Hi, I'm Derick. Welcome to PHP internals news, a podcast dedicated to explaining the latest developments in the PHP language. This is Episode 89. Today I'm talking with Larry Garfield and Joe Watkins about a partial function application RFC that they're proposing with Paul Crevela and Levi Morrison. Larry, would you please introduce yourself?

Hello World. I'm Larry Garfield or Crell on most social medias. I'm a staff engineer for Typo3 the CMS. And I've been getting more involved in internals these days, mostly as a general nudge and project manager.

And hello, Joe, would you please introduce yourself as well?

Hi, I'm Joe, or Krakjoe, I do various PHP stuff. That's all there is to say about that really.

I think you do quite a bit more than just a little bit. In any case, I think for this RFC, you, you wrote the implementation of it, whereas Larry, as he said, did some of the project management, I'm sure there's more to it than I've just paraphrased in a single sentence. But can one of you explain in one sentence, or if you must, maybe two or three, what partial function applications, or I hope for short, partials are?

Partial function application, in the broadest sense, is taking a function that has some number of parameters, and making a new function that pre fills some of those parameters. So if you have a function that takes four parameters, or four arguments, you can produce a new function that takes two arguments. And those other two you've already provided a value for in advance.

Okay, I feel we'll get into the details in a moment. But what are its main benefits of doing this? What would you use this for?

Oh, there's a couple of places that you can use partial application. It is what got me interested. It's very common in functional programming. But it's also really helpful when you want to, you have a function that like, let's say, string replace takes three arguments, two of which are instructions for what to replace, and one of which is the thing in which you want to replace. If you want to reuse that a bunch of times, you could build an object and pass in constructor values and save those and then call a function. Or you can just partially apply string replace with the things to search for, and the things to replace with and get back a function that takes one argument and will do that replacement on it. And you can then reuse that over and over again. There are a lot of cases like that, usually use in combination with functions that wants a callback. And that callback takes one argument. So array map or array filter are cases where very often you want to give it a function that takes one argument, you have a function that takes three arguments, you want to fill in those first ones first, and then pass the result that only takes one argument to array map or a filter, or whatever. So that's the one of the common use cases for it.