swift 5.1 evaluate a closure in the wrong way - swift

Question

I'm having an issue with evaluation of one line of code
if i break it down to two lines, it's working , but in one line of code, it's just evaluate in a 'new' to a 'wrong' way.
my main reason for asking this question, is not to solve it, I know I can use parenthesis to solve it, and break it to Two line, but don't want to solve it, I just want to know why its evaluated like this , and if there's a solution for this : some setting to patch , in Order THAT it will work in ONE LINE OF CODE :
Heres the code that working in Two lines
Heres the code that trying to do the same thing, but rise an error as you can see:
full code of both working and not working :
class ClosuresStack {
var dic = Dictionary<String,(()->String)->String >()
subscript(_ str:String)-> (()->String)->String {
get {
return dic[str]!
}
set {
dic[str] = newValue
}
}
}
func createClosuresStak() -> ClosuresStack {
let cs = ClosuresStack()
func takesAClosureReturnA_string(_ closure:()->String) ->String {
return closure() + " Two"
}
cs["C"] = takesAClosureReturnA_string
return cs
}
let c = createClosuresStak()["C"]
let str = c{"One"}
print(str) // print: One Two
let c = createClosuresStak()["C"]{"One"} // error -->
now, I want to somehow understand how to change it that it will work in ONE LINE OF CODE : meaning that the evaluation of 'createClosuresStak()["C"]{"One"}' will create a closure after ["C"] , and then from that point writing the {"One"}
will make it a full evaluate of the line :
let c = createClosuresStak()["C"]{"One"}
making 'c' a String
if that's not possible, I need to know it Too , tnx :)
UPDATE
tnx for the comments , its help me understand the problem more clearly :
1) im understanding that the createClosuresStak()["C"]{"One"}
acutely trying to add the string 'One' as another parameter to the sub script , and there for the error from the compiler was that is cannot subscript (String,()->String} , 'C' as the string inside the [] , and the other parameter {"One"} -> BUT , isn't that some kind of a bug?, been that i'm using [] ,Cleary the compiler need to 'understand' that I want to subscript a String, also by power of inferring that swift has,
2) now I'm still trying to get that syntax to work as it is so I try to change some things, in order to get it to work :
so I created a function that take a string, and return a dictionary of type : Dictionary<String,()->String>, and then trying so subscript it
and the compiler don't rise an error that way :
func closuresDictionary(_ s:String) -> Dictionary<String,()->String> {
var dic = Dictionary<String,()->String>()
func foo()->String {
return s + " Two"
}
dic["C"] = foo
return dic
}
let c = closuresDictionary("One")["C"]{ "SomeString" }
c is now a closure of type ()->String which does noting with string that I put inside, so the syntax works, but the outcome is not doing anything.
when im changing the return type of the dictionary to a different closure : (String)->String instead of ()->String , im getting the same old error, that I'm trying to subscript a (String,(String)->String)
and I need a function that will take the string inside the {} , and create something from it meaning that I need to subscript to return a closure of (String)->String
its seems like there's no way to do that
im adding two more pictures of my last trying in order to get this line of code in current syntax to work
the wanted syntax working but the outcome is not an outcome not doing any thing with the string inside the {}:
same error, by changing the function to (String)->String

Answer 1

Your example:
let c = createClosuresStak()["C"]{"One"}
is using trailing closure syntax.
Trailing closure syntax works by including the trailing closure as an additional parameter to a function call. Subscripting an array is really a function call under the hood (to a function called subscript), and Swift is trying to pass that closure as a second parameter to the subscripting call, which is what the error is explaining:
Cannot subscript a value of type 'ClosuresStack' with an argument of type '(String, () -> String)'.
In other words, you can't pass both "C" and the closure {"One"} to the subscripting function.
There are at least 3 ways to fix this and still put it on one line:
Option 1: Use an explicit call to pass the closure instead of using trailing closure syntax
Wrap the closure in () to make the call explicit:
let c1 = createClosuresStak()["C"]({"One"})
print(c1)
Option 2: Wrap the createClosureStak()["C"] in parentheses
That lets Swift know the subscripting only gets "C" as a parameter and allows trailing closure syntax to work as expected:
let c2 = (createClosuresStak()["C"]){"One"}
print(c2)
Option 3: Add .self to the result before the trailing closure syntax:
That again finishes the subscripting call and avoids the confusion.
let c3 = createClosuresStak()["C"].self {"One"}
print(c3)
Personally, I would choose option one, because trailing closure syntax is unnecessary syntactic sugar that clearly is not working here.