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

[Nicholas Maccharoli]

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

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) // 50foo.clamped(to: 200...300) //
200foo.clamped(to: 0...150) // 100// Half-Open range variant
foo.clamped(to: 0..<50) // 49foo.clamped(to: 200..<300) //
200foo.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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