You can mutate constant properties in init. Swift 1.0 Beta 3.

InitMutation.swift

class SomeClass {
    let someArray: Array<String>
    
    init() {
        self.someArray = []
        self.someArray.append("Fine") // No error
    }
    
    func someFunction() {
        self.someArray.append("Not Fine") // Error
    }
}