[Extensibility] Writing DSLs

The CoffeeScript grammar is pretty flexible when it comes to writing a DSL. Implicit parentheses, commas and objects combine to give you an expressive language which compiles to good old JavaScript. The examples on this page assume a good level of knowledge of CoffeeScript.

Implicit objects can be nested below a function call

Let's look at an example of what we are trying to accomplish before we get started:

# DSL
describe 'Whiskey'
  age: 18
  brand: 'Jack'

# Usage
glass = new Whiskey()
puts glass.age  # 18

To implement our custom describe function let's first look at the generated JavaScript for it:

describe('Whiskey')({
  age: 18,
  brand: 'Jack'
});

We have a call to the function itself and a following one to its returned value. CoffeeScript makes it easy to implement such chaining:

# Implementation
describe = (name) -> (properties) ->
  window[name] = class
    constructor: ->
      (@[name] = value) for name, value of properties

Implicit parentheses turn code into natural language

Example:

# DSL and usage
a = keyA: 'valueA'
b = keyB: 'valueB'

extend b, using a

puts b.keyA  # 'valueA'

This compiles to fairly simple JavaScript:

extend(a, using(b));

To implement this we only need a few lines of CoffeeScript:

# Implementation
using  = (obj) -> obj
extend = (obj, using) -> (obj[key] = value) for key, value of using