On 2016-06-10 11:39:34 +0000, Leonardo Pessoa via swift-evolution said:
I would vow to remove where clauses altogether. Most of the times I
used it in ifs, fors and switches I ended up thinking it hardened
readability of my code (perhaps because of placement, but I do not
think alternatives made it more readable) and so it was my decision to
stop using where whenever possible in my code. Since Erica's
performance test pointed out using guard is nearly as fast as using
where and the core team is removing wheres from ifs, I think the ones
in fors could go next.
Interestingly, I find where clauses in `for` to be a useful tool to
*improve* readibility. To me, it flows much better to write & read this:
for (start, duration) in days where calendar.isDateInWeekend(start) {
...
}
rather than this:
for (start, duration) in days {
guard calendar.isDateInWeekend(start) else { continue }
...
}
or this:
for (start, duration) in days.filter({ calendar.isDateInWeekend($0.0)
}) {
...
}
Of these three, I'd argue the first is the easiest to understand for a
person who's never seen Swift code before. (Admittedly I doubt that's
something languages should be optimized for, though.) The extra
punctuation and the use of $0 instead of the loop's bindings makes the
third approach seem especially noisy & scary.
Obviously, editorial tools like this aren't essential, and I could
easily adapt to live without such syntactic sugar. But I do not like
the idea of systematically ridding the language of such charming
constructs when there is no technical reason to do so. SE-0099 fixed a
technical problem with the syntax of a shorthand construct, without
removing it from the language. It did not bring back Swift 1's `if`
pyramids.
Moreover, Erica is absolutely right about `switch`: while `where`
clauses are arguably non-essential in `for` loops, `switch` statements
would lose a great deal of their expressive power without them. For
example, in the following snippet (taken from a lexer in SwiftPM), all
cases are logically at the same level. I think it's a win that Swift
allows them to be expressed as such, and I'd strongly object to a
proposal that removed this capability.
switch c {
case UInt8(ascii: "\n"): ...
case UInt8(ascii: "#"): ...
case let c where c.isSpace(): ...
case UInt8(ascii: "\""): ...
case let c where c.isNumberInitialChar(): ...
case let c where c.isIdentifierChar(): ...
case UInt8(ascii: ","): ...
case UInt8(ascii: "="): ...
case UInt8(ascii: "["): ...
case UInt8(ascii: "]"): ...
case UInt8(ascii: "."): ...
default: ...
}
Obviously, we could rewrite this to use a single `if` ladder, but I
think most would not find that an improvement.
(The same argument applies to `catch` statements, although with less force.)
As long as `where` is kept in `switch` and `catch`, and as long the
grammar is unambiguous, I think there is little point to removing where
clauses from `for`. To improve consistency, I'd not mind if SE-0099
kept `where` as an alternative way to spell `,` in `if` statements, too.
--
Károly
@lorentey
_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution
