> On May 4, 2016, at 6:19 PM, bwm003--- via swift-users <swift-users@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
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