swift: Assign function type to variable - swift

Question

struct System {
var method: (() -> ())?
var curMethod: Int
init() {
method = nil
curMethod = 0
}
mutating func method1() {
curMethod = 1
}
mutating func method2() {
curMethod = 2
}
}
var sys = System()
sys.method = System.method1
sys.method!()
I get an error cannot assign value of type (inout System) -> () -> ()' to type '(() -> ())?. What am I doing wrong?

Answer 1

First of all, your line sys.method = System.method1 is wrong, as it would require method1 to be a static function (like a class function) rather than an instance function, so it should be changed to sys.method = sys.method1. But this isn't allowed - the error is "error: partial application of 'mutating' method is not allowed".
If you make System a class (rather than a struct), it will work if you replace the System.method1 with sys.method1.
The reason for this is that a mutating func is actually quite a lot more than a simple function under the hood - it is a curried function (curried with a compiler generated function) that effectively creates a new copy of the struct with the "new" value - hence, you A) can't access it it directly ("partial application is not allowed") and B) you can't assign it to a ()->() variable.

Answer 2

So, there're 3 variants suggested by participants. Everything is working, and using class instead of struct seems to me less complicated.
struct System1 {
var method: (() -> ())?
var curMethod: Int
init() {
method = nil
curMethod = 0
}
mutating func method1() { curMethod = 1 }
mutating func method2() { curMethod = 2 }
}
struct System2 {
var method: ((inout System2) -> ())?
var curMethod: Int
init() {
method = nil
curMethod = 0
}
mutating func callCurrentMethod() { method?(&self) }
mutating func method1() { curMethod = 1 }
mutating func method2() { curMethod = 2 }
}
class System3 {
var method: (() -> ())?
var curMethod: Int
init() {
method = nil
curMethod = 0
}
func method1() { curMethod = 1 }
func method2() { curMethod = 2 }
}
var struct1 = System1()
var struct2 = System2()
var class1 = System3()
print(struct1.curMethod)
let curried = System1.method1
let unsafe = curried(&struct1)
unsafe()
print(struct1.curMethod)
print(struct2.curMethod)
struct2.method = { $0.method1() }
struct2.callCurrentMethod()
print(struct2.curMethod)
print(class1.curMethod)
class1.method = class1.method1
class1.method!()
print(class1.curMethod)