[swift-users] Strange Behavior With Closures

[Joe Groff]

> On May 4, 2016, at 6:19 PM, bwm003--- via swift-users <swift-users at swift.org> wrote:
> 
> Hi Everyone, 
> I was playing around in Xcode 7.3 with this code...
> 
> /**
> 
>  *  Represents a future value of type U, from a computation that has not been done yet.
>  */
> public struct Promise<T, U> {
>     var futureValue: T -> U
>     
>     public func then<T,U,V>(closure: U -> V) -> Promise<T, V> {
>         return Promise<T, V>(futureValue: {
>             input in
>             let value = futureValue(input)
>             return closure(value)
>         })
>     }
> }
> 
> 
> and came across the following error:
> 
> error: cannot invoke value of type 'T -> U' with argument list '((T))'
> 
> Can someone explain what is going on here? It looks like my closure should be accepting a value of T, but for some reason the types aren't matching up.

You've shadowed Promise<T, U>'s generic type parameters T and U with new T and U type parameters on `then`. Remove those, and this should work:

public struct Promise<T, U> {
    var futureValue: T -> U
    
    public func then<V>(closure: U -> V) -> Promise<T, V> {
        return Promise<T, V>(futureValue: {
            input in
            let value = futureValue(input)
            return closure(value)
        })
    }
}

If you have time, we'd appreciate a bug report about producing a better diagnostic here. We should warn or error when generic parameters are shadowed like this.

-Joe