1

I am recently learning the source code of the Bool type in Swift. I have defined a Coin structure (shows below) imitating the source code. Similar to Bool, there are two values of true and false. I want to assign two values to Coin, on and off. It is hoped that the following effects can be achieved during initialization:

let coin = on

However, it is obviously not possible now, what can I do to achieve a direct initialization method like

let display = false?

At the same time, I also want to know where the two reserved keywords, false and true, are defined. I didn't find any relevant content in the Bool.swift file. I hope you can guide me.

The Coin structure is defined below:

public protocol ExpressibleByCoinLiteral {
    associatedtype CoinLiteralType: _ExpressibleByIntCoinLiteral
    init(coinLiteral value: CoinLiteralType)
}

public protocol _ExpressibleByIntCoinLiteral {
    init(_intCoinLiteral value: Int)
}

public struct Coin {
    public var _value: Int

    public init() {
        self._value = 0
    }
    
    @_transparent
    public init(_ value: Int) {
        self._value = value
    }
    
    @inlinable
    public init(_ value: Coin) {
        self = value
    }
}

extension Coin: _ExpressibleByIntCoinLiteral, ExpressibleByCoinLiteral {
    public init(_intCoinLiteral value: Int) {
        self._value = value
    }
    
    @_transparent
    public init(coinLiteral value: Coin) {
        self = value
    }
}

extension Coin: LosslessStringConvertible {
    public init?(_ description: String) {
        if description == "on" {
            self = on
        } else if description == "off" {
            self = off
        } else {
            return nil
        }
    }
}
2

They are the tokens for Boolean literals, defined in Token.h (C++):

#ifndef SWIFT_TOKEN_H
#define SWIFT_TOKEN_H

#include "swift/Basic/SourceLoc.h"
#include "swift/Basic/LLVM.h"
#include "swift/Parse/Token.h"
#include "swift/Config.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/StringSwitch.h"

namespace swift {

enum class tok : uint8_t {
#define TOKEN(X) X,
#include "swift/AST/TokenKinds.def"

  NUM_TOKENS
};
...

Note that Token.h defines the TOKEN macro and then includes the file TokenKinds.def, which defines a bunch of macros for different kinds of token.

1 Like
3

Possibly you're just playing around in order to learn the stdlib implementation, in which case disregard this. But if you are actually trying to implement this in your (non-stdlib) code, then the canonical way to do this is:

enum State {
    case on
    case off
}

let coin = State.on
// Or equivalently:
let coin: State = .on

You could create global constants if you really want to mimic boolean more closely, although that'd be a bit non-ideomatic:

enum State {
    case on
    case off
}

let on = State.on
let off = State.off

let coin = on

…though I don't think that'll work as you want with optionality.

3 Likes
4

Thank you so much

1 Like