I'm not sure what you're objecting to about this. Is it the very
appearance of curly braces?
I went to bed thinking that maybe I should have explained that better,
and I guess I was right. ;^) Here's why I think we should do something
here.
From what I can tell, mapping or filtering on a single property is
exceptionally common. I ran a few regexes on the Swift code present on
my machine (basically the stuff in the Swift repos, plus my
open-source projects, plus my closed-source stuff, plus various
playgrounds and things) to see how common different kinds of `map`,
`filter`, and `flatMap` closures were:
2142 OP { …$0… }
1835 OP(function) or OP(some.method)
589 OP { $0.property } or OP { $0.some.property }
564 OP { $0.property }
525 OP { function(…$0…) } or OP { some.method(…$0…) }
186 OP { $0.method(…) }
153 OP { function($0) } or OP { some.method($0) }
100 OP { $0 as SomeType } or OP { $0 as? SomeType } or OP { $0 as! SomeType }
52 OP { $0.method() }
35 OP { collection[…$0…] } or OP { some.collection[…$0…] }
20 OP { collection[$0] } or OP { some.collection[$0] }
13 OP { $0! }
(Simple regex-based match of `map`, `flatMap`, and `filter`
calls. Permits various spacing schemes and `try`. If you want to run
it on a more representative sample, the script is here, requires
fish(1):
https://gist.github.com/brentdax/2a8ee2705c39e9948aafedbd81b1366f\)
So, at least in my unscientific sample, more than a quarter of
map/filter/flatMap calls which use `$0` simply look up a property or
chain of properties on it.
Totally believable.
If we want to make something about `map`
and friends more convenient, this seems like a good place to look.
(Using a straight function is a few times more common than a property,
but given that this is also the syntax used to abstract over a closure
body, I'm not sure how much weight to put on that fact.)
So what's wrong with what we have now? Syntactic weight. Consider this expression:
person.map { $0.name }
The "loudest" parts of this expression are the closure brackets and
the `$0`, but they are actually the *least* important.
That's funny, my eye skips right over them and focuses on “.name” Not
kidding at all. And “name” is a single fairly short, un-chained
identifier.
They do not express anything about what this line of code *does*; they
exist solely to tell the compiler how to do it. They are pure glue
code, and serve only to obscure the actual intent. Compare that to:
person.map(\.name)
Here, we still have a glue character (the `\`), but it's just one, and
it's relatively inconspicuous compared to something like `$0`.
Meh. This in particular doesn't look like a major improvement.
That's not *too* bad, though. It gets a lot worse when the key path is
actually in a variable:
array.map { $0[keyPath: prop] }
Again, look at how much of this line is given over to adapting a line
of code to the compiler—and how little that actually matters when
understanding what the line does. The most important thing in that
expression is `prop`, but it's completely lost in this sea of
irrelevant syntax. Compare to:
array.map(prop)
Yes, that's a lot of extra syntax. But again, you've used an
abbreviated, single identifier for the property and a
short, non-descriptive identifier for the array. Let's make this a
fair/realistic comparison:
gradeHistories.map { $0[keyPath: \.average] }
vs.
gradeHistories.map(\.average)
Yep, I agree that passing a keypath directly is still much nicer in this
case.
Which puts that piece of information front and center.
If there was an argument that the case was too complex to handle
nicely, I think we could justify leaving it alone.
It's not the “too complex” criterion I'd want to talk about—it's the
“how often do you need it” criterion. If it's rare, it doesn't matter
so much (I don't have an opinion about whether it is in fact rare).
That's essentially what happened with the much-requested placeholder
syntax:
Sorry, I may have missed that discussion.
Lots of people wanted it, but critics pointed out the fundamental
ambiguity of the syntax, and after spending gallons of electrons
arguing about it, the proponents pretty much backed off. But key paths
don't have that problem—they always work the same way and are
completely unambiguous, so there's no scope for misinterpretation. And
the cases key paths can handle appear to be about as common as the
cases placeholder syntax was intended for.
Nor is there a strong argument that the suggested behavior is
fundamentally "weird". It's pretty natural to think of a
`KeyPath<Root, Value>` as being a `(Root) -> Value` function, and the
`foo.map(\.bar)` syntax reads pretty straightforwardly as long as you
know what `map` does.
It's not fundamentally weird. I'm just not sure it's important. If it
actually is important, as I said, I feel very strongly that it shouldn't
require anyone to create an overload of map, because that quickly leads
to overloading everything that takes a closure.
There's one more reason I think we should do this. It is not about the
technology; it is not even really about the ergonomics. It's more
about language "marketing", for lack of a better term.
I think we were all surprised by the SE-0110 backlash. But in
hindsight, I think it's pretty easy to explain. During the Swift 3 and
4 cycles, we systematically stripped conveniences and sugar from
higher-order functions. We have good reasons to do this; we want to
pare things down, fix the foundations, and then build up new
conveniences. Eat your vegetables now and you can have dessert later.
But we're entering our second year of eating nothing but
vegetables. It's very difficult to maintain a strict diet forever,
especially when—like the vast majority of Swift's users who don't
participate in evolution or implementation—you don't really see the
point of it. It's hard to blame them for being tired of it, or for
complaining when yet another tasty food is pulled off the menu.
Offering a treat like this on occasion will help ease the pain of
losing the things we *need* to take away. And this is a particularly
good candidate because, although it's a convenience for higher-order
functions—which is where the pain is felt—it has little to do with
parameter handling, the area where we actually need to remove things
and refactor. It's like a dessert of ultra-dark chocolate—it's a treat
that doesn't set the actual goal back very far.
In the abstract, "fundamentals now, sugar later" is the right
approach. But it can't be considered "right" if the users won't accept
it. So let's look for opportunities to add conveniences where we
can. Maybe this isn't the right feature—subtyping is always a bit
perilous—but we should be on the lookout for features like this one,
places where we can improve things for our functional programming fans
without obstructing our own efforts to clean up parameter handling.
These are all good arguments. For me it's a question of priorities and
long-term, irrevocable impacts.
By the way, if you're worried about whether subtyping will fly, I've
recently been thinking there might be a role for a “promotion” operator
that enables lossless “almost-implicit” conversions, e.g.:
someNumber^ is equivalent to numericCast(someNumber)
\.someKeyPath^ is equivalent to { $0\.someKeyPath }
someSubstring^ is equivalent to String(someSubstring)
etc.
This convenience can be implemented by anyone today for keypaths, and
will provide nearly the syntax you're looking for. This is exactly the
sort of thing I'd love to see become a widespread existing practice
before we incorporate it in the standard library, so we could properly
judge its impact on real code.
So, there are lots of options worth exploring before we jump on map and
flatmap and start adding one-off convenience overloads for keypaths.
···
on Thu Jul 06 2017, Brent Royal-Gordon <brent-AT-architechies.com> wrote:
On Jul 6, 2017, at 9:13 AM, Dave Abrahams <dabrahams@apple.com> wrote:
--
-Dave