different situations we can consider.
(1) Sometimes there is no answer.  Typically a search.
    In this case, Maybe is appropriate.
(2) Sometimes you asked a question which fails to have an answer
    for a reason.
    In this case, Either is appropriate.
(3) Sometimes you asked a sensible question for which the system
    might have been expected to produce an answer, but something
    went wrong. Numeric overflow, database connection shut down
    unexpectedly, hard drive developed a bad block.
    In this case, an exception is appropriate.

And of course there are other reasons to use Either.  Think of
divide-and-conquer:  classify :: Problem -> Either SubProblems EasyProblem.
Because Gleam's Result isn't Haskell's Either in terms of connotations for
human beings, even if they are basically the same to a computer.

On Fri, 29 May 2020 at 22:26, 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?
> Have a nice day,
> ~Marten / Qqwy
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