How does one make an optional closure in swift? -

How does one make an optional closure in swift? -

i'm trying declare argument in swift takes optional closure. function have declared looks this:

class promise{ func then(onfulfilled: ()->(), onreject: ()->()?){ if allow callablerjector = onreject { // stuff! } } }

but swift complains "bound value in conditional must optional type" "if let" declared.

you should enclose optional closure in parentheses. scope ? operator.

func then(onfulfilled: ()->(), onreject: (()->())?){ if allow callablerjector = onreject { // stuff! } }

swift optional-parameters