withoutActuallyEscaping example question

Using Swift
1

Hi,

One of the motivating examples for withoutActuallyEscaping looks like the following. This predictably doesn’t work because "use of non-escaping parameter 'predicate' may allow it to escape"

func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
  return Array(array.lazy.filter { predicate($0) })
}

The solution is to use withoutActuallyEscaping. This works and produces the expected results.

func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
  return withoutActuallyEscaping(predicate) { predicate in
    Array(array.lazy.filter({predicate($0)}))
  }
}

What I find puzzling is the below example compiles and runs correctly. It seems like it should be the same compiler error as in the first example.

func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
  return Array(array.lazy.filter(predicate))
}

If you understand why this is so, it would be very helpful.

Thank you and best wishes,
Ray

2

Hi Ray,

There are two overloads of filter() available on ‘array.lazy’; the version that takes an escaping closure and returns a LazyFilterCollection, and the version that takes a non-escaping closure and returns [Int].

In the first example, we pick the LazyFilterCollection-returning overload, because the literal closure { predicate($0) } can be coerced to both an escaping or a non-escaping closure type, and in the absence of additional constraints we go with the overload from a concrete type over an overload in a protocol extension. After the overload has been picked we validate the body of the closure, and notice that it is invalid because whole the closure is already known to be @escaping, it references the non-@escaping ‘predicate’.

In the second example, ‘predicate’ is known to be non-@escaping, which rules out the first overload completely, so we go with the second overload and perform a non-lazy filter.

I would argue this is somewhat confusing, but it might be difficult to change the overload resolution rules in a way where the first overload is always chosen.

Slava

···

On Mar 26, 2017, at 12:13 AM, Ray Fix via swift-users <swift-users@swift.org> wrote:

Hi,

One of the motivating examples for withoutActuallyEscaping looks like the following. This predictably doesn’t work because "use of non-escaping parameter 'predicate' may allow it to escape"

func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
  return Array(array.lazy.filter { predicate($0) })
}

The solution is to use withoutActuallyEscaping. This works and produces the expected results.

func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
  return withoutActuallyEscaping(predicate) { predicate in
    Array(array.lazy.filter({predicate($0)}))
  }
}

What I find puzzling is the below example compiles and runs correctly. It seems like it should be the same compiler error as in the first example.

func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
  return Array(array.lazy.filter(predicate))
}

If you understand why this is so, it would be very helpful.

Thank you and best wishes,
Ray
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swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users

3

Here’s another fun one: the following type checks despite the literal closure; we’ve constrained the return type of filter() to [Int], and not LazyFilterCollection<Int>, so again the type checker picks the overload that takes the non-escaping closure:

func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
  return array.lazy.filter { predicate($0) }
}

Slava

···

On Mar 26, 2017, at 1:14 AM, Slava Pestov via swift-users <swift-users@swift.org> wrote:

Hi Ray,

There are two overloads of filter() available on ‘array.lazy’; the version that takes an escaping closure and returns a LazyFilterCollection, and the version that takes a non-escaping closure and returns [Int].

In the first example, we pick the LazyFilterCollection-returning overload, because the literal closure { predicate($0) } can be coerced to both an escaping or a non-escaping closure type, and in the absence of additional constraints we go with the overload from a concrete type over an overload in a protocol extension. After the overload has been picked we validate the body of the closure, and notice that it is invalid because whole the closure is already known to be @escaping, it references the non-@escaping ‘predicate’.

In the second example, ‘predicate’ is known to be non-@escaping, which rules out the first overload completely, so we go with the second overload and perform a non-lazy filter.

I would argue this is somewhat confusing, but it might be difficult to change the overload resolution rules in a way where the first overload is always chosen.

Slava

On Mar 26, 2017, at 12:13 AM, Ray Fix via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

Hi,

One of the motivating examples for withoutActuallyEscaping looks like the following. This predictably doesn’t work because "use of non-escaping parameter 'predicate' may allow it to escape"

func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
  return Array(array.lazy.filter { predicate($0) })
}

The solution is to use withoutActuallyEscaping. This works and produces the expected results.

func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
  return withoutActuallyEscaping(predicate) { predicate in
    Array(array.lazy.filter({predicate($0)}))
  }
}

What I find puzzling is the below example compiles and runs correctly. It seems like it should be the same compiler error as in the first example.

func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
  return Array(array.lazy.filter(predicate))
}

If you understand why this is so, it would be very helpful.

Thank you and best wishes,
Ray
_______________________________________________
swift-users mailing list
swift-users@swift.org <mailto:swift-users@swift.org>
https://lists.swift.org/mailman/listinfo/swift-users

_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users

4

It seems like we could just not take escaping-ness into account at all, and only diagnose if it mismatches. We'd have to decide if we want that behavior, though.

Jordan

···

On Mar 26, 2017, at 01:14, Slava Pestov via swift-users <swift-users@swift.org> wrote:

Hi Ray,

There are two overloads of filter() available on ‘array.lazy’; the version that takes an escaping closure and returns a LazyFilterCollection, and the version that takes a non-escaping closure and returns [Int].

