I think you misunderstood. The `action` closure is invoked with no arguments, but the invocation `self.isWarningEnabled.toggle()` does pass one argument, namel

I think you misunderstood. The `action` closure is invoked with no arguments, but the invocation `self.isWarningEnabled.toggle()` does pass one argument, namely the `self` value from the closure's lexical context (`.toggle()` needs to know which instance to manipulate). If you replace the no-argument closure with a function that expects one argument (`self.isWarningEnabled.toggle`), you get the partial application the error mentions (“partial application” is passing fewer arguments to a function than it expects).