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[SR-4979] SubSequence is not a Sequence, can't be zipped? #47556

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pcantrell opened this issue May 23, 2017 · 3 comments
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[SR-4979] SubSequence is not a Sequence, can't be zipped? #47556

pcantrell opened this issue May 23, 2017 · 3 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. standard library Area: Standard library umbrella

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@pcantrell
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Previous ID SR-4979
Radar None
Original Reporter @pcantrell
Type Bug
Status Resolved
Resolution Done
Environment

Tested with:

• Swift 3
• a recent Swift 4 snapshot

…in Xcode 8.

Additional Detail from JIRA
Votes 0
Component/s Standard Library
Labels Bug
Assignee None
Priority Medium

md5: 308b88d2afc5e35dcf218fc68935c491

relates to:

  • SR-1445 Recursive protocol constraints

Issue Description:

This code works:

extension Array {
  var adjacentPairs: AnySequence<(Iterator.Element, Iterator.Element)> {
    return AnySequence(
      zip(self, dropFirst()))
  }
}

let a = ["car", "bon", "fire", "fly", "way", "farer"]
for (w0, w1) in a.adjacentPairs {
  print(w0 + w1)
}

But change that extension from Array to Sequence, and it doesn’t compile:

extension Sequence {  // ← only change
  var adjacentPairs: AnySequence<(Iterator.Element, Iterator.Element)> {
    return AnySequence(
      zip(self, dropFirst()))  // ← compile error here
  }
}

The error is:
@rintaro
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Mannequin

rintaro mannequin commented May 24, 2017

This is FIXME thing.
https://github.com/apple/swift/blob/master/stdlib/public/core/Sequence.swift#L333-L346

For now, you can workaround this with extra where clause:

extension Sequence where SubSequence : Sequence, SubSequence.Iterator.Element == Iterator.Element {
    var adjacentPairs: AnySequence<(Iterator.Element, Iterator.Element)> {
        return AnySequence(zip(self, dropFirst()))
    }
}

@pcantrell
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Author

Ah, right, I see that this is the primary motivation for SE-0157.

Thanks for the workaround, Rintaro!

@slavapestov
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This has been fixed on master by @DougGregor.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
This issue was closed.
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Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. standard library Area: Standard library umbrella
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@slavapestov @pcantrell