FunctionBuilders

Stringify.swift

func stringify(@StringBuilder _ value: () -> Stringified) {
    print(value().value)
}

struct Stringified {
    let value: String
    init(_ value: String) {
        self.value = "stringified " + value
    }
}

@_functionBuilder
struct StringBuilder {
    static func buildBlock() -> Stringified {
        return Stringified("empty")
    }
}

extension StringBuilder {
    static func buildBlock(_ value: String) -> Stringified {
        return Stringified("string \(value)")
    }
    
    static func buildBlock(_ value: String...) -> Stringified {
        return Stringified("string \(value.joined(separator: ","))")
    }
}

stringify {
    "1"
    "2"
}

// stringified string 1,2

extension StringBuilder {
    static func buildBlock(_ value: Int) -> Stringified {
        return Stringified("int \(value)")
    }
    
    static func buildBlock(_ value: Int...) -> Stringified {
        return Stringified("int \(value.map(String.init).joined(separator: ","))")
    }
}

stringify {
    1
    2
}

// stringified int 1,2

extension StringBuilder {
    static func buildBlock<T>(_ value: T) -> Stringified {
        return Stringified("generic \(value)")
    }
    
    static func buildBlock<T>(_ value: T...) -> Stringified {
        return Stringified("generic \(value.map({ "\($0)" }).joined(separator: ","))")
    }
}

stringify {
    true
    false
}

// Following code does not compile

// stringified generic true,false

stringify {
    
}

// Cannot convert value of type '() -> ()' to expected argument type '() -> Stringified'
// Expected "stringified "

stringify {
    "1"
}

// Cannot convert value of type 'String' to closure result type 'Stringified'
// Expected "stringified 1"

stringify {
    1
}

// Cannot convert value of type 'Int' to closure result type 'Stringified'
// Expected "stringified 1"

stringify {
    true
}

// Cannot convert value of type 'Bool' to closure result type 'Stringified'
// Expected "stringified true"