[As described below, this is Part C of a 3-part course. Participants should complete Parts A and B first -- Part C "dives right in" and refers often to material from Part A and Part B.] 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 of Part A. Part B assumes successful completion of Part A. 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. 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.
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Programming Languages, Part C
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Section 8 and Homework 6 (First Module with Ruby)
Let's get started programming with Ruby, including learning about Ruby's variant of (almost) function closures and the "inheritance and overriding" that is the essence of object-oriented programming. The welcome message has a few additional comments about "what makes Ruby different" and how to approach the (rather different) homework assignment, so let's get started...
Meet the Instructors
Dan GrossmanProfessor
Computer Science & Engineering
[music] In this segment, I want to talk a little bit more about which parts of a
Ruby program can access and use which other parts of a program since that's an
essential aspect of any programming language.
As we know, hiding things is essential for modularity and abstraction.
That's why we studied, for example, the module system in ML.
And object-oriented programming languages generally have various ways to take what
Ruby calls instance variables, methods, classes, etc.
And make them available to only part of a program.
So we can use Ruby as a good example of the sort of decisions that languages tend
to. To make.
So, Ruby does one thing that I actually like quite a lot, which the, it emphasizes
that object state is always private. So, instance variables can only ever be
accessed by methods on the actual object that has that instance variable.
Not even another object of the same class. So as a result when we access or assign to
an instance variable we always write @foo. We never write e.@foo, because you're not
allowed to access @foo for any object other than self.
And so self dot would be redundant, and you're not even allowed to write it.
So, to make objects stay publicly visible, when that is what we want, when we want
the outside world to know the content, to be able to update the contents of an
instance variable, then we need to define our own methods to do that.
And we can do that explicitly and these methods are usually called getter methods
when they return the contents of a field and setter methods when they update the
contents of the field. So in Ruby we could very easily return the
contents of the foo instance variable with a method like get_foo whose expression
just looks up the contents and returns it. And similarly set_foo could take some
argument x and assign to the instance variable because the method is allowed to
access at foo and in this way we can provide access via the set foo method.
Now that said, this is common enough in Ruby that there's some built in support
for it and further more, the conventional names are slightly different than I showed
on the previous slide. The convention in Ruby is that if you want
a getter method for the instance variable @foo, just call the method foo.
And if you want a setter method call it foo equals.
It turns out in Ruby your allowed to have a method end with the equals character,
and that's the convention for a setter method.
This convention actually goes one step further.
There's a cute piece of syntactic surgar in Ruby, which is if you do have a method
that ends in equals, when you write a call to that method your allowed to have.
White space, space characters between the non equals character and the equal.
So, it turns out in Ruby, you can write e.foo space equals some expression.
And that's the exact same thing as foo without the space.
It is literally a method call to the foo, foo equals method.
Nothing more, it just makes using setters look nicer.
Furthermore people felt that writing getters and setters like this was three
lines long and that was just too long. So there are other ways you can accomplish
the same thing in ruby. If you want getter methods foo and bar for
instance variables at foo and at bar. You can just write attr underscore reader,
it's short for attribute reader, colon, foo, comma separated, and then the names
of all the methods preceded with colons that you want getter methods for.
And, I'm not going to explain what the colons mean, and why this is the exact
syntax, but you're welcome to use this shorter form.
Form. And if you want getters and setters, you
just say attr underscore accessor, and if you said colon foo that would define both
the method foo, and the method foo equal. But these are just shorthand versions.
All we are doing are defining getter and setter methods for accessing the instance
variables in a way that makes them truly private to the object.
And it's only the methods that are giving the outside world an Indirect way to
access them. So why private object state?
Well it turns out most people would agree that requiring these instance variables to
be private to just the object makes the object more in an object oriented
programming style. That it let's you focus on the interface
to an object what method you can call. Without knowing how the object is actually
represented. Why is that good?
It's for all of our normal abstraction and modularity reasons.
That way later the class implementation could change and clients are not messed up
by there being different instance variables or different implementation of
the same methods. In fact it hides from clients how the
actual instance variables are storing the data.
I have acute example here on the slide where maybe I give clients a method
celsius_temp= which they would probably assume was a setter method for an instance
variable, celsius temp. But maybe internally the class finds it
much easier to store temperatures in kelvin, and it simply implements this
setter method by writing to a kelvin temperature instance variable
inappropriately shifted value. In fact you can do this for different
classes, that represent internal state in entirely different ways and this is
related to the idea of duck typing, that we'll talk about in a slightly future
segment. So that's the story for instance
variables. Now let's talk about methods.
So it turns out Ruby has three different visibilities, three different rules that
it can apply to methods and you can choose for each method which one you want.
So you can have private methods which, just like private instance variables, can
only be used by other methods of the same object.
You could have protected which cannot be used by just anyone.
A protected method can only be used by objects of the same class or any
subclasses. And we still have to discuss subclasses.
So it doesn't have to be the object itself, but it does have to be the same
class or a sub-class. And then there's public, and a public
method can be called by anyone who has access to the object.
If you have the object, it has a public method m, you can call m on it.
So, the default for methods is public, and that makes some sense.
The entire purpose of an object is to call methods on it, but there are various ways,
there are always various ways in Ruby, various ways to change the visibility of
an object. Let me just show you one way.
Here on this next slide. So when your define some class foo, and
you're going to have a bunch of method definitions, the default as I mentioned,
is public. So when you start defining methods, all
those methods will be public. But in your class definition, you can use
the keyword protected and once you write that word in between methods, then all the
following methods will have the protected visibility instead of public.
And similarly, you could then switch back to public, so it's a keyword as well.
And then that would make these the method definitions after this public be public,
and there's a keyword private. So basically what you do is when you have
a method definition, to figure out its visibility, you find the most recent use
of protected public or private, and that's what it is.
Uh,and at the beginning of the class it's like there's an, an implicit public to get
things started. So that's really the entire story on
method visibility to the extent I want to get into it.
There is one detail which is for private methods, it turns out since the only
methods that are going to able to call that method are methods on the same
object, you can always just write m or m of args.
Because as we know that shorthand for self.m, that's a short way of writing a
method call on the same object. But for private methods, that's the only
call that is allowed and so Ruby actually forces you to use the shorthand.
Any other object here would not be allowed by under visibility rules.
You would think it would allow self dot, just because you wanted to remind yourself
that it was a call on the same object. But, in fact, it really does not allow it
even syntactically. You cannot write the self dot on a private
method. You just have to call the method with the
short form that does not indicate which object you're calling the method on.
And that's everything we need to know about method visibility in Ruby.
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