;; Look! I'm redefining addition! (locally)
100 + 10 where
   a + b = a - b

;; Currying a function using an external library
;; This could take a few seconds to load, be patient    
require: lodash -> curry
curry! add(x, y) =
   x + y
{1, 2, 3}.map(add(10))

;; Parsing and displaying Markdown
;; This could take a few seconds to load, be patient
require: markdown -> parse
grocery-list = """
Grocery list
* Potatoes
* Rice
div[raw] % parse(grocery-list)

;; Plotting with flot
;; This could take a few seconds to load, be patient
load "https://code.jquery.com/jquery-2.1.3.min.js"
load "http://www.flotcharts.org/flot/jquery.flot.js"
globals: $
$out.elem.style.height = "500px" ;; making room
$.plot($out.elem) with {
   1..100 each i -> {i, Math.sin(i / 10)}
   1..100 each i -> {i, Math.sin(i / 10 - 1)}
   1..100 each i -> {i, Math.sin(i / 10) + 0.25}

;; Trivial canvas-based paint app
;; Try setting the color with ctx.stroke-style = "red"
require: offset
var active = false
offx(e) = e.client-x - offset(canvas).left
offy(e) = e.client-y - offset(canvas).top
canvas = dom with canvas %
   width and height = "300px"
   style = "border:2px solid black; cursor:crosshair"
   onmousedown(e) =
      active = true
      ctx.move-to(offx(e), offy(e))
   (onmouseup and onmouseout)(e) =
      active = false
   onmousemove(e) =
      if active:
         ctx.line-to(offx(e), offy(e))
ctx = canvas.get-context("2d")
ctx.line-width = 3
print canvas

;; snake game. Sorry for the code density, I code-golfed it a bit.
require: /dom
snake(len, tl, interval) =
   map = 1..len each x -> 1..len each y -> 0
   r() = Math.floor(Math.random() * len)
   spwn(value and {x, y} is {r(), r()}) =
      if{map[x][y], spwn(value), (map[x][y] = value, {x, y})}
   var {{px, py}, dx, dy, score} = {spwn{1}, 0, 0, 0}
   1..5 each i -> spwn(-1)
   canvas = dom with canvas %
      width and height = '{len * tl}px'
      tabindex = 1000
      onkeydown(e) =
         {dx, dy} = {39={1,0},37={-1,0},40={0,1},38={0,-1}}[e.which]
   ctx = canvas.get-context("2d")
   loop = set-interval(f, interval) where f() =
      map[px][py] = score + 1
      {px, py} = {u(px,dx), u(py,dy)} where u(p,d) = (p+d+len) mod len
      if dx or dy: match map[px][py]:
         < 0 -> (spwn(-1); score += 1)
         > 0 -> (print 'Game Over! {score} points!'; clear-interval(loop))
         0 -> 0...len each x -> 0...len each y when map[x][y]>0 -> map[x][y]-=1
      map[px][py] = score + 1
      ctx.clear-rect(0, 0, canvas.width, canvas.height)
      0...len each x -> 0...len each y when map[x][y] ->
         ctx.fill-style = if{map[x][y] < 0, .blue, .red}
         ctx.fill-rect(tl * x, tl * y, tl, tl)
   set-timeout(-> canvas.focus(), 10)
   print "Snake: use arrow keys to move"
   print canvas
snake(15, 30, 150)

The Earl Grey programming language

Enter or Ctrl-EnterEvaluate an expression
Shift-EnterInsert a new line
Up/Down or Ctrl-Up/Ctrl-DownNavigate history
Ctrl-LClear the screen

Alright, so you have stumbled upon this page for some reason.

Good! Stay right here!

Start with the tutorial: next
Just show me cool stuff: cool

Variables and expressions

For the purpose of this tutorial, I will assume you are already familiar with an existing language such as JavaScript, Python or Ruby, which have a similar feature set.

