CODEWITHSUNDEEP
iosswiftprotocols
lochiwei
lochiwei

Reputation: 1358

Swift: Using `var` leads to a compiler warning, using `let` leads to compiler errors?

I have defined a CanStack protocol with an associated type named Item:

CanStack.swift

// protocol definition
protocol CanStack: 
  ExpressibleByArrayLiteral, CustomStringConvertible
{
  associatedtype Item
  var items:[Item] { get }
  init()
  mutating func push(_ items: [Item])
  mutating func pop() -> Item?
}

// protocol extension (default behavior)
extension CanStack {

  public var isEmpty: Bool { return items.isEmpty }

  // init by array
  public init(_ items:[Item]) {
    self.init()
    self.push(items)
  }

  // init by variadic parameter
  public init(_ items:Item...){
    self.init()
    self.push(items)
  }

  // push items by variadic parameter
  public mutating func push(_ items:Item...){
    self.push(items)
  }

}

// conform to ExpressibleByArrayLiteral 
extension CanStack {
  public init(arrayLiteral items:Item...){
    self.init()
    self.push(items)
  }
}

// conform to CustomStringConvertible
extension CanStack {
  public var description: String {
    return "["
      + items.map{"\($0)"}.joined(separator:", ")
      + " ⇄ in/out"
  }
}

and defined a StackStruct struct conforming to this protocol, this generic structure has a type parameter Item (exactly the same name with the associated type above):

StackStruct.swift

public struct StackStruct<Item> {

    public private(set) var items = [Item]()
    public init() { }

    mutating public func push(_ items:[Item]) {
        self.items += items
    }

    @discardableResult
    mutating public func pop() -> Item? {
        return items.popLast()
    }

}

// adopt CanStack protocol
extension StackStruct: CanStack { }

and then I defined another class Stack confoming to the protocol too:

Stack.swift

public class Stack<Item> {

    public private(set) var items = [Item]()
    public required init() {}

    public func push(_ newItems:[Item]) {
        items += newItems
    }

    @discardableResult
    public func pop() -> Item? {
        return items.popLast()
    }

}

// adopt CanStack protocol
extension Stack: CanStack { }

and I have 3 test cases:

TestCases.swift


func testStackStruct() {
    // init
    var s1: StackStruct = [1,2,3] // expressible by array literal
    var s2 = StackStruct([4,5,6]) // init by array
    var s3 = StackStruct(7, 8, 9) // init by variadic parameter

    // push
    s1.push([4,5])    // array
    s2.push(10, 11)   // variadic
    s3.push(20)       // variadic

    // pop
    for _ in 1...4 { s1.pop() }
    s2.pop()
    s3.pop()

    // print these stacks
    example("stack struct", items:[s1,s2,s3])
}

func testStackClass_Var() {
    // init
    var s4: Stack = [1,2,3] // ⚠️ warning: s4 was never mutated; consider changing to let ...
    var s5 = Stack([4,5,6]) // init by array
    var s6 = Stack(7, 8, 9) // init by variadic parameter

    // push
    s4.push([4,5])    // array
    s5.push(10, 11)   // variadic
    s6.push(20)       // variadic

    // pop
    for _ in 1...4 { s4.pop() }

    // print these stacks
    example("stack class", items: [s4,s5,s6])
}

func testStackClass_Let() {
    // init
    let s7: Stack = [1,2,3] // expressible by array literal
    let s8 = Stack([4,5,6]) // init by array
    let s9 = Stack(7, 8, 9) // init by variadic parameter

    // push
    s7.push([4,5])    // array
    s8.push(10, 11)   // ⛔ Error: Extra argument in call
    s9.push(20)       // ⛔ Error: Cannot convert value of type 'Int' to expected argument type '[Int]'

    // pop
    for _ in 1...4 { s7.pop() }

    // print these stacks
    example("stack class", items: [s7,s8,s9])
}

and I can run the first test case testStackStruct() without any problem:

output of testStackStruct()

[1 ⇄ in/out
[4, 5, 6, 10 ⇄ in/out
[7, 8, 9 ⇄ in/out

and run the testStackClass_Var() case with only a compiler warning:

⚠️ warning: s4 was never mutated; consider changing to let ...

output of testStackClass_Var()

[1 ⇄ in/out
[4, 5, 6, 10, 11 ⇄ in/out
[7, 8, 9, 20 ⇄ in/out

but the testStackClass_Let() case can't even compile successfully, I got two compiler errors:

s8.push(10, 11)  // ⛔ Error: Extra argument in call
s9.push(20)      // ⛔ Error: Cannot convert value of type 'Int' to expected argument type '[Int]'

The only difference between the testStackClass_Var() case and testStackClass_Let() case is whether I used a var or let to declare those stack instances.

I can't figure out where or what I was doing wrong, can somebody help? thanks.

p.s.

my little helper function:

example.swift

import Foundation   // string.padding() needs this

// print example
// usage:
//    example("test something", items: [a,b,c]) {
//      // example code here ...
//    }
public func example<T>(
    _ title: String,         // example title
    length: Int      = 30,   // title length
    items: [T]?      = nil,  // items to print
    run: (()->Void)? = nil   // example code (optional)
){
    // print example title
    print(
        ("----- [ " + title + " ] ")
            .padding(toLength: length, withPad: "-", startingAt: 0)
    )

    // run example
    if let run = run { run() }

    // print items if provided
    if let items = items {
        items.forEach{ print($0) }
    }

    // new line
    print()
}

Upvotes: 4

Views: 126

Answers (2)

Maxim Aba
Maxim Aba

Reputation: 89

but the testStackClass_Let() case can't even compile successfully, I got two compiler errors:

 s8.push(10, 11)  // ⛔ Error: Extra argument in call
s9.push(20)      // ⛔ Error: Cannot convert value of type 'Int' to expected argument    type '[Int]'

The only difference between the testStackClass_Var() case and testStackClass_Let() case is whether I used a var or let to declare those stack instances.

It is all about using mutating functions on Swift structs. Here is a detailed answer: Thanks to Natasha-the-Robot

Upvotes: 0

Martin R
Martin R

Reputation: 540005

Your

 public func push(_ newItems:[Item]) {

is a method of public class Stack<Item>, which is a reference type, therefore you can call it on a constant:

let s4: Stack = [1,2,3]
s4.push([4,5])

On the other hand, the variadic method

public mutating func push(_ items:Item...)

is an extension method of protocol CanStack, which can be adopted by structs as well, therefore it requires a variable. That is why

let s8 = Stack([4,5,6]) // init by array
s8.push(10, 11)   // Error: Extra argument in call

does not compile.

Here is a shorter example demonstrating the problem:

protocol P {
    mutating func foo()
    mutating func bar()
}

extension P {
    mutating func bar() {}
}

class C: P {
    func foo() {}
}

let c = C()
c.foo()
c.bar() // Cannot use mutating member on immutable value: 'c' is a 'let' constant

The underlying reason (as I understand it from the sources listed below) is that a mutating func called on a reference type is not only allowed to mutate the properties of self, but also to replace self by a new value.

It would compile if P is declared as a class protocol instead (and the mutating keyword is removed):

protocol P: class {
    func foo()
    func bar()
}

extension P {
    func bar() {}
}

Related resources:

Upvotes: 3

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