Hello World – How It Works¶
Let’s look at our
helloworld code again:
actor Main new create(env: Env) => env.out.print("Hello, world!")
Let’s go through that line by line.
This is a type declaration. The keyword
actor means we are going to define an actor, which is a bit like a class in Python, Java, C#, C++, etc. Pony has classes too, which we’ll see later.
The difference between an actor and a class is that an actor can have asynchronous methods, called behaviours. We’ll talk more about that later.
A Pony program has to have a
Main actor. It’s kind of like the
main function in C or C++, or the
main method in Java, or the
Main method in C#. It’s where the action starts.
new create(env: Env) =>
This is a constructor. The keyword
new means it’s a function that creates a new instance of the type. In this case, it creates a new Main.
Unlike other languages, constructors in Pony have names. That means there can be more than one way to construct an instance of a type. In this case, the name of the constructor is
The parameters of a function come next. In this case, our constructor has a single parameter called
env that is of the type
In Pony, the type of something always comes after its name and is separated by a colon. In C, C++, Java or C#, you might say
Env env, but we do it the other way around (like Go, Pascal, Rust, TypeScript, and a bunch of other languages).
It turns out, our
Main actor has to have a constructor called
create that takes a single parameter of type
Env. That’s how all programs start! So the beginning of your program is essentially the body of that constructor.
Wait, what’s the body? It’s the code that comes after the
This is your program! What the heck is it doing?
In Pony, a dot is either a field access or a method call, much like other languages. If the name after the dot has parentheses after it, it’s a method call. Otherwise, it’s a field access.
So here, we start with a reference to
env. We then look up the field
out on our object
env. As it happens, that field represents stdout, i.e. usually it means printing to your console. Then, we call the
env.out. The stuff inside the parentheses are the arguments to the function. In this case, we are passing a string literal, i.e. the stuff in double quotes.
In Pony, string literals can be in double quotes,
", in which case they follow C/C++ style escaping (using stuff like \n), or they can be triple-quoted,
""" like in Python, in which case they are considered raw data.
What’s an Env, anyway? It’s the “environment” your program was invoked with. That means it has command line arguments, environment variables, stdin, stdout, and stderr. Pony has no global variables, so these things are explicitly passed to your program.
Really, that’s it. The program begins by creating a
Main actor, and in the constructor, we print “Hello, world!” to stdout. Next, we’ll start diving into the Pony type system.