This weekend at campjs one of my projects has been porting my Haskell Game of Life to CoffeeScript, with type annotations courtesy of Ristretto.
https://github.com/liammclennan/coffeescript-game-of-life
Ristretto
Given a function from string to int:
number_of_chars = (s) -> s.length
Ristretto can enfore the types given the following annotated version:
number_of_chars = T 'number_of_chars :: String -> Int', (s) ->
s.length
The format of the type annotation will be familiar to Haskellers. Read :: as ‘has the type’. All types to the left of a -> are function arguments and the type to the right of the last -> is the return type. So the above function number_of_chars has the type String to Int.
Ristretto can also define data types. For example:
T 'typedef Person :: { name: String, age: Int }'
class Person
constructor: (@name, @age) ->
The JavaScript equivalent is:
T('typedef Person :: { name: String, age: Int }');
function Person(name,age) {
this.name = name;
this.age = age;
}
Game of Life
Here is my Game of Life implementation:
require 'coffee-script'
T = require './ristrettoAssert'
_ = require 'underscore'
# Define a type for a living cell
T 'typedef Cell :: {x:Int, y:Int}'
class Cell
constructor: (@x,@y) ->
# seed the initial living cells
r_pentomino = [new Cell(2, 1), new Cell(3, 1), new Cell(1,2), new Cell(2,2), new Cell(2,3)]
# determine if a coordinate (x,y) is a neighbour of a cell
is_neighbour = T('is_neighbour :: Cell -> Int -> Int -> Bool', (c, x, y) ->
Math.abs(x-c.x) < 2 and Math.abs(y-c.y) < 2 and (not (x is c.x and y is c.y))
)
living_neighbours = T('living_neighbours :: [Cell] -> Int -> Int -> Int', (cs,x,y) ->
(cell for cell in cs when is_neighbour cell, x, y).length
)
is_on = T('is_on :: [Cell] -> Int -> Int -> Bool', (cs,x,y) ->
(cell for cell in cs when cell.x is x and cell.y is y).length > 0
)
largest_x = T 'largest_x :: [Cell] -> Int', (cs) ->
_.max(cs, (i) -> i.x).x
largest_y = T 'largest_y :: [Cell] -> Int', (cs) ->
_.max(cs, (i) -> i.y).y
alive_next_generation = T 'alive_next_generation :: [Cell] -> Int -> Int -> Bool', (cs,x,y) ->
false
(is_on(cs, x, y) and 2 <= living_neighbours(cs, x, y) <=3) or ((not is_on(cs, x, y)) and (living_neighbours(cs,x,y) is 3))
tick = T 'tick :: [Cell] -> [Cell]', (cs) ->
ncs = []
for x in [0..largest_x(cs)+1]
for y in [0..largest_y(cs)+1]
ncs.push(new Cell(x,y)) if alive_next_generation cs, x, y
ncs
visualize = T 'visualize :: [Cell] -> String', (cs) ->
result = ''
for y in [0..largest_y(cs)+1]
for x in [0..largest_x(cs)+1]
result += if is_on(cs,x,y) then 'x' else '-'
if x is largest_x(cs)+1
result += '\n'
result
[1..50].reduce((p,c,i) ->
console.log visualize p
tick p
, r_pentomino)
Thoughts
While re-implementing game of life my experience was that ristretto frequently alerted me to type errors. Without Ristretto these errors would have manifested later in the program with a greater distance between the problem and the runtime error.
Ristretto does not provide the full benefits of a static type checker. It is more like a type-based design-by-contract library. Personally, I have been frustrated by type errors in Javascript, which led me to implement a basic runtime type checker for JavaScript, but I think that Ristretto is a better and more logically consistent solution.
Output
The output of my CoffeeScript Game of Life for the first 50 generations takes 22s on my Chromebook and is printed below. To generate the full 1103 generations before this pattern stabilises will likely require a better algorithm.
