[swift-evolution] Fixing the confusion between non-mutating algorithms and single-pass sequences

[Dmitri Gribenko]

Hi,

I'd like to continue the discussion of the issue raised by David Waite
inhttp://thread.gmane.org/gmane.comp.lang.swift.evolution/21295/:

> My main motivation for proposing this is the potential for developer confusion. As stated during one of the previous threads on the naming of map, flatMap, filter, etc. methods on Sequence, Sequence has a naming requirement not typical of the rest of the Swift standard library in that many methods on Sequence may or may not be destructive. As such, naming methods for any extensions on Sequence is challenging as the names need to not imply immutability.

I'd like to focus on a particular point: methods on Sequence can
consume elements, but the APIs are not markedmutating.

Dave Abrahams, Max Moiseev, and I have discussed this issue and we
agree this problem is severe and worth solving, we also think that the
likely solutions would be source-breaking, so it is important that we
discuss it now.

We have discussed a few options.

- Rejected option: remove Sequence, let IteratorProtocol model
single-pass data streams

- Rejected option: use a syntactic marker, like sequence.consumedIn.map {}

- Rejected option: mutating APIs on Sequence, non-mutating APIs on Collection

Proposed: rename Sequence to IterableOnce or TraversableOnce. We think
that Sequence does not convey the single-pass restriction clearly. The
term "sequence" has been used in math (as in "integer sequence"), and
since the math domain does not have mutation, "sequence" can be
understood to mean "multi-pass", since you can traverse a sequence of
integers an arbitrary number of times.

We think that only the last option is viable in the Swift language as
it exists now, without creating an undue burden for API vendors and
users.

For more details about rejection options, please see the full writeup:
https://gist.github.com/gribozavr/47f4717f3afc762549383e94da7f748b

Dmitri

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