How should one start a tutorial, usually? Man, I don't know. But I know there are two very useful things programming languages can do:

  1. Doing math! Like 1 + 1 (<- psst! you can click these!) You can do math with Earl Grey. The operator precedence you learned at school (well I hope you did) will work just fine: (mod is modulo)

    99 + 7 * (18 - 3) mod 10

  2. Putting stuff in variables! Sometimes you just need to put something in a variable so that you can use it later. A variable can contain dashes:

    my-variable = 1 + 2 + 3

    Why don't you put your name in a variable so you won't forget it? A string is delimited by double quotes. My name is "Olivier" for example.

Type next to get to the next topic.


Variables are nice, but a variable can only hold one thing, and that's a bummer. If only there was a way to hold more than one thing in a thing.

Oh, that's right, there is!

Arrays store sequences of things. An array in Earl Grey is denoted by curly braces:

  • Empty array: {}
  • Array of numbers: {12, 87, -4}
  • Array of stuff: {"Hello", 123, {"nested", "things", 10 * 10}}

Accessing members of an array may be done using square brackets, starting from zero: {18, 9, 71}[0]. You can also omit the brackets: {18, 9, 71} 0.

Type next to get to the next topic.


Now, what can you do on arrays? Well, all operations supported in JavaScript are also supported, and the syntax is the same. See here for a list. This being said, EG defines a few additional operations and shortcuts:

(Reminder: you can click these)

{1, 2, 3, 4}.reverse() {1, 2} ++ {5, 6} {10, 8, 71, 13, 24}[2...] {10, 8, 71, 13, 24}[... -2] {10, 8, 71, 13, 24}[2 ... -2] enumerate({53, 87, -22, 41}) neighbours({53, 87, -22, 41}) zip({1, 2, 3, 4}, {11, 22, 33, 44}) {1, 2, 3, 4, 5} each x -> x * x

Type next to get to the next topic.


Objects map names, which are strings, numbers or symbols, to stuff, which can be anything. Like arrays, objects are declared with curly brackets, but with the addition of the = or => operator:

  • Empty object: {=}
  • My name and age: {name = "Olivier", age = 30}
  • Also acceptable: {"name" => "Olivier", "age" => 30}

To get the value of an object's field, use square brackets, the dot operator, or nothing at all:

  • {name = "Olivier"}["name"]
  • {name = "Olivier"} "name"
  • {name = "Olivier"}.name

.name and "name" are actually the same thing. If a string could be a variable name, then you can put a dot in front instead of quotes around it.

Type next to get to the next topic.


Numbers, strings, arrays and objects are amazing, but you won't get very far without better abstractions. A function lets you encapsulate behavior so that you don't repeat yourself. For instance, define a function to square a number:

square(x) = x * x

Simple, eh? You can use it like this: square(5). And if you want to define a long function, then you can indent the body:

square(x) = print 'I am squaring {x}!' x * x

Type next to get to the next topic.


Sometimes you may need to give a function to another function to do stuff. For example, the for-each method of arrays takes a function to call on every member of the array.

Depending on the situation, there are four ways to do it. Yes, four. Do you have a problem with that?

  1. Declare a named function, especially if you need it elsewhere:

    f(x) = print x {1, 2, 3}.for-each(f)

  2. Inline, for small functions

    {1, 2, 3}.for-each(x -> print x)

  3. with operator, to avoid brackets and indent the body, when the function is the last argument

    {1, 2, 3}.for-each with x -> print x

  4. where operator, if it is clearer/more practical to name the function, especially when the function isn't the last argument

    print "watch for it..." set-timeout(f, 1000) where f() = print "there it is!"

Type next to get to the next topic.


if executes code conditionally. Its syntax is the same as in Python:

if 1 + 1 == 2: "Everything is fine!" elif 2 + 2 == 4: "I think this was just a fluke." else: "Oh no! Mathematics are falling apart!"

if returns its result. There is also an alternative syntax for one-liners:

if{1 + 1 == 2, "yay", "boooo!"}

Type next to get to the next topic.


while loops work as they do in Python:

var i = 10 while i >= 0: print i i -= 1

Note that the var keyword is necessary above because i is a mutable variable.

C-style for loops are available, if for some reason you want to use them:

for (var i = 10; i >= 0; i--): print i

break and continue inside loops work as they do in other languages.

Type next to get to the next topic.


The each operator is the generic way to loop over sequences such as arrays or ranges.

