[swift-evolution] mutating/non-mutating suggestion from a Rubyist

Dave Abrahams dabrahams at apple.com
Tue Apr 26 15:02:46 CDT 2016

on Tue Apr 26 2016, Thorsten Seitz <tseitz42-AT-icloud.com> wrote:

> Am 23.04.2016 um 10:27 schrieb Pyry Jahkola via swift-evolution
> <swift-evolution at swift.org>:
>     I'd like to second James Campbell's suggestion of a `mutate` keyword.
>     Clarifying comments inline below:
>         On 23 Apr 2016, at 00:24, Dave Abrahams via swift-evolution
>         <swift-evolution at swift.org> wrote:
>         This is not a new idea. Something almost identical to this has been
>         explored and discussed quite thoroughly already:
>         <https://github.com/apple/swift/blob/master/docs/proposals/Inplace.rst>.
>         In fact, it was implmented and later reverted because it raised
>         language-design questions for which we had no good answers.
>     I don't know if the following are particularly good answers, but I'll try
>     anyway:
>         I don't believe the choice of glyph (& vs =) affects any of the
>         fundamental issues:
>         * Should the x.=f() syntax be required for *every* mutating method
>         invocation?
>     Allow me to ask it differently: Should some specific syntax be required for
>     every mutating method? — Yes.
> I think I like that idea.
>     Should the syntax be `x.=f()`? — Not necessarily. I kinda like James
>     Campbell's idea of a `mutate` keyword. Consider the following:
>     var numbers = [5, 12, 6, 2]
>     mutate numbers.append(10)
>     mutate numbers.sort()
>     if let biggest = mutate numbers.popLast() {
>     print("The biggest number was:", biggest)
>     }
>     So `mutate` would work much like `try` but—unlike `try` which can move
>     further to the left—`mutate` would have to always prefix the mutating
>     receiver. 
> That doesn't look so bad (we might shorten 'mutate' to 'mut', though I don't
> think that would be really necessary).

We've already discussed this whole question length, specifically
considered the direction of an almost-identical language feature, and
ended up settling on the “form/ed/ing” naming conventions.  If there is
some new information since then, it would be possible to handle
something like this via a new evolution proposal.  However, anything
like that is way out of scope for Swift 3, and in the absence of new
information, it isn’t a productive use of time to rehash it at all.


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