Opaque result types

[Douglas_Gregor]

@Moximillian

• It's easier to reason what the code is doing, when trying to understand somebody else's code

Having a function with a result type that looks like this:

LazyMapSequence<
  LazyFilterSequence<
    LazyMapSequence<Elements, ElementOfResult?>
  >,
  ElementOfResult
>

is not helping anyone reason about the code. It is flooding them with irrelevant details. Better to say "it's a Collection whose Element type is ElementOfResult".

@Moximillian

There's less accidental errors, because we are not relying on the compiler to infer things correctly

There really isn't any interesting inference here. The return statement can spell out the type explicitly, or leave it inferred, but callers are unaffected either way.

@Moximillian

We can use the same approach, if at some point there can be multiple opaque types in same statement

Personally, I don't consider multiple opaque types to be important enough to base other design decisions on it. It's left as a discussion (not a specific part of the proposal) because I suspect it would become more of an anti-pattern.

@Moximillian

Swift is supposed to have ease of use and progressive disclosure as priorities. Trying to understand "_.Element" or ".Element" and what they refer to is much harder than understanding named type, such as "O.Element", because you can find the other places where that type is mentioned.

Maybe, but having a name that's used in the declaration but not available elsewhere is also confusing. Frankly, I wouldn't want the name in the declaration (e.g., in the generated interface or documentation) unless clients can use the name.

@Moximillian

If the main motivation for opaque types is to be used in libraries to hide concrete types, it's not a vast amount of code (compared to all end user app code). So spending couple of seconds to write a type is ok.

The motivation is two-fold: don't have to write these complicated composed types, and can hide them from clients. @gregtitus made a good argument that the former is the more important motivation, and I tend to agree.

Doug