1..100 each i -> i * i

each accepts an indented block, but also multiple clauses, executed in order. A "clause" maps some kind of pattern to a body. There are many kinds of patterns. Can you figure out what these do? (click to verify)

{1, "banana", 6, {98}, {2, 3}, "mess", {=}} each Number? i -> i + 1 String? s when s.ends-with("s") -> s + "es" String? s -> s + "s" {x} -> x + 1 {x, y} -> x + y else -> null

break and continue can be used to manipulate the stopping condition and the results. For instance, this computes the squares of odd numbers less than ten:

1..100000 each i when i > 10 -> break i when i mod 2 == 0 -> continue i -> i * i

Type next to get to the next topic.

Pattern matching

As you just saw, each's clauses let you match patterns against values. But you can do that almost everywhere in EG! It would be long to explain all possible patterns, but all these examples should give you an idea:

{x, {y, z}} = {1, {2, 3}} {x, 5} = {1, 2} ;; This will fail! String? s = 4 ;; This will fail! Array! a = 4 ;; *Create* an array if needed n > 0 = 8 ;; Try with -1 instead of 8 double(Number! x) = x + x double! x = "9" {x} or x = 9 ;; Try with {9} instead of 9 a and b = 100 ;; Set two variables at the same time {=> name} = {name = "Peter"} {p1 => {x => x1, y => y1}, p2 => {x => x2, y => y2}} = {p1 = {x = 1, y = 1}, p2 = {x = 8, y = 3}}

Type next to get to the next topic.

...Pattern matching

match specializes in matching:

match {7, 8}: {} -> 0 {x} -> x {x, y} -> x + y

Sometimes it can be useful to match on a function argument immediately, so as shorthand you can put the match keyword directly inside the arguments list:

fact(match) = 0 -> 1 n -> n * fact(n - 1) fact(10)

You can nest match inside patterns indefinitely:

silly(match, y) = {match} -> {match} -> {match} -> x -> x * y silly({{{5}}}, 7)

The same trick can be used with each:

square(each x) = x * x square(1..10)

Type next to get to the next topic.


EG comes with a mini-language to define "documents". Assuming you know HTML, it is relatively straightforward:

strong % "I AM YELLING!"

span[raw] % "this works in a pinch"

ol % type = "i" li % "Collect Underpants" li % "?" li % "Profit"

.card % style = "border: 5px solid black;" h2 % "This is not part of the tutorial!" p % "Do you know your multiplication table?" table % 1..10 each row -> tr % 1..10 each col -> td % row * col

These objects can be converted to HTML with the /html package:

require: /html html(em % "hello")

Type next to get to the next topic.


Use require to import libraries. If you use it here, it will import from the npm repositories. Pretty neat! (Give a few seconds for the package to be fetched the first time, it can be a bit slow).

require: markdown div[raw] % markdown.parse("This *works*! Yay!")

You can check the contents of the package just by typing markdown.

Type next to get to the next topic.

Asynchronous programming

Sometimes you need to wait for data, or for some event to happen in order to keep doing something. Maybe you're waiting for a file to be read from the hard drive, for a file to be retrieved online, for a database transaction to complete, and so on. And while you wait, you don't want to stop the whole application, that would just be silly!

Enter asynchronous programming.

By default the interactive interpreter will wait for an asynchronous call to finish before yielding back control to you. If you want to avoid waiting, the trick is to wrap the call(s) in {}s.

An asynchronous function is marked with the keyword async, and it can freely wait on any sub-operation with await.

async f(seconds) = await wait(seconds * 1000) print 'I have waited {seconds} seconds.' {f{5}, f{1}, f{2}}

Notice that the above does not take 8 seconds to finish, as one would expect from purely sequential execution. Each operation finishes in its own time.

EG's implementation is based on ES6's generators and promises, but it looks much nicer :) If a library function advertises that it returns a Promise object, then await will work.

A lot of functionality, especially in the node ecosystem, uses a different (horrendous) interface based on callbacks. Thankfully, assuming they follow protocol, these functions can be converted with the promisify builtin.

Type next to get to the next topic.

The End

The tutorial is over :)

You can restart it with next, or simply keep playing around in the interactive interpreter. Cheers!