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From the course by University of Washington

Programming Languages, Part A

834 ratings

University of Washington

Programming Languages, Part A

834 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] There's just one more topic you need for Homework 3 and that's a little

bit of guidance on how to find functions in ML standard library.

So this doesn't really have a lot to do with the rest of the section other than

the standard library does have a lot of useful and in good style high order

functions. Well we decided to make it a small part

of Homework3 because learning how, new programming languages and how to use them

does involve learning how to find things in libraries.

So ML like most programming languages has a standard library.

This is just code that is always provided.

in a particular way with any implementation of the language.

I like to make this distinction that there are two sorts of things that you

put in a standard library when you're designing your programming language.

First, you put things that if you do not put them.

Noone using your language, will be able to do things.

If you don't give a way to open a file, on the computer the program is running

on, you can't write code that will do it some other way.

You need libraries for setting timers, libraries for accessing the network,

libraries for printing things out. These are things that you can't build out

of other pieces like lists and numbers and strength.

The other thing you put in a standard library is actually optional relatively

speaking. These are just things that are so common

that having a standard definition is appropriate.

Since ML programmers are going to use List.map, if we define it in the library,

in 1 place, with 1 name, with 1 set of arguments in a particular order, then

everyone can use it, and we can all read each others code more easily, and we

don't all have to rewrite the same function for every program we're writing.

So those are the two things one puts in a standard library.

And as I mentioned, you should be comfortable in any language seeking out

documentation and gaining some intuition on where to look.

it doesn't make sense to always every time you need a function expect that

someone like a course instructor will tell you exactly what function to use and

where to look for it. For it.

Okay. So standard ML has documentation for its

standard library, it's actually quite a but more primitive than the documentation

for most modern programming languages. But it will meet our needs, and it's at

this URL you will see here. Here.

Now, it's organized into things called structures and signatures.

This is using ML's module system, which we're going to study just a little bit in

the next section. But we can use the library before we

really understand structures and signatures.

And on homework 3 you just need to look things up in a few places.

Things related to strings, characters so the char structure, lists, and they've

separated out list pair. Those are functions that operate over two

different lists. 'Kay, once you find something the way you

use it is exactly like we have been using library functions already.

Whatever structure it's in, you write the name of the structure then a dot, and

then the function, so List.map, String.issubstring, or anything else.

So I do have a web browser open here. This is the URL I've pointed you to, and

there's a nice long list of things. There's functions for Arrays, some things

for the Command Line. Some things for oh, INetSock is probably

for accessing the network. One of the ones that we'll be interested

in is the List structure where we have lots of stuff over lists.

It gives a little synopsis of what this module is about.

and then it lists all the different functions that are defined, just like the

REPL would print them out. And the below that it has the actual

documentation. Giving the documentation of the semantics

of what these functions will do. So, for example, I never told you there

is a function last, that takes a list l and returns last element of l.

It raises a particular exception if the list is empty.

And so on and so forth. Okay.

So that's the documentation. There is no full substitute for proper

written documentation about how a library should be used, in that form.

But I wanted to point out one other thing.

Which is, when you're programming with a REPL you can often save yourself a little

bit of time by using the REPL to remind yourself some useful partial information

about how things work. Like what functions are defined.

what their types are what the order of arguments are.

And today I'm going to show this I'm not showing you anything new feature of the

REPL I'm just pointing out that you can use it for this purpose which you might

not have thought of and it's so useful to do that that some REPLs have gone to the

next step and they actually have added special commands that let you access the

full documentation. For libraries written in the language,

ML's REPL does not have that to my knowledge, so I just use it in sort of

the way REPL's can always do, I just use it to look things up.

So here's my REPL and I know that if I type, if I have a function defined like f

n = x + 1, if I just type F. I say, oh, that's a function, and it's

typed int arrow int. Well guess what? If I type List.map, it

nicely reminds me the type of List.map. Oh, look.

The 2 arguments are curried, not tupled. Maybe that's why I was getting an error

message, so this is just a convenient thing to do.

You can even guess, oh. List.last, was that, was that the name of

the function? Oh, yes, it was. Right? You can kind of guess at function

names if you want. if you think it's called fold left, it

turns out you're wrong. You get and unbound variable that's not

in the library. You're like, oh, is it called fold? No,

it turns out it's called fold L. Now at some point you should stop

guessing and messing around, and you should look up the documentation.

But you know, this is fairly convenient. It turns out with one other trick, I'm

not going to explain why this works, you can actually get the REPL to print out

all the bindings. for a particular structure and once we

know a little bit more about the module system this might make more sense.

So for example, if you knew there was a list structure, and you just wanted to

without going to your web browser look up everything, you can write the keyword

structure, this is all ML code it's just features I haven't shown you.

x = list And this is not very useful, it tells you that the signature, which is

kind of the type of a structure is listed all capitals.

And now if I say signature, spelled out English word, acts equals list.

So I'm putting here whatever was after the colon on the previous line and hit

return. While it prints out all that first part.

Now I can't go that next step, I don't get that actual documentation but you

might still find this useful. Anyway that's your brief guide to

programming against libraries and that should be everything you need to complete

Homework 3.

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