Swift

5.8    Nested Functions

Yeah, you heard right: Swift allows you to nest child functions into parent functions, not unlike a dream within a dream as in the Inception movie. In case you missed the movie, Listing 5.49 features a simple example in which the parent isNameValid function contains nested child functions isLongEnough, hasNoSpaces, and hasNoNumbers.

// Functionfunc isNameValid(_ name: String) -> Bool {    func isLongEnough(_ name: String) -> Bool {        return name.count >= 3    }        func hasNoSpaces(_ name: String) -> Bool {        return !name.contains(" ")    }        func hasNoNumbers(_ name: String) -> Bool {        return !name.contains { $0.isNumber }    }        return isLongEnough(name) && hasNoSpaces(name) && hasNoNumbers(name)}// Main flowisNameValid("John")     // true