What is 'Currying'?

Question

I\'ve seen references to curried functions in several articles and blogs but I can\'t find a good explanation (or at least one that makes sense!)

Answer 1

An example of currying would be when having functions you only know one of the parameters at the moment:

For example:

func aFunction(str: String) {
    let callback = callback(str) // signature now is `NSData -> ()`
    performAsyncRequest(callback)
}

func callback(str: String, data: NSData) {
    // Callback code
}

func performAsyncRequest(callback: NSData -> ()) {
    // Async code that will call callback with NSData as parameter
}

Here, since you don't know the second parameter for callback when sending it to performAsyncRequest(_:) you would have to create another lambda / closure to send that one to the function.