> On Jan 16, 2017, at 8:10 PM, Dave Abrahams via swift-evolution > <swift-evolution@swift.org> wrote: > > > on Mon Jan 16 2017, Charles Srstka <cocoadev-AT-charlessoft.com> wrote: > >> On Jan 16, 2017, at 6:42 PM, Dave Abrahams via swift-evolution >> <swift-evolution@swift.org> wrote: >>> >>> >>> on Mon Jan 16 2017, Charles Srstka <swift-evolution@swift.org >>> <mailto:swift-evolution@swift.org>> wrote: >>> >> >>>>> On Jan 16, 2017, at 7:49 AM, Chris Eidhof via swift-evolution >>>>> <swift-evolution@swift.org> wrote: >>>>> >>>>> Hi, >>>>> >>>>> How does everyone feel about adding a second version of `reduce` to >>>> >>>>> `Sequence`? Instead of a `combine` function that's of type `(A, >>>>> Element) -> A`, it would be `(inout A, Element) -> ()`. This way, we >>>>> can write nice functionals algorithms, but have the benefits of >>>>> inout (mutation within the function, and hopefully some copy >>>>> eliminations). >>>>> >>>>> IIRC, Loïc Lecrenier first asked this on Twitter. I've been using it >>>>> ever since, because it can really improve readability (the possible >>>>> performance gain is nice, too). >>>>> >>>>> Here's `reduce` with an `inout` parameter, including a sample: >>>>> https://gist.github.com/chriseidhof/fd3e9aa621569752d1b04230f92969d7 >>>>> >>>>> -- >>>>> Chris Eidhof >>>> >>>> I did this in my own private code a while ago. There is one drawback, >>>> which is that Swift’s type >>>> inference system isn’t quite up to handling it. For example, doing this >>>> results in an “ambiguous >>>> reference to member” warning: >>>> >>>> range.reduce([Int]()) { $0.append($1) } >>> >>> The diagnostic could be better, but the compiler shouldn't let you do >>> that, because it requires passing an unnamed temporary value ([Int]()) >>> as inout. >> >> No it doesn’t. The signature of the method is: >> >> func reduce<A>(_ initial: A, combine: (inout A, Iterator.Element) -> ()) -> A >> >> The unnamed temporary value is “initial” here, which is not passed as inout; >> the inout parameter is >> the first argument to the “combine” closure. The value represented by the >> ‘initial’ parameter is >> passed to the closure, true, but only after being stored in a not-unnamed >> ‘var’ variable, as you can >> see from the source of the proposed method: >> >> func reduce<A>(_ initial: A, combine: (inout A, Iterator.Element) -> ()) -> >> A { >> var result = initial >> for element in self { >> combine(&result, element) >> } >> return result >> } >> >> Therefore, I don’t understand this objection. >> >>>> One would think that the type of this closure should be clear: >>>> >>>> 1) The closure calls append(), a mutating function, so $0 must be inout. >>>> >>>> 2) The closure doesn’t return anything, which should rule out the >>>> default implementations of reduce, >>> >>> The closure *does* return something: (), the empty tuple >> >> But it’s not what it’s supposed to return. Sequence’s implementation >> of reduce, which the compiler thinks matches the above, is declared >> like this: >> >> public func reduce<Result>(_ initialResult: Result, _ >> nextPartialResult: (Result, Self.Iterator.Element) throws -> Result) >> rethrows -> Result >> >> The closure is supposed to return Result, which in this case would be >> [Int]. It doesn’t, so I’m not sure why the compiler is thinking this >> is a match. > > Okay, sounds like I'm totally wrong! Has to happen at least once in a > lifetime, doesn't it? 😉 > > So please file bug reports for these issues.
This one should already be fixed in master. If it isn't, definitely file a new one! -Joe _______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution
