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A prototype-based programming language inspired by Self, Io, Ruby and Smalltalk

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Espresson is an experimental dynamic, prototype-based, object-oriented programming language that compiles down to JavaScript.

While it is object-oriented it's still very different from most modern languages, for instance you won't find any classes or variables, instead there are only objects and slots, even lexical scope is implemented entirely in terms of language primitives.

The way you communicate with objects in Espresso is by sending messages of which there are three kinds: unary, binary and keyword.

Espresso also has, although currently very limited, support for predicate dispatch.


npm install -g espresso-language

This will install the Espresso compiler (esc) globally on your machine.

To run a Espresso script just type esc, if you would rather compile it to a js file type esc --compile which will create a JavaScript file with the same basename in the current directory (name.js in this case).

Executing esc without any arguments launches a (very basic) REPL.

For more information run esc --help

Note: This requires that you have nodejs v0.8.8 and npm installed already.

Hello, world!

So, let's look att how Epresso works. We'll start with the ever present Hello, world:

"Hello, world!" println //=> "Hello, world!"

Here we are sending the unary message println to a string object containing Hello, world!. In this case, since println is a method, it results in a method call but it could just as well have been a regular slot access and it would have looked just the same.

Now, let's expand this example and introduce a few more concepts:

Greeter = Object clone
Greeter greet: name with: message := {
  "#{message}, #{name}!" println

Greeter greet: "world" with: "Hello" //=> "Hello, world!"

We begin by sending the clone message to the prototypical Object object, this returns a brand new object who's prototype points to Object and then assign it to Greeter. Next we define the greet:with: message on Greeter, the := operator signifies to the compiler that this should be treated as a method. Finally we send the greet:with: message to the Greeter object. One thing you may note is that the name of this message and it's arguments are mixed togheter, this is called a keyword message, but the name actually is greet:with: and you can validate this by asking Greeter for a reference to it rather than executing it, like so:

greet = Greeter get: 'greet:with:
greet call: "world", "Hello" //=> "Hello, world!"

The get: message returns a method reference that we can call:. It takes a variadic number of arguments and will by default execute in the context of it's original receiver, in this case Greeter, though you can override that choice by sending the call:as: message instead.

Now, you may be wondering about Greeter and greet since I earlier told you that there are no variables in Espresso, but they sure do look like variables don't they? Well, actually they are slots on an object, in this case the top-level object Lobby.

Lobby println // => Lobby
                  >>> Greeter
                  >>> greet

With that out of the way, let's return to our friend the Greeter and extend it further:

Greeter = Object clone
Greeter greet: someone with: message := {
  "#{name} says: #{message}, #{someone name}" println

g1 = Greeter clone: @{ name = "G1" }
g2 = Greeter clone: @{ name = "G2" }

g1 greet: g2 with: "Hello" //=> "G1 says: Hello, G2!"

What we see here is a common architectural technique in prototype based languages, namely separating data from behavior. Our prototypical Greeter object depends on a name slot being defined but it never defines it rather it expects that any object which clones it does, in this case that would be g1 and g2.

As you can see, g1 can now greet g2 with a nice message. But there's a slight problem here, and that is, our greet:with: method is very specific in that it requires it's first argument to be an object with a name slot. What if we wanted it to call it with a string instead?

g1 greet: "world" with: "Hello" //=> "g1 says: Hello, nil!"

Hmm, not exactly what we wanted. Now we could solve that using a conditional inside the greet:with: method but then it wouldn't be possible to extend it further without changing that method. This is where predicate dispatch comes in handy:

Greeter greet: someone with: message := {
  "#{name} says: #{message}, #{someone}!" println

Greeter greet: someone @{understands?: 'name} with: message := {
  greet: someone name with: message

g1 greet: "world" with: "Hello" //=> "P1 says: Hello, world!"
g1 greet: g2      with: "Hello" //=> "P1 says: Hello, P2!"

Now we have two implementations of greet:with: a specific one which requires a named object and a general one that accepts any object. The great thing about this is that it can be extended further, even by third party code, without needing to change the original definition.