the-egg-language: A JavaScript repository from mikew1

Goals for this repo

Implement, explore & perhaps extend The Egg Language as described by Marijn Haverbeke in Eloquent Javascript.

See and experience how any computer language^* is in fact implemented... in some other computer language!

Here, Egg is written in javascript (javascript is perhaps an uncommon choice for this kind of thing, but it doesn't matter).

Here's a quick introduction to Egg.

To set a variable:

define(word, 2)

Note assignment is not done with =, it's define that does that, which can be confusing.

It's worth pausing to let that sink in.

To get the variable back:

print(word)
> 2

Now something much harder. Here's a function definition for timesBy, a higher order function, which returns a function. Function defininitions take the form fun(args... body), with however many args you want. The thing we define below, timesBy, is a function (fun), that has one arg, howMuch. Its body, which is the same as its return value, then follows, in the form of a function definition. So, timesBy is function which makes functions which timesBy howMuch you specify.

Define timesBy:

define(timesBy,
    fun(howMuch, define(anon, 
                   fun(byWhat, *(howMuch, byWhat))
                 )
        )
)

Call timesBy to get a function back:

define(timesBy4, timesBy(4))

Call the function just made:

print(timesBy4(5))
> 20

Or do both steps at once, i.e. call the factory then immediately call the resulting function.

You might have seen this form in javascript.

print(timesBy(4)(5))
> 20

To reiterate, if you ended the line after the (4) above, what you'd have is a function, so the (5) after that is another invocation.

Higher order functions are by no means the meat and potatoes of the language implementation. They're pretty incidental, and I don't think Marijn even pointed out that Egg supported them. The interesting stuff is a lot simpler than that. I just wanted turn up the gas for the introduction.

Why write some other language in javascript?

In Marijn's own words:

Building your own programming language is surprisingly easy (as long as you do not aim too high) and very enlightening.

Eloquent Javascript is a wonderful book, by the way! The role of this repository is to consolidate the code which Marijn has provided in Chapter 12, then explore & extend it.

My own why do this right now is that writing a DSL/language like this helps build a strong foundation for Lisp/Clojure, which I'm currently learning.

How to explore

Clone the repo & check out the commit history to see the language being constructed.

Recommended prerequisites

Be OK with regular expressions & recursion and/or willing to refresh.

Want to know how a programming language is created!

What to try to break/change

Everything. There's loads of scope to modify the code here.

Todos

allow comments (easy)
add arrays (easy)
consider how to convert it to a compiler
consider where macros would fit if this was a Lisp
consider how to make a REPL
write a little text adventure in egg based on a Lisp/Scheme example.
since egg is already in PN, why not move the first opening parens to the left, to be closer to S-expr & more lispish. Am finding the half-way house syntax rather unreadable. Have never seen a language quite like this!
shorten define to def & function to fn

current syntax:

do(
   define(x, 10),
   if(>(x, 5),
     print("large"),
     print("small")))

consider change to new syntax:

(do 
  (def x, 10),
  (if (> x, 5),
    (print "large"),
    (print "small")))

if above works, remove commas or make them optional, to get:

(do 
  (def x 10)
  (if (> x 5)
    (print "large")
    (print "small")))

consider anon functions. If name is "anon", make actually anonymous, unreferencable? So, how is lambda defined?