[swift-evolution] [Review] SE-0109: Remove the Boolean protocol

Tony Allevato allevato at google.com
Wed Jun 29 15:55:46 CDT 2016

On Tue, Jun 28, 2016 at 11:44 PM Douglas Gregor via swift-evolution <
swift-evolution at swift.org> wrote:

> Hello Swift community,
> The review of SE-0109 "Remove the Boolean protocol" begins now and runs
> through July 4, 2016. The proposal is available here:
> https://github.com/apple/swift-evolution/blob/master/proposals/0109-remove-boolean.md
> Reviews are an important part of the Swift evolution process. All reviews
> should be sent to the swift-evolution mailing list at
> https://lists.swift.org/mailman/listinfo/swift-evolution
> or, if you would like to keep your feedback private, directly to the
> review manager. When replying, please try to keep the proposal link at the
> top of the message:
> Proposal link:
> https://github.com/apple/swift-evolution/blob/master/proposals/0109-remove-boolean.md
> Reply text
> Other replies
> <https://github.com/apple/swift-evolution#what-goes-into-a-review-1>What
> goes into a review?
> The goal of the review process is to improve the proposal under review
> through constructive criticism and, eventually, determine the direction of
> Swift. When writing your review, here are some questions you might want to
> answer in your review:
>    - What is your evaluation of the proposal?
> +1. Having two similarly named types is confusing to newcomers (especially
those from languages where "Boolean" is the preferred term instead of
"Bool") and the Boolean protocol has very little use.

>    - Is the problem being addressed significant enough to warrant a
>    change to Swift?
> Yes.

>    - Does this proposal fit well with the feel and direction of Swift?
> Yes. Indeed, the fact that any type can conform to Boolean and get an
implicit conversion in if statements and Boolean expressions is very
un-Swift-like; the language currently prefers explicit and safe conversions
instead of implicit ones. I would be surprised by a language that doesn't
let me implicitly convert two similar types like Int32 and Int but would
let me implicitly convert anything I wanted to Boolean.

Giving users the ability to implicitly treat anything as a Boolean in those
contexts harms the readability of code, IMO. It forces readers to track
down the meaning in a place that isn't evident from that line of code. If I
see something like this:

    struct SomeValue: Boolean { ... }

    func foo(x: SomeValue) {
        if x {

...where do I go to find the meaning of this? The user isn't querying
.boolValue explicitly, so I can't hover over that and get to its
documentation. Maybe they didn't even document that property from the
conformance, so I have to go on a fishing expedition through the type's
documentation to figure out which values map to true and which map to false.

I like that Swift tends away from such "magic" and forces the user to say
what they mean. If a developer was writing an arbitrary type where they
wanted to use this, I would want to know why they think that's better than
*saying what they mean*—in many cases, I would expect that a property like
"isValid", "isEmpty", "exists", or any number of other assertions would
clarify the meaning and read better in a world where we put so much
emphasis on API design/naming.

>    - If you have used other languages or libraries with a similar
>    feature, how do you feel that this proposal compares to those?
> Many languages provide implicit conversions between some of its types and
boolean contexts (Javascript, Python, PHP), or a way to define custom
conversions (C++), and it's almost always clever, mysterious, or horrible.
I definitely don't want to see Swift going down the road of users writing
or extending types to make them work magically as if they were booleans. An
extra property/method call or comparison is not onerous. Let's nip this in
the bud now.

>    - How much effort did you put into your review? A glance, a quick
>    reading, or an in-depth study?
> Briefly read the proposal.

> More information about the Swift evolution process is available at
> https://github.com/apple/swift-evolution/blob/master/process.md
> Thank you,
> -Doug
> Review Manager
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
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