Partial application of map function

Asked Feb 24 '21 at 18:09

Active Feb 24 '21 at 18:09

Viewed 110 times

Just defined a generic map function in Swift:

func map<T, S: Sequence>(_ f: @escaping (S.Element) -> T) -> (S) -> [T] {
    return { xs in xs.map(f) }
}

However, when I try to partially apply the identity function...

map({ $0 })

the compiler says:

No exact matches in call to global function 'map'

How can I solve this issue ?

asked Feb 24 '21 at 18:09

F. Zer

1

Idk why it’s give that particular error, but one of the issues here is at S and T are undetermined at your call site – Alexander Feb 24 '21 at 18:58
Thank you, @Alexander. Makes sense. Do you know a way to partially apply a function in this manner ? – F. Zer Feb 24 '21 at 19:03
You’ll have to provide the type data, either by adding type annotations on your closure (I.e. specify the parameter and return type) or on the return type of map – Alexander Feb 24 '21 at 19:05
Something like this, @Alexander ? `let f : ([Int]) -> [Int] = map( {$0} )` ? – F. Zer Feb 24 '21 at 19:10
@F.Zer can you please add some more context to your question? – Leo Dabus Feb 24 '21 at 19:10
How can I add a type annotation to the closure ? – F. Zer Feb 24 '21 at 19:10
I guess partial application is not going to be very useful if I have to annotate everything beforehand. – F. Zer Feb 24 '21 at 19:11
2

`{ x: Int -> [Whatever] in }` – Leo Dabus Feb 24 '21 at 19:11
Sure, @LeoDabus. What kind of context ? – F. Zer Feb 24 '21 at 19:11
I just tested your suggestion, but it does not work, @LeoDabus. For example: `map({ x: (Int) -> Int in x })`. It says: Cannot find x in scope. – F. Zer Feb 24 '21 at 19:15
@F.Zer my bad you need to put it inside parentheses `(x: Int)` => `let f: ([Int]) -> [Int] = map { (x: Int) -> Int in x }` – Leo Dabus Feb 24 '21 at 20:13
1

Good. Now, it works perfectly, @LeoDabus. – F. Zer Feb 24 '21 at 21:05

0 Answers0