-----
--xx-
-xx--
--x--
-----
-----
-xxx-
-x---
-xx--
-----
--x--
-xx--
x--x-
-xx--
-----
-xx--
-xxx-
x--x-
-xx--
-----
-x-x-
x--x-
x--x-
-xx--
-----
--x---
xx-xx-
x--x--
-xx---
------
-xxx--
xx-xx-
x--xx-
-xx---
------
xx-xx-
x-----
x---x-
-xxx--
------
xx----
x--xx-
x-xx--
-xxx--
--x---
------
xx----
x--xx-
x-----
------
-xxx--
------
xx--
x---
----
-xx-
--x-
--x-
----
xx---
xx---
-x---
-xx--
--xx-
-----
xx---
--x--
-----
-x-x-
-xxx-
-----
-x---
-x---
--x--
-x-x-
-x-x-
--x--
-----
-----
-xx--
-xx--
-x-x-
-x-x-
--x--
-----
-----
-xx--
x--x-
xx-x-
-x-x-
--x--
-----
------
-xx---
x--x--
xx-xx-
xx-x--
--x---
------
------
-xx---
x--xx-
---xx-
x--xx-
-xx---
------
-------
-xxx---
-x--x--
--x--x-
-x--x--
-xxx---
-------
--x----
-xxx---
-x--x--
-xxxxx-
-x--x--
-xxx---
--x----
-------
-xxx---
-x-x---
x----x-
xx---x-
x----x-
-x-x---
-xxx---
-------
-x-x----
xx-xx---
x-x-x---
xx--xxx-
x-x-x---
xx-xx---
-x-x----
--x-----
--------
xx-xx-
x---x-
--x---
x-x-x-
--x---
x---x-
xx-xx-
--x---
------
xx-xx-
x-x-x-
------
--x---
------
x-x-x-
xxxxx-
-xxx--
------
xxxxx-
x-x-x-
-x-x--
------
-x-x--
x-x-x-
x---x-
x---x-
--x---
------
x-x-x--
x---x--
-xxx---
-------
-xxx---
x-x-x--
x---xx-
-x-x---
-------
-x-x---
x---x--
-xxx---
-------
-xxx---
x-x-xx-
x-x-xx-
----x--
-------
-------
x---x--
-xxx---
-------
-xxxx--
x----x-
-------
---xxx-
-------
-------
-xxx---
-xxx---
----x--
-xxxx--
-xxxx--
-----x-
----x--
----x--
-------
--x----
-x-x---
-x--x--
----x--
-x---x-
-x---x-
--x--x-
----xx-
-------
--x-----
-x-x----
--xxx---
----xx--
----xx--
-xx-xxx-
-----xx-
----xx--
--------
--x-----
-x--x---
--x--x--
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---x----
---x----
----xxx-
--------
-------
-xxx---
-------
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-------
---x-x-
----xx-
-----x-
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--x-----
--x-----
--x-----
--------
--------
--------
-----x--
-----xx-
----xx--
--------
--------
-xxx----
--------
--------
--------
--------
-----xx-
------x-
----xxx-
--------
--x------
--x------
--x------
---------
---------
---------
-----xx--
----x--x-
-----xx--
-----x---
---------
---------
-xxx-----
---------
---------
---------
---------
-----xx--
----x--x-
----xxx--
-----xx--
---------
--x------
--x------
--x------
---------
---------
---------
-----xx--
----x--x-
----x--x-
----x-x--
---------
---------
-xxx-----
---------
---------
---------
---------
-----xx--
----x--x-
---xx-xx-
-----x---
---------
--x------
--x------
--x------
---------
---------
---------
-----xx--
---xx--x-
---xx-xx-
----xxx--
---------
---------
-xxx-----
---------
---------
---------
---------
----xxx--
---x---x-
-------x-
---xx-xx-
-----x---
---------
--x-------
--x-------
--x-------
----------
----------
-----x----
----xxx---
----xx-x--
---xx--xx-
----xxxx--
----xxx---
----------
----------
-xxx------
----------
----------
----------
----xxx---
----------
-------xx-
---x----x-
--------x-
----x--x--
-----x----
----------
--x--------
--x--------
--x--------
-----------
-----x-----
-----x-----
-----xxx---
-------xx--
--------xx-
-------xx--
-----------
-----------
-xxx-------
-----------
-----------
-----------
----xx-----
-----x-xx--
---------x-
---------x-
-------xxx-
-----------
--x---------
--x---------
--x---------
------------
------------
----xxx-----
----xxx-x---
---------x--
---------xx-
--------xx--
--------x---
------------
------------
-xxx--------
------------
------------
-----x------
----x-xx----
----x-xx----
-----x--xxx-
----------x-
--------x-x-
--------xx--
------------
--x----------
--x----------
--x----------
-------------
-----xx------
----x--x-----
----x----x---
-----xxxxxx--
--------x-xx-
--------x-x--
--------xx---
-------------
-------------
-xxx---------
-------------
-------------
-----xx------
----x-x------
----x----xx--
-----xxx---x-
------x----x-
-------xx-xx-
--------xx---
-------------
--x-----------
--x-----------
--x-----------
--------------
-----xx-------
----x-x-------
----x--x--x---
-----xxx---x--
-----x--x--xx-
-------xx-xx--
-------xxxx---
--------------