In the first example, we pick the LazyFilterCollection-returning overload, because the literal closure { predicate($0) } can be coerced to both an escaping or a non-escaping closure type, and in the absence of additional constraints we go with the overload from a concrete type over an overload in a protocol extension. After the overload has been picked we validate the body of the closure, and notice that it is invalid because whole the closure is already known to be @escaping, it references the non-@escaping ‘predicate’.

In the second example, ‘predicate’ is known to be non-@escaping, which rules out the first overload completely, so we go with the second overload and perform a non-lazy filter.

I would argue this is somewhat confusing, but it might be difficult to change the overload resolution rules in a way where the first overload is always chosen.

5

Wow. Thanks for the explanation! I didn’t realize I was toggling between different overloads but that makes sense now.

Thanks again,
Ray

PS: It is sort of a strange example in the first place (being lazy and then immediately eager.) I don’t think it is a use case that is compelling enough to consider overload resolution rules over.

···

On Mar 26, 2017, at 1:21 AM, Slava Pestov <spestov@apple.com> wrote:

Here’s another fun one: the following type checks despite the literal closure; we’ve constrained the return type of filter() to [Int], and not LazyFilterCollection<Int>, so again the type checker picks the overload that takes the non-escaping closure:

func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
  return array.lazy.filter { predicate($0) }
}

Slava

On Mar 26, 2017, at 1:14 AM, Slava Pestov via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

Hi Ray,

There are two overloads of filter() available on ‘array.lazy’; the version that takes an escaping closure and returns a LazyFilterCollection, and the version that takes a non-escaping closure and returns [Int].

In the first example, we pick the LazyFilterCollection-returning overload, because the literal closure { predicate($0) } can be coerced to both an escaping or a non-escaping closure type, and in the absence of additional constraints we go with the overload from a concrete type over an overload in a protocol extension. After the overload has been picked we validate the body of the closure, and notice that it is invalid because whole the closure is already known to be @escaping, it references the non-@escaping ‘predicate’.

In the second example, ‘predicate’ is known to be non-@escaping, which rules out the first overload completely, so we go with the second overload and perform a non-lazy filter.

I would argue this is somewhat confusing, but it might be difficult to change the overload resolution rules in a way where the first overload is always chosen.

Slava

On Mar 26, 2017, at 12:13 AM, Ray Fix via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

Hi,

One of the motivating examples for withoutActuallyEscaping looks like the following. This predictably doesn’t work because "use of non-escaping parameter 'predicate' may allow it to escape"

func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
  return Array(array.lazy.filter { predicate($0) })
}

The solution is to use withoutActuallyEscaping. This works and produces the expected results.

func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
  return withoutActuallyEscaping(predicate) { predicate in
    Array(array.lazy.filter({predicate($0)}))
  }
}

What I find puzzling is the below example compiles and runs correctly. It seems like it should be the same compiler error as in the first example.

func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
  return Array(array.lazy.filter(predicate))
}

If you understand why this is so, it would be very helpful.

Thank you and best wishes,
Ray
_______________________________________________
swift-users mailing list
swift-users@swift.org <mailto:swift-users@swift.org>
https://lists.swift.org/mailman/listinfo/swift-users

_______________________________________________
swift-users mailing list
swift-users@swift.org <mailto:swift-users@swift.org>
https://lists.swift.org/mailman/listinfo/swift-users

6

That seems like a worthy thought.

···

on Mon Mar 27 2017, Jordan Rose <jordan_rose-AT-apple.com> wrote:

On Mar 26, 2017, at 01:14, Slava Pestov via swift-users <swift-users@swift.org> wrote:

Hi Ray,

There are two overloads of filter() available on ‘array.lazy’; the

version that takes an escaping closure and returns a
LazyFilterCollection, and the version that takes a non-escaping
closure and returns [Int].

In the first example, we pick the LazyFilterCollection-returning
overload, because the literal closure { predicate($0) } can be
coerced to both an escaping or a non-escaping closure type, and in
the absence of additional constraints we go with the overload from a
concrete type over an overload in a protocol extension. After the
overload has been picked we validate the body of the closure, and
notice that it is invalid because whole the closure is already known
to be @escaping, it references the non-@escaping ‘predicate’.

In the second example, ‘predicate’ is known to be non-@escaping,
which rules out the first overload completely, so we go with the
second overload and perform a non-lazy filter.

I would argue this is somewhat confusing, but it might be difficult
to change the overload resolution rules in a way where the first
overload is always chosen.

It seems like we could just not take escaping-ness into account at
all, and only diagnose if it mismatches. We'd have to decide if we
want that behavior, though.

--
-Dave

7

Say we want to handle predicate errors:

func myFilter(array: [Int], predicate: (Int) throws -> Bool) rethrows -> [Int] {
    return try withoutActuallyEscaping(predicate) { predicate in
        let matching = try array.lazy.filter({try predicate($0)})
        return Array(matching)
    }
}

Will the laziness be lost in this case?

1 Like
8

Yes, you won't get the lazy behavior when you pass a throwing closure to filter(_:)—none of the lazy versions accept throwing closures, so it falls back to the original Sequence version. With a throwing closure, the entire filter operation has to happen right away in order to know whether it throws or not, so it can't be lazy.

1 Like
9

One of the pains with lazy is that you can't throw, this was also discussed in this thread about the proposed Stream API addition.

1 Like
10

Thanks for clarifying this for me :blush:

1 Like