How can I change the textual representation displa

Question

How can I modify the textual output that gets displayed in string interpolation?

The Printable protocol looks the most obvious but it's ignored in both String Interpolation and when printing the instance, e.g:

struct Point : Printable
{
    var x = 0
    var y = 0

    var description : String {
        return "(\(x), \(y))"
    }

    func toString() -> String {
        return description
    }
}

Likewise the toString() convention has no effect either:

var p = Point(x: 10, y: 20)

println(p)                   // V11lldb_expr_05Point (has 2 children)
println("\(p)")              // V11lldb_expr_05Point (has 2 children)
println(p.description)       // (10, 20)
println("\(p.description)")  // (10, 20)

The behavior is different again in PlayGround which uses its own String representation for structs, i.e:

p // {x 10, y 20}

Is there a way I can change how an instance is displayed?

Answer 1

Relevant Apple Swift Docs

Apple provides this example:

struct MyType: Printable {
    var name = "Untitled"
    var description: String {
        return "MyType: \(name)"
    }
}

let value = MyType()
println("Created a \(value)")
// prints "Created a MyType: Untitled"

If you try this in playground, you will get the same issue that you're getting (V11lldb_expr...). In playground, you get the description on the right hand side when you call the initializer, but the println doesn't return something legible.

Out of playground, however, this code behaves as you would expect. Both your code and the sample code from Apple above print the correct description when used in a non-playground context.

I don't think you can change this behavior in playground. It could also just be a bug.

EDIT: I'm pretty sure that this is a bug; I submitted a bug report to Apple.

UPDATE: In Swift 2, instead of Printable, use CustomStringConvertible (relevant doc link).

struct MyType: CustomStringConvertible {
    var name = "Untitled"
    var description: String {
        return "MyType: \(name)"
    }
}

let value = MyType()
println("Created a \(value)")
// prints "Created a MyType: Untitled"

Answer 2

This appears to be a bug in the playground. If you actually compile and run the programme normally it prints:

(10, 20)
(10, 20)
(10, 20)
(10, 20)

as expected.

You should report this at https://bugreport.apple.com

Answer 3

As an alternative in Swift 5+ you can extend the String.StringInterpolation

struct Point {
    var x : Int
    var y : Int
}

extension String.StringInterpolation {
    mutating func appendInterpolation(_ value: Point) {
        appendInterpolation("\(value.x):\(value.y)")
    }
}

This will change the value for print("\(p)") but not for print(p) - which will still use the description

Answer 4

If you open the console view : View -> Assistant Editor -> Show Assistant Editor you can see expected print lines Checked in xCode 6.3.2 with Yosimite 10.10

Answer 5

Swift 2 - 4

Summary

Conform to the CustomStringConvertible protocol and add description:

var description: String {
    return "description here"
}

---

Example

You can create some structs:

struct Animal : CustomStringConvertible {
    let type : String

    var description: String {
        return type
    }
}

struct Farm : CustomStringConvertible {
    let name : String
    let animals : [Animal]

    var description: String {
        return "\(name) is a \(self.dynamicType) with \(animals.count) animal(s)."
    }
}

If you initialize them:

let oldMajor = Animal(type: "Pig")
let boxer = Animal(type: "Horse")
let muriel = Animal(type: "Goat")

let orwellsFarm = Farm(name: "Animal Farm", animals: [oldMajor, boxer, muriel])

The custom descriptions will appear in your playground:

See also CustomDebugStringConvertible, which you can use for more verbose output during debugging.

---

Usage Note

You can initialize a String from any type without implementing this protocol. For example:

For this reason, the docs say:

Using CustomStringConvertible as a generic constraint, or accessing a conforming type's description directly, is therefore discouraged.