Promisekit: How to avoid long compilation time

[QuentinArnault]

• PromiseKit version:
• Xcode version: 8.3.2

Hi,

I love to use PromiseKit but compilation times can be very long as soon as you chain 4 then in a method. Have you any tricks in order to avoid this compilation extra times?

Regards,
Quentin

[mxcl]

I've not personally noticed this.

[nathanhosselton]

I've also not seen this. Can you provide more detail? An example would be even better.

[mxcl]

Like, a test project that proves it would be good.

[QuentinArnault]

I guess this is not directly a PromiseKit. Compilation times go exponential if you chain generic functions that returns Promises.

With this function
func genericPromise<T>(_ value: T) -> Promise<T> { return Promise { fulfill, _ in DispatchQueue.main.async { fulfill(value) } } }

I wrote 5 promises chains with 1, 2, 3, 4, 5 successive calls.

```func testWith1GenericFunctions() {
_ = genericPromise("a string").then {
print($0)
}
}

func testWith2GenericFunctions() {
_ = genericPromise("a string").then {
return genericPromise($0)
}.then {
print($0)
}
}

func testWith3GenericFunctions() {
_ = genericPromise("a string").then {
return genericPromise($0)
}.then {
return genericPromise($0)
}.then {
print($0)
}
}

func testWith4GenericFunctions() {
_ = genericPromise("a string").then {
return genericPromise($0)
}.then {
return genericPromise($0)
}.then {
return genericPromise($0)
}.then {
print($0)
}
}

func testWith5GenericFunctions() {
_ = genericPromise("a string").then {
return genericPromise($0)
}.then {
return genericPromise($0)
}.then {
return genericPromise($0)
}.then {
return genericPromise($0)
}.then {
print($0)
}
}
```

Compilation times are:

• testWith1GenericFunctions: 29 ms
• testWith2GenericFunctions: 7 ms
• testWith3GenericFunctions: 50 ms
• testWith4GenericFunctions: 450 ms
• testWith5GenericFunctions: 3276 ms

So I think that there is no bugs in PromiseKit, I just would like to have advice on how to optimise this situations with a readable solution.
PromiseTestCompilationTime.zip

[mxcl]

Possibly if you specify the return types of the closures it helps?

[QuentinArnault]

Indeed, with this function
```

func testWith5GenericFunctionsWithReturnTypes() {
_ = genericPromise("a string").then { value -> Promise in
return genericPromise(value)
}.then { value -> Promise in
return genericPromise(value)
}.then { value -> Promise in
return genericPromise(value)
}.then { value -> Promise in
return genericPromise(value)
}.then { value -> Void in
print(value)
}
}```

Compilation time goes down from 3240ms to 21ms.

Thanks.

[mxcl]

Interesting, thanks for bringing this to our attention.

[mxcl]

If you will, please report this to http://bugs.swift.org

I cannot, it's quite possible that people at Apple will not listen to me anymore.

[QuentinArnault]

I'll try with swift 4 if the problem is still present. But maybe it is unavoidable, genericity has a price.

[mxcl]

Obviously I'm not the expert, but my layman's observation is this looks like an O(n) situation. Linear chain.

[mxcl]

I find Swift 4 much improves this.