Unused code — a variable, parameter, or literally some code — is harmful. Not because it does something in a wrong way — by definition it does not — but since it sits unused it tells that it is very likely it not by design, rather by omission. Hence Skila reports an error in such case — so far I implemented check for unused parameters. If it is intended, because you have to implement some interface method and in your particular implementation this or that parameter does not play any role, mark it explicitly:
def foo(@x : Int) : Int ...
The name of the parameter is “x”, and “@” character is here as an attribute. Of course the check works in other way too — you cannot use such parameter.
Skila 