str

is an implicitly predefined variable

I am sorry. What do you mean by

implicitly predefined variable

? Do you mean it has been defined earlier in the code? Where is it defined?

thats the

implicit

part, its nowhere defined

...
val str = "some string"
...
fun reformat(str: String,
             normalizeCase: Boolean = true,
             upperCaseFirstLetter: Boolean = true,
             divideByCamelHumps: Boolean = false,
             wordSeparator: Char = ' ') {
...
}
...
reformat(str, true, true, false, '_')

Oh okay so it has been defined somewhere earlier in the code.