[swift-evolution] Proposal: An Either Type in the STL

Kevin Ballard kevin at sb.org
Sat Dec 12 00:43:10 CST 2015

Can you provide any examples where you think it's demonstrably better to
have an Either than to just implement your own two-variant type,
assuming that we do already have a Result type to handle results?

I haven't gone trawling through GitHub for Either, but glancing through
that search you provided, it looks like it's basically just people
trying to reproduce Haskell concepts in Swift, which isn't particularly
meaningful. The only non-FP justification I saw was robrix/Either which
demonstrated using an Either to hold the results of a computation, which
of course is what Result does, and really the only reason it's called
Either is because of FP anyway.

-Kevin Ballard

On Fri, Dec 11, 2015, at 08:09 PM, Developer via swift-evolution wrote:
>> I do not support the addition of Either. As far as I am aware,
>> the only
compelling argument in favor of an Either type is "because Haskell has
one", but I believe it's commonly recognized that Haskell's Either is
not particularly good.
> I have yet to read counterarguments about the addition of Either to
> the Haskell base library.  And though it may be tempting to assume,
> because TypeLift is oriented towards declarative programming, that we
> wish to see Swift and Haskell enter some kind of unholy union (excuse
> the pun), I based this proposal not off of a desire to see more FP,
> but to see less duplicate implementations
> (https://github.com/search?utf8=✓&q=Either+language%3Aswift&type=Code&ref=searchresults)
> .  If GitHub is any indicator, the Swift community wants an Either,
> and it seems fitting to me that the language standardize on one and
> allow the community to instead focus their efforts on writing
> interesting extensions to such a common type.
> If you need a less anecdotal argument, consider the type of program
> `throws` can represent versus the type of program `Either` can
> represent.  On the one hand, yes, the latter does have certain
> conventions that encourage it to duplicate cases that the former
> already provides, but Either is supposed to properly represent a
> computation that can produce multiple discrete results - an if-else as
> a value.  If you wish to support more arities, nest down the left or
> right lobe of the type as you would nest branches.  If you want to
> start supporting higher-order variants, you start to run into inclusion-
> exclusion problems (see how we implemented a `Those` type
> https://github.com/typelift/Swiftz/blob/master/Swiftz/Those.swift)
>> It's a weird name for results (which Result
covers), and for other cases it's usually better just to define your own
two-variant enum anyway. Rust provides some evidence in favor of this,
as this was the rationale for why Rust has a Result<T,E> but no Either,
and it turns out there has been no need to add an Either.
> I’ll admit, the name is definitely less than ideal, but it should be a
> simple thing to change (I’m only interested in the structure here):
> XOr, Choice, Variant, Alternative, V come to mind.
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