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[SR-8779] UnsignedInteger constraint on RandomNumberGenerator is not ideal #51287

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Azoy opened this issue Sep 18, 2018 · 3 comments
Open

[SR-8779] UnsignedInteger constraint on RandomNumberGenerator is not ideal #51287

Azoy opened this issue Sep 18, 2018 · 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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@Azoy
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Azoy commented Sep 18, 2018

Previous ID SR-8779
Radar None
Original Reporter @Azoy
Type Bug
Additional Detail from JIRA
Votes 0
Component/s Standard Library
Labels Bug
Assignee None
Priority Medium

md5: 7065369b6f42acb03b6e059f6af05a15

Issue Description:

Lack of a better bug title, these extension methods on RandomNumberGenerator:

public func next<T: FixedWidthInteger & UnsignedInteger>() -> T {}

public func next<T: FixedWidthInteger & UnsignedInteger>(upperBound: T) -> T {}

are somewhat difficult to work with. Here is the implementation of randomElement() on Collection:

  public func randomElement<T: RandomNumberGenerator>(
    using generator: inout T
  ) -> Element? {
    guard !isEmpty else { return nil }
    let random = generator.next(upperBound: UInt(count))
    let index = self.index(
      startIndex,
      offsetBy: numericCast(random)
    )
    return self[index]
  }

Granted the numericCast might be unnecessary and you can achieve the same using Int(random), but removing the UnsignedInteger constraint allows for an implementation like so:

  public func randomElement<T: RandomNumberGenerator>(
    using generator: inout T
  ) -> Element? {
    guard !isEmpty else { return nil }
    let random = generator.next(upperBound: count)
    let index = self.index(
      startIndex,
      offsetBy: random
    )
    return self[index]
  }

This is just one example of where removing this constraint is helpful.
@Azoy
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Azoy commented Sep 18, 2018

cc: @stephentyrone

@belkadan
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You're supposed to use Int.random(in: 0...count, using: &generator), not call next directly. See SE-0202 for more details.

@stephentyrone
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We have considered something like having .next( ) return any FixedWidthInteger, but basically "what Jordan said". Most use cases should be using the methods that take a range.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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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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@stephentyrone @belkadan @Azoy