Xcode Swift: extension of array: "can't invoke.. with argument list .. "

Motivation: Trying to construct an 'contains' function that return true or false depending on if the array contains the parameter supplied in object.

Error: Can't invoke 'exist' with argument list of type '((T) -> Bool)'

I can't tell if the problem is in extension of Array,or in the closures. Both expressions in contains() have similar error.

The error looks ridiculous since ((T) -> Bool) is exactly what I have written for parameter of 'exist'

extension Array{    
    func exist(requirement:(T) -> Bool) -> Bool{
        for a in self{
            if(requirement(a)){
                return true
            }
        }
        return false
    }
    func contains(a:T) -> Bool{
        //return exist{return $0==a}
        return exist{(s0:T)->Bool in return s0==a}
    }
}