[swift-evolution] [draft] Add `clamped(to:)` to the stdlib

[Nate Cook]

Hi Nick,

I think this might have come up in an earlier thread, but clamped(to:) with a half-open range argument brings up the question of how to clamp to an empty range. As proposed, the implementation of this method would result in an error when passed an empty range:

    foo.clamped(to: 0..<0)
    // fatal error: Can't form Range with upperBound < lowerBound

Is that the desired behavior? Providing documentation for both new methods would help make it clear what the expected functionality is.

Thanks!
Nate


> On Mar 13, 2017, at 9:16 PM, Nicholas Maccharoli via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Swift-Evolution,
> 
> After considering the indispensable feedback from the community here is a slightly more polished draft.
> 
> If anyone is interested the draft can be downloaded here:
> https://github.com/Nirma/swift-evolution/blob/clamp_function/proposals/NNNN-add-clamp-function.md <https://github.com/Nirma/swift-evolution/blob/clamp_function/proposals/NNNN-add-clamp-function.md>
> 
> Thank you so much, 
> 
> - Nick
> 
> 
> Add clamp(to:) to the stdlib
> 
> Proposal: SE-NNNN <https://github.com/Nirma/swift-evolution/blob/clamp_function/proposals/NNNN-filename.md>
> Authors: Nicholas Maccharoli <https://github.com/Nirma>
> Review Manager: TBD
> Status: Awaiting review
> During the review process, add the following fields as needed:
> 
> Decision Notes: Rationale <https://lists.swift.org/pipermail/swift-evolution/>, Additional Commentary <https://lists.swift.org/pipermail/swift-evolution/>
> Bugs: SR-NNNN <https://bugs.swift.org/browse/SR-NNNN>, SR-MMMM <https://bugs.swift.org/browse/SR-MMMM>
> Previous Revision: 1 <https://github.com/apple/swift-evolution/blob/...commit-ID.../proposals/NNNN-filename.md>
> Previous Proposal: SE-XXXX <https://github.com/Nirma/swift-evolution/blob/clamp_function/proposals/XXXX-filename.md>
>  <https://github.com/Nirma/swift-evolution/blob/clamp_function/proposals/NNNN-add-clamp-function.md#introduction>Introduction
> 
> This proposal aims to add functionality to the standard library for clamping a value to a ClosedRange. The proposed function would allow the user to specify a range to clamp a value to where if the value fell within the range, the value would be returned as is, if the value being clamped exceeded the upper or lower bound in value the value of the boundary the value exceeded would be returned.
> 
> Swift-evolution thread: Add a clamp function to Algorithm.swift <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170306/thread.html#33674>
>  <https://github.com/Nirma/swift-evolution/blob/clamp_function/proposals/NNNN-add-clamp-function.md#motivation>Motivation
> 
> There have been quite a few times in my professional and personal programming life where I reached for a clampedfunction and was disappointed it was not part of the standard library.
> 
> Having functionality like clamped(to:) added to Comparable as a protocol extension would benefit users of the Swift language who wish to guarantee that a value is kept within bounds.
> 
>  <https://github.com/Nirma/swift-evolution/blob/clamp_function/proposals/NNNN-add-clamp-function.md#proposed-solution>Proposed solution
> 
> The solution proposed is simply that there be a clamped(to:) function added to the Swift Standard Library. The function would behave much like its name describes.
> 
> Given a clamped(to:) function existed it could be called in the following way, yielding the results in the adjacent comments:
> 
> let foo = 100
> 
> // Closed range variant
> 
> foo.clamped(to: 0...50) // 50
> foo.clamped(to: 200...300) // 200
> foo.clamped(to: 0...150) // 100
> 
> // Half-Open range variant
> 
> foo.clamped(to: 0..<50) // 49
> foo.clamped(to: 200..<300) // 200
> foo.clamped(to: 0..<150) // 100
>  <https://github.com/Nirma/swift-evolution/blob/clamp_function/proposals/NNNN-add-clamp-function.md#detailed-design>Detailed design
> 
> The implementation of clamped(to:) that is being proposed is composed of two protocol extensions; one protocol extension on Comparable and another on Strideable.
> 
> The implementation for clamped(to:) as an extension to Comparable accepting a range of type ClosedRange<Self>would look like the following:
> 
> extension Comparable {
>     func clamped(to range: ClosedRange<Self>) -> Self {
>         if self > range.upperBound {
>             return range.upperBound
>         } else if self < range.lowerBound {
>             return range.lowerBound
>         } else {
>             return self
>         }
>     }
> }
> The implementation of clamped(to:) as an extension on Strideable would be confined to cases where the stride is of type Integer. The implementation would be as follows:
> 
> extension Strideable where Stride: Integer {
>     func clamped(to range: Range<Self>) -> Self {
>         return clamped(to: range.lowerBound...(range.upperBound - 1))
>     }
> }
>  <https://github.com/Nirma/swift-evolution/blob/clamp_function/proposals/NNNN-add-clamp-function.md#source-compatibility>Source compatibility
> 
> This feature is purely additive; it has no effect on source compatibility.
> 
>  <https://github.com/Nirma/swift-evolution/blob/clamp_function/proposals/NNNN-add-clamp-function.md#effect-on-abi-stability>Effect on ABI stability
> 
> This feature is purely additive; it has no effect on ABI stability.
> 
>  <https://github.com/Nirma/swift-evolution/blob/clamp_function/proposals/NNNN-add-clamp-function.md#effect-on-api-resilience>Effect on API resilience
> 
> The proposed function would become part of the API but purely additive.
> 
>  <https://github.com/Nirma/swift-evolution/blob/clamp_function/proposals/NNNN-add-clamp-function.md#alternatives-considered>Alternatives considered
> 
> Aside from doing nothing, no other alternatives were considered.
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> https://lists.swift.org/mailman/listinfo/swift-evolution

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