Introduction

Evy is an interpreted, event-oriented, dynamic, general purpose functional language. It is built on the paradigm of asynchronous application design, where components communicate with each other by publishing and subscribing to events.

Examples

A simple Hello, world program in Evy:

print "Hello, world!"

A familiar scene:

@@ ask question
  @ reply question answer
    print ">" answer
  broadcast question
  
@@ broadcast question
  print question

@ broadcast question="What is your name?"
  reply question answer="My name is Sir Launcelot of Camelot."
@ broadcast question="What is your quest?"
  reply question answer="To seek the Holy Grail."
@ broadcast question="What is your favorite color?"
  reply question answer="Blue."
  reply question answer="Yellow."
  reply question answer="AAUUUGGGGGHH!"

ask question="What is your name?"
ask question="What is your quest?"
ask question="What is your favorite color?"

Using the HTTP API:

@ http_response
  print method url status body

http_request "/foo"

Tutorial

Let's walk through the basic syntax of an Evy program:

hello

This program publishes the hello event.

hello
goodbye

This program publishes the hello event and the goodbye event, but in no particular order.

hello
  goodbye

This program publishes the hello event, then publishes the goodbye event.

@ hello

This program subscribes to the hello event.

@ hello
  goodbye

This program publishes the goodbye event after the hello event occurs. Note that the hello event is not actually published in this program.

@ hello
  goodbye
hello

This program publishes the goodbye event after the hello event occurs. Subscriptions are given priority over publications, so the hello event is not published until after the subscription occurs.

hello
@ hello
  goodbye

This program is functionally identical to the previous one. It demonstrates how priority is given to subscriptions.

print "Hello, world!"

This program publishes the print event with an unnamed parameter whose value is "Hello, world!".

http_request url="/test/user.json"

This program publishes the http_request event with a named url parameter whose value is "/test/user.json".

@ http_request url
  print url

This program subscribes to http_request events that contain a named url parameter. This subscription publishes the print event with a named url parameter whose value is equal to the value of the most recently published url parameter. In this case, it is the value published by the http_request event.

http_request method="GET" url="/test/user.json"

This program publishes the http_request event with two named parameters: method and url.

@ http_request url
  print url
http_request method="GET" url="/test/user.json"

This program subscribes to http_request events that contain a named url parameter. The subscriber defined here will match even though the http_request event contains both a method parameter and a url parameter.

@ http_request url
  print method url
http_request method="GET" url="/test/user.json"

Even though the subscriber does not define the method parameter, because it is included with this publication, the subscriber has access to it. Though this is not recommended (you should always declare your dependencies!), it can make some code more readable.

Background

Programming UI for desktop, mobile, and web applications involves a great deal of event handling. From low-level user interactions such as key press and mouse movement, to object lifecycle events such as creation and destruction, to domain-specific events such as login success and specific page accesses, event handling has become a significant part of application development.

Evy is inspired by the heavy usage of event-driven development in JavaScript, especially in frameworks such as jQuery and Node.js. These frameworks have seen a great deal of success, largely in part due to their simplification of event handling.

I believe that an entire application can be built around the principle of publishing and subscribing to events. I also believe that this architecture would result in an inherently efficient and scalable application. Evy is my attempt to prove this to be true.

Design

Evy's grammar has very few rules:

EVENT: IDENTIFIER (' ' CONTEXT)*;
CONTEXT: (IDENTIFIER '=')? (STRING|INT|FLOAT|BOOLEAN);
IDENTIFIER: ('!'|'#'..'&'|'('..'/'|':'..'~')('!'|'#'..'&'|'('..'~')*

Every statement in Evy is an event. Identifiers can be virtually any printable character, with the following exceptions:

• An identifier may not begin with a number.
• An identifier may not contain a single-quote ('), a double-quote ("), or whitespace.