Optional: Closure Idioms Without Closures

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From the course by University of Washington

Programming Languages, Part A

830 ratings

University of Washington

Programming Languages, Part A

830 ratings

This course is an introduction to the basic concepts of programming languages, with a strong emphasis on functional programming. The course uses the languages ML, Racket, and Ruby as vehicles for teaching the concepts, but the real intent is to teach enough about how any language “fits together” to make you more effective programming in any language -- and in learning new ones. This course is neither particularly theoretical nor just about programming specifics -- it will give you a framework for understanding how to use language constructs effectively and how to design correct and elegant programs. By using different languages, you will learn to think more deeply than in terms of the particular syntax of one language. The emphasis on functional programming is essential for learning how to write robust, reusable, composable, and elegant programs. Indeed, many of the most important ideas in modern languages have their roots in functional programming. Get ready to learn a fresh and beautiful way to look at software and how to have fun building it. The course assumes some prior experience with programming, as described in more detail in the first module. The course is divided into three Coursera courses: Part A, Part B, and Part C. As explained in more detail in the first module of Part A, the overall course is a substantial amount of challenging material, so the three-part format provides two intermediate milestones and opportunities for a pause before continuing. The three parts are designed to be completed in order and set up to motivate you to continue through to the end of Part C. The three parts are not quite equal in length: Part A is almost as substantial as Part B and Part C combined. Week 1 of Part A has a more detailed list of topics for all three parts of the course, but it is expected that most course participants will not (yet!) know what all these topics mean.

From the lesson

Section 3 and Homework 3 -- and Course Motivation

This section is all about higher-order functions -- the feature that gives functional programming much of its expressiveness and elegance -- and its name! As usual, the first reading below introduces you to the section, but it will make more sense once you dive in to the lectures. Also be sure not to miss the material on course motivation that we have put in a "lesson" between the other videos for this week and the homework assignment. The material is "optional" in the sense that it is not needed for the homeworks or next week's exam, but it is still very highly encouraged to better understand why the course (including Parts B and C) covers what it does and, hopefully, will change the way you look at software forever.

Meet the Instructors

Dan Grossman
Professor
Computer Science & Engineering

[MUSIC] This segment and the next couple, which are all optional fit together and

are about taking some of our closure idioms and porting them to languages that

don't actually have closures. So the idea here is that closures and

higher-order programming Programming with functions like map and filter is great.

And it's particularly easy when your language supports closures.

Because you can very easily create things with private data, pass them around.

Function types are exactly what we want. But if your language doesn't have

closures you can still program in this style.

Now it's often much more painful. You often have to create what

conceptually is your closure environment manually.

But by showing you how to do this in languages you might be familiar with we

can see some of the connections between different programming styles.

So one segment is going to do this in an object oriented style using java, and the

key idea there is to have interfaces, that just have one method, so that's kind

of like the function code, the call me. Part of an interface of, of a closure.

And then in C, where we have function pointers but we don't have closures, we

can pass the environment explicitly as an extra argument.

So these are optional segments because you need to be a bit of an expert in Java

to understand the Java one. And I bit of an expert in C to understand

the C one. Even if you do know some Java or C, you

may find this a bit over your head, that's okay.

I want to put these segments up, so you can refer back to them someday.

If you're in these languages, and you find yourself wanting to program in a

functional style, then these segments might help you, point you in the right

direction. So it shows some connections between the

languages and the features It can help you understand what closures and objects

are really all about, and at the end of the day it is a bit clumsy so it may make

you just wish that more programming languages actually had closures, so that

we didn't have to simulate them and incode the same idioms.

In more painful ways. So this segment just shows the ML code

that we're then going to translate, which is often called porting, to Java or C.

It's just a little list library. I'm not going to use ML's built in lists

because I want to show all the code, right? And I want to find a fuller

comparison because we're going to write everything ourselves in all three

languages. And then the next segments are

independent. You can watch one or the other or both or

neither And then show how to write the same code in those other languages.

So here we are. I'm just going to define my own

polymorphic data constructor. Alpha my list, two constructors.

Cons which has two parts, the head of the list, which is an alpha.

And the tail of the list, which is an alpha my list.

Or empty for the empty list. These are just plain old constructors.

So I can write a function map. I'm using a curried style here, but

that's not essential. That takes in a function and one of these

my lists, so not ML's built in lists but one of these my lists.

If it's empty we return empty. If it's not empty then we cons on F

applied to X and mapping F across Xs. That is mapped over the my list type.

Filter is similar, it's just the appropriate if then L.

I just want to expand this out here so you can see it.

Indent it a little more nicely. Empty list is empty list, if f of x, then

Cons x onto filtering f across x's, otherwise ignore x and just filter f

across x's. And finally, length, which is easier than

any of them. Empty list returns zero, non empty list

Returns one plus the length of the rest of the list.

Then a couple clients that are using these functions to define their own more

specific functions. How about something that takes a My List

of integers and doubles all the elements. Then we would just use the map function

that's no longer quite on the screen, but is above.

Remember two curried arguments So I'm doing a partial application here.

I'm just calling it with, an anonymous function that takes a number, and

multiplies it by 2. And then, how about another function that

just takes a, a list. A my list, of course.

And, in N. and returns how many.

elements of the list are n, okay? And so, one way I could do this in terms of the

functions I've written, it's not the most efficient way to compute this but it does

work, is to first filter the list. They so that we only have the elements

that are n. And then compute its length.

Alright? So the result of this called a filter will be a list that has nothing in

it but a bunch of the number n. Then however long that number, that list

is, is the number of n's that were in the original list.

So we're going to write that code in this style, in Java, and C in the upcoming

segments.

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