ExpressibleByStringInterpolation

Use string interpolation to include one or more expressions in a string literal, wrapped in a set of parentheses and prefixed by a backslash. For example:

let price = 2
let number = 3
let message = "One cookie: $\(price), \(number) cookies: $\(price * number)."
print(message)
// Prints "One cookie: $2, 3 cookies: $6."

Add new interpolation behavior to existing types by extending DefaultStringInterpolation, the type that implements interpolation for types like String and Substring, to add an overload of appendInterpolation(_:) with their new behavior.

For more information, see the DefaultStringInterpolation and StringInterpolationProtocol documentation.

To create a new type that supports string literals and interpolation, but that doesn’t need any custom behavior, conform the type to ExpressibleByStringInterpolation and implement the init(stringLiteral: String) initializer declared by the ExpressibleByStringLiteral protocol. Swift will automatically use DefaultStringInterpolation as the interpolation type and provide an implementation for init(stringInterpolation:) that passes the interpolated literal’s contents to init(stringLiteral:), so you don’t need to implement anything specific to this protocol.

If you want a conforming type to differentiate between literal and interpolated segments, restrict the types that can be interpolated, support different interpolators from the ones on String, or avoid constructing a String containing the data, the type must specify a custom StringInterpolation associated type. This type must conform to StringInterpolationProtocol and have a matching StringLiteralType.

For more information, see the StringInterpolationProtocol documentation.