Urlang is JavaScript with a sane syntax
Urlang is a language designed to allow straightforward translation to JavaScript. Think of Urlang as JavaScript with sane syntax and JavaScript semantics. JavaScript in this context is short for ECMAScript 5 in strict mode.
Although the constructs of Urlang and JavaScript map almost one-to-one, a little sugar was added:
- function definitions allow default arguments
- let expressions
Even though the syntax of Urlang is Racket-like, remember that the semantics is standard JavaScript. This means in particular that tail calls build context.
The following examples are compiled using the urlang form.
(urlang <module> ...)
The urlang form compiles the modules. The result of compiling
a module is saved a file whose path is the module-name with .js
added.
The urlang form is controlled by the following parameters:
(current-urlang-run? #t) ; compile and run (using node)
(current-urlang-echo? #t) ; print JavaScript to screen
(current-urlang-console.log-module-level-expr? #t) ; use conole.log to print module-level exprs
> (urlang
(urmodule fact ; module name
(export fact) ; fact is exported
(import + - * = displayln ref console)
(define (fact n) (if (= n 0) 1 (* n (fact (- n 1)))))
(fact 5)))
The generated JavaScript:
"use strict";
function fact(n){return (((n===0)===false)?(n*(fact((n-1)))):1);};
console.log((fact(5)));
exports.fact=fact;
The output from Node:
"120\n"
Urlang macro transformers receive and produce standard Racket syntax objects. This implies that standard tools such as syntax-parse are available.
Consider an Urlang version of cond:
SYNTAX (cond [e0 e1 e2 ...] ... [else en]),
like Racket cond except there is no new scope
The urlang macro transformer is an standard (phase 0) Racket function.
(define-urlang-macro cond
(Ξ» (stx)
(syntax-parse stx
[(_cond [else e0:Expr e:Expr ...])
#'(begin e0 e ...)]
[(_cond [e0 e1 e2 ...] clause ...)
(syntax/loc stx
(if e0 (begin e1 e2 ...) (cond clause ...)))]
[(_cond)
(raise-syntax-error 'cond "expected an else clause" stx)])))
The macro can now we be used:
> (urlang
(urmodule sum-example
(define (even? x) (=== (% x 2) 0))
(var (sum 0) x (a (array 1 2 3 4 5)) (i 0) (n a.length))
(while (< i n)
(:= x (ref a i))
(cond
[(even? x) (+= sum (ref a i))]
[else "skip"])
(+= i 1))
sum))
The generated JavaScript:
"use strict";
function even_p(x){return ((x%2)===0);};
var sum=0,x,a=[1,2,3,4,5],i=0,n=a.length;
while((i<n)){(x=a[i]);(((even_p(x))===false)?"skip":(sum+=a[i]));(i+=1);};
console.log(sum);
The output from Node:
"6\n"
The heart of the system is a compiler written using the Nanopass compiler Framework. The compiler is exported as a function
compile : urlang-module -> JavaScript
that compiles an urlang module and produces JavaScript, that can be evaluated by the Node.js platform (or be embedded in a web page).
The Urlang module to be compiled can be represented
- as a syntax object
- as a Nanopass structure (representing an Lurlang program)
Use 1) to program in Urlang directly.
Use 2) if you intend to use Urlang as a compiler backend.
[Note: Nanopass is a framework for implementing compilers.]
The intended use of Urlang is to use 1) to write (generate) a Racket runtime in JavaScript. The middle-end of the Racket-to-JavaScript compiler will produce output as Nanopass structures, so 2) will be used as the backend for the Racket-to-JavaScript compiler.
Internally the function expand
expand : syntax -> LUrlang
will parse and expand its input and produce an LUrlang representation.
Note that expand allows the user to extend the input language
using define-urlang-macro. An Urlang macro is a syntax to syntax
transformation implemented as a normal Racket function.
This allow you to use all of the standard Racket macro machinery.
Main functions:
expand : syntax -> Lurlang
expand the input and produce a fully expanded Urlang program
represented as a Lurlang structure
compile : syntax ->
Expand and compile. The output is written to standard out.
eval : syntax -> value
expand, compile and run the input (an Urlang module represented as a syntax object)
Running means that `node` is used to run the generated JavaScript.
Having Urlang as a #lang language allows
- macros (using full Racket at compile time)
- export of defined names
- easier testing
In the grammar below:
xstands for a non-keyword identifierfstands for an identifier defined as a function
<module> ::= (urmodule <module-name> <module-path> <module-level-form> ...)
<module-level-form> ::= <export> | <import> | <definition> | <statement>
<export> ::= (export x ...)
<import> ::= (import x ...)
<definition> ::= (define (f <formal> ...) <body>)
| (define x <expr>)
<formal> ::= x | [x <expr>]
<statement> ::= <var-decl> | <block> | <while> | <do-while> | <if> | <break> | <expr>
<var-decl> ::= (var <var-binding> ...)
<block> ::= (block <statement> ...)
<var-binding> ::= x | (x e)
<while> ::= (while <expr> <statement> ...)
<do-while> ::= (do-while <expr> <statement> ...)
<if> ::= (sif <expr> <statement> <statement>)
<break> ::= (break) | (break label)
<body> ::= <statement> ... <expr>
<expr> ::= <datum> | <reference> | <application> | <sequence>
| <ternary> | <assignment> | <let> | <lambda> | <dot>
<ternary> ::= (if <expr> <expr> <expr>)
<reference> ::= x
<application> ::= (<expr> <expr> ...)
<sequence> ::= (begin <expr> ...)
<assignment> ::= (:= x <expr>)
<let> ::= (let ((x <expr>) ...) <statement> ... <expr>)
<lambda> ::= (lambda (<formal> ...) <body>)
<keyword> ::= define | begin | urmodule | if | := | ...se code...
<datum> ::= <fixnum> | <string> | #t | #f
<identifier> an identifier that is not a keyword
<fixnum> an integer between -2^53 and 2^53
<module-name> a symbol or string
Some application are special cases:
(ref e0 e1) becomes e0[e1]
(ref e0 "str") becomes e0.str
(array e ...) becomes [e,...]
Property access with dot notation is rewritten to use bracket syntax in the parser.
Example: object.property becomes object["property"]
If e0 evaluates to value strictly equal to false, then e2 otherwise e1.
Note: The JavaScript becomes ((e0===false) ? e2 : e1)
Binds x to undefined and y to 3.