The Evy Programming Language

Evy is an interpreted, event-oriented, dynamic, general purpose functional language. Evy's simple, expressive syntax makes it especially suitable for creating domain-specific languages, such as conversation trees for video games and UI workflows for applications.

Evy closely models the publisher-subscriber pattern. Each statement either publishes or subscribes to an event. Events can have any number of parameters, which can be key-value pairs, or just values. Value-only parameters are dynamic: they act as values unless or until a matching key comes along, after which the parameter is transformed into a key-value pair.

Learning by Example

As with any new language, it's generally easier to learn by example than by studying descriptions of the language or by parsing syntax. Here are some practical examples of Evy in action.

Hello, world!

print "Hello, world!"

This does exactly what you would expect: it prints the phrase "Hello, world!" to the screen. What you're doing in this line of code is publishing the print event with a single parameter: the string "Hello, world!".

The following program does exactly the same thing, but in a slightly different way:

print Hello, world!

Notice that I've left out the quotes. It still does exactly the same thing as the first example, but why? What you're doing in this line of code is publishing the print event with two parameters this time: the value Hello, and the value world!. The print event is handled natively, by joining the values of each of its parameters with a space.

Subscriptions

Let's say you want to cause the program to exit whenever the phrase "exit" is printed...

@ print exit
  exit

Now, if the print event is published with a first parameter of exit, you will exit the program.

Conversation tree

Now, let's say we want to implement a conversation tree for a game we're developing. The Evy script might look something like this:

@@ Greeting

  NPC says="Greetings, stranger!"

  Consider saying="Do I know you?"
  Consider saying="Hello, sir."

  @ Player says="Do I know you?"

    NPC says="We met in a dream a long, long time ago."

    ...

  @ Player says="Hello, sir."

    NPC says="You don't remember me, do you?"

    ...

Your game would interact with this script by implementing subscribers for the NPC and Consider events and publishers for the Greeting and Player events, as described below:

• The subscriber for NPC would cause the NPC to say the value of the says parameter.
• The subscriber for Consider would present an option for the player to say the value of the saying parameter.
• You would publish a Greeting event when the player begins to interact with the NPC.
• You would publish a Player event whose says parameter matches one of the options presented to the player via the Consider event.

More Examples

...TODO: provide some more practical examples...