When external parameter name is needed to call swift functions

In the following cases, the parameter name is required when calling a swift function

1. when defining a regular function without external parameter name, the caller does not specify the first parameter's name, but need to specify all following parameter's name

    func join0(string: String, toString: String, withJoiner: String) -> String {

        return string + toString + withJoiner

    }

    join0("hello", toString: "world", withJoiner: ", ")

2. if the function's first as well as other parameters have external names, (internal name and external name are indicated by a space),

    func join1(string s1: String, toString s2: String, withJoiner: String) -> String {

        return s1 + withJoiner + s2

    }

   

    join1(string: "hello", toString: "world", withJoiner: ", ")

3. if the function's first parameters starts with '#', it indicates internal name and external name are same

    func join2(#string: String, toString: String, withJoiner: String) -> String {

        return s1 + joiner + s2

    }

   

    join1(string: "hello", toString: "world", withJoiner: ", ")

 

4. if the function's first parameters has default value, the caller must specify the parameter name to call it

    func join3(string: String ="abc", toString: String, withJoiner: String) -> String {

        return s1 + joiner + s2

    }

   

    join3(string: "hello", toString: "world", withJoiner: ", ") 

 5.In any case, if the parameter's external name is '_', then caller does not need specify the parameter's name,

    func join4( _ string: String = "abc", _ toString: String, _ withJoiner: String) -> String {

        return string + withJoiner + toString

    }

    

       join4("hello", "world", ", ")

    Sometimes, if a parameter has a default value, but does not want to have an external name, then, explicitly specify '_' as external parameter name as shown below:

• func join(s1: String, s2: String, _ joiner: String = " ") -> String {
• return s1 + joiner + s2}
• join("hello", "world", "-")