Notes from Functional Brighton joint meetup with Async JS, 28 June 2012.
Richard Dallaway, @d6y
A small functional language that compiles to JavaScript.
- It's a functional language
- Statically typed
- Has type inference
- Aims to have readbale JavaScript output
Plan: very brief show what it looks like, say why it's interesting, give specific examples of the interesting bits.
Most of this is from the IEEE paper paper. Check the Links at the end.
Interact via the REPL, roy -r to run code, or compile and see the JS output.
$ ./roy
Roy: Small functional language that compiles to JavaScript
Brian McKenna <brian@brianmckenna.org> (http://brianmckenna.org/)
:? for help
roy> let double x = x * 2
roy> double 21
42 : Number
Interacting with JavaScript:
$ cat example.roy
let double x = x * 2
console.log (double 21)
$ ./roy -r example.roy
42
or...
$ ./roy example.roy
Roy info: Identifier "console" on line 2 is unknown - dynamically typed
$ cat example.js
var double = function(x) {
return x * 2;
};
console.log(double(21));
//@ sourceMappingURL=example.js.map
$ node example.js
42
There are a billion things in JavaScript that Roy won't know about. console is an example: it is treated as untyped and just output as is.
-
It's not trying to port an existing language and libs (that'd be a lot of work)
-
It's not ignoring JavaScript. Anything not recognised is assumed to be a JavaScript entity.
-
Written in JavaScript, so you can compile in the browser.
-
Types - you write type annotations when you want to restrict; no more falsyness.
-
Structural types
-
Option
-
Pattern matching
-
Possibilities for making async call code readable.
First, saves you from things like:
js> 7 + true + "hi"
"8hi"
roy> 7 + true
Error: Type error on line 0: Boolean is not Number
Type inference:
roy> 7
7 : Number
This is compiled to a Javascript var:
roy> let greeting = "Hello"
roy> :t greeting
String
Fucnctions:
roy> let double x = x * 2
roy> :t double
Function(Number, Number)
roy> let identity x = x
roy> :t identity
Function(#ba, #ba)
roy> let repeat s n = if n == 0 then s else s ++ (repeat s n - 1)
roy> repeat "*" 10
*********** : String
roy> :t repeat
Function(String, Number, String)
roy> repeat 10 "*"
Error: Type error on line 0: Number is not String
Adding types:
Boolean, Number, String, Structure, Array, Tuple, Function
roy> type Person = {name: String, age: Number}
roy> let birthday (p: Person) = p.age + 1
roy> :t birthday
Function({name: String, age: Number}, Number)
roy> birthday {name:"Bob", age:99}
100 : Number
roy> birthday 7
Error: Type error on line 0: Number is not Person
Structure type are useful with type inference too, if you leave off the p: Person in birthday:
roy> birthday {name: "Bob", age: 99}
100 : Number
roy> birthday {fruit: "Banana", age: 99}
100 : Number
Duck typing. Anything that has an age property will work here, which is a little safer than passing arbitrary objects around and hoping they have the properties you want. Good for JSON?
Can do {name: "Bob"} with {age: 99}
I've drawn a Cons diagram if you need it.
Cons data type, map, filter...
data List a = Nil | Cons a (List a)
let nil = Nil()
let xs = Cons 1 (Cons 2 (Cons 3 nil))
console.log(xs)
let map f l = match l
case (Cons v r) = Cons (f v) (map f r)
case Nil = nil
let filter f xs = match xs
case Nil = nil
case (Cons h t) = if (f h) then
Cons h (filter f t)
else
filter f t
let head xs = match xs
case (Cons h t) = h
let double x = x * 2
let rs = filter (\x -> x > 3) (map double xs)
let rs = filter (\x -> x > 3) (map (\x -> x * 2) xs)
console.log rs
let pretty xs = match xs
case Nil = ""
case (Cons h t) = h ++ " " ++ (print t)
console.log (pretty rs)
data Option a = Some a | None
let optionMonad = {
return: λx →
Some x
bind: λx f → match x
case (Some a) = f a
case None = None ()
}
let m = (do optionMonad
x ← Some 1
y ← Some 3
return x + y
)
match m
case (Some x) = console.log x
case None = console.log "I need both x and y"
let m2 = (do optionMonad
x ← Some 1
y ← None ()
return x + y
)
match m2
case (Some x) = console.log x
case None = console.log "I need both x and y"
Produces...
$ node test/fixtures/good/option_monad.js
4
I need both x and y
Anything with bind and return function is Roy's monad representation.
Note the Unicode.
Chain async calls:
let print x = console.log(x)
let deferred = {
return: $.when
bind: \x f ->
let defer = $.Deferred ()
x.done (\v -> (f v).done defer.resolve)
defer.promise ()
}
let v = do deferred
ip <- $.ajax 'http://cfaj.freeshell.org/ipaddr.cgi'
country <- $.ajax 'http://api.hostip.info/country.php'
return (ip ++ country)
v.done print
Compiles to:
var print = function(x) {
return console.log((x));
};
var deferred = {
"return": $.when,
"bind": function(x, f) {
var defer = $.Deferred();
x.done(function(v) {
return f(v).done(defer.resolve);
});
return defer.promise();
}
};
var v = (function(){
var __monad__ = deferred;
return __monad__.bind($.ajax('http://cfaj.freeshell.org/ipaddr.cgi'), function(ip) {
return __monad__.bind($.ajax('http://api.hostip.info/country.php'), function(country) {
return __monad__.return((ip + country));
});
});
})();
v.done(print);
//@ sourceMappingURL=deferredmonad.js.map
Initial commit was May 2011. Brian McKenna. Experimental.
- IEEE article is a great starting point for the language. Highly recommended.
- Seriously, read that IEEE article.
- The blog has lots of short feature introduction posts.
- Roy home page contains an in-browser kind of REPL.
- Latest source is on BitBucket.
- ...but confusingly also on GitHub -- the issues on GitHub contain lots of discussion and ideas.
- There's a bundle for Sublime and Textmate.
- The mailing list is on Google Groups.
- Video interview with Brian McKenna Nov 2011.
- Another video
- @puffnfresh and @roylangjs on Twitter