Partial Function Application and bind()

Transcript

1. Partial Function Application and bind() Daniel Brook-Roberge October 22, 2015 Engineering Meeting
2. What is Partial Function Application? • Partial application is an operation on a function and one or more parameter values, that produces a new function with the given parameter values “fixed”. • partial(f, 5) → g(x) • g(x) == f(5, x)
3. Currying and Partial Application • Currying is another operation on a function, that converts a function of multiple parameters into a series of functions of a single parameter: f(x, y, z) → f(x)(y)(z) • Functions are automatically curried in several functional languages, such as Haskell and F#. • A curried function is trivial to partially apply, as we can just execute a subset of the functions: f(2) → g(y, z) = f(x, y, z) • As such, currying and partial application are often confused.
4. Function.bind() in Javascript • ES5 provides a function method bind() that can be used for partial application. • The first parameter to bind() is bound to this inside the function, and subsequent parameters are bound to the function parameters in order.
5. Partial Application and Callbacks • Partial application is very useful for callbacks, as it can provide a callback with parameters set at registration time alongside parameters set at call time.
6. DRYing Code with Partial Application • Partial application can be useful to remove code duplication while preserving an existing API.
7. Example from Web Push Client
8. Partial Application and Function Parameters • Partial application is useful when combined with arguments that are functions. • This allows a function to be configurable for a variety of applications.
9. Summary • Partial function application is a technique for specifying some of the parameters to a function separately from the function call itself. • We can use it to customize functions for callbacks or to modularize logic for a DRYer codebase. • Javascript provides Function.bind for partially applying a function. • This is a technique that I find myself using all the time now that I am familiar with it.