Operator overloading and syntactic macros for JavaScript
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Divergence Rebase

Rebase is a Divergence module that provides operator overloading and syntactic macros in pure JavaScript. There are a few relatively unobtrusive limitations (see Caveats below), but it is able to run on its own source code successfully.

I'm writing a guide that covers both Rebase and Divergence usage; see the Divergence guide.

Note that this project is no longer under much maintenance and has been superseded by Caterwaul. Caterwaul addresses a number of deficiencies in Rebase, including implementing all of JavaScript syntax and working on IE6. It also provides a much more sophisticated macro system.

String interpolation

Any string inside a rebased function (see below) can have expressions inside of it, and those expressions will be interpolated just like they are in Ruby. For example, this string:

'foo is #{3 + 5}'

will be translated to ('foo is ' + (3 + 5) + ''). You can use any expression inside a #{} block as long as it contains no } characters -- the parser is uninsightful about handling those and will chop off the interpolated expression at the first closing brace it sees.

Naturally this happens only inside rebased functions; this code illustrates that:

(function () {return 'foo is #{3 + 5}'}) ()                 // => 'foo is #{3 + 5}'
d.rebase (function () {return 'foo is #{3 + 5}'}) ()        // => 'foo is 8'

You can use overloaded operators and other syntactic macros in the expressions and they will be handled correctly:

d.rebase (function () {return 'foo is #{(x >$> x + 1) (5)}'}) ()    // => 'foo is 6'

Note that even though it seems valid, you can't use an interpolated string as a hash key, because string interpolation rewrites the literal into a parenthesized expression:

d.rebase (function () {return {'foo is #{3 + 5}': 'bar', bif: 'baz'}})      // => SyntaxError: invalid object key: ('foo is ' + (3 + 5) + '')

Rebase isn't context-aware enough to detect or fix this. A reasonable workaround, however, is to fold over d.init, merging objects:

d.rebase (function () {
  return d.init ('foo is #{3 + 5}'.maps_to('bar'), {bif: 'baz'});
}) ();                                                                      // => {'foo is 8':'bar', bif:'baz'}

Operator overloading

Consider this function:

(function (x) {return x << 4}) ([1, 2, 3])            // Nothing useful

To add an operator, you just set a member of the prototype:

Array.prototype['<<'] = function () {this.push.apply (this, arguments); return this};
d.rebase (function (x) {return x << 4}) ([1, 2, 3])   // => [1, 2, 3, 4]

You can also create new operators using 'sandwich identifiers', which are identifiers that, when placed between two binary operators, will become part of those operators. For example:

d.rebase.sandwiches['foo'] = true;
d.rebase (function (x) {return x >foo> y})    // => x['>foo>'](y)

You can overload these operators in exactly the same way.

You can also rebase expressions, just like eval:

d.rebase ('[1, 2, 3] << 4')   // => [1, 2, 3, 4]

However, these expressions don't close over surrounding variables. There are two ways to work around this. One is to use eval explicitly:

eval (d.rebase.local ('[1, 2, 3] << 4'))    // [1, 2, 3, 4]

The other way is to preload a function:

d.rebase (function (x, y, z) { ... x y z ... }).fn (x, y, z);

Note that you can't say it this way:

d.rebase ((function (x, y, z) { ... }).fn (x, y, z))

because then you'd be rebasing the proxy function that fn() creates. Instead, you have to rebase the original and proxy the result with the preloaded arguments. (See the Divergence guide if this usage of fn() seems unfamiliar.)

Code can be protected from alteration as well. You do this by using the literal keyword:

d.rebase (function () {
  [1, 2, 3] << 4;
  var f = x >$> x + 1;
  var y = literal (3 << 4);

In this case, the expression 3 << 4 will remain untransformed; that is, no macros will be run on it and no operator overloading will be performed. This can be useful for optimization.

By default, Rebase won't transform the strings that Divergence promotes into functions. If you do want to use operator overloading and other macros inside these strings, however, you can:

[1, 2, 3].map ('((x, y) >$> x + y).fn($0)')         // Nothing useful

[1, 2, 3].map ('((x, y) >$> x + y).fn($0)')         // Returns an array of functions

Preserving lexical closure

JavaScript doesn't support dynamic scoping very uniformly, but eval lets you hack around that. If you want to keep the scope chain of a rebased function intact, you can use d.rebase.local and eval to do that:

(function () {
  var x = 5;
  var f = eval (d.rebase.local (function () {return y >$> x + y})) ();
  f(4)        // => 9
}) ();


  1. Method calls are a lot slower than operators, so Rebase will slow your code down by quite a bit.
  2. Postfix increment/decrement expressions followed by a binary operator, such as x++ + 5, are not parsed correctly (they fail a sanity check).
  3. Statement-mode function definitions aren't allowed -- you have to use var f = function () ... instead of function f () ...
  4. Rebased functions aren't closures. They're re-evaluated at the global scope, which means that any closed-over variables will have to be passed in explicitly. However, all sub-functions inside a rebased function will close over variables within the rebased function's scope.
  5. SpiderMonkey JS does aggressive constant-folding, including replacing certain arithmetic expressions with NaN if it can determine that the types won't work out. In particular, this includes bit-shifting, multiplying, dividing, etc. by a non-numeric literal. So for reliable operation, you should use variables instead of literals to avoid these issues.
  6. Nullary return doesn't get parsed correctly. You always need to return something, even if it's just undefined.
  7. do {} while () loops aren't handled. Handling these makes while context-sensitive, and Rebase's parser is purely precedence-oriented.
  8. Floating-point numbers with signed exponents are not parsed correctly. They will be parsed as numbers that end with e or E and that get added to or subtracted from other numbers.

Rebase uses a series of functions installed on the prototypes of all standard types in order to mimic the default behavior. Sometimes these functions will not quite behave the same way due to autoboxing; if you find such a case, let me know.

Syntactic macros

Rebase also lets you transform the syntax tree in arbitrary ways. Any function in the d.rebase.macros array will be run on each syntax node. This lets you do a number of useful things, including inserting debugging information, tracing things (see the Divergence debugging module), or coming up with new meanings for existing operators.

The value passed into a macro will have one of three types:

  1. A syntax tree node
  2. A string representing some token of input -- in this case it will be a boxed String object and have line and character attributes indicating the position from which it was read
  3. undefined, meaning that there isn't anything there

Syntax tree nodes each have a parent, an operator called op (whose arity, precedence, etc. can be checked by the functions and hashes inside d.rebase), and an array of operands called xs. Note that the syntax tree constructed by Rebase is not complete or necessarily even correct; its only purpose is to provide some minor degree of abstraction above a string. (A new feature is that each token generated by the lexer knows its position; these are stored in the line and character attributes on the string.)

Tokens are represented exactly as typed. This includes nullary keywords such as break and continue, strings, regular expressions, numbers, booleans, etc.

Undefined is encountered in situations where the JavaScript grammar isn't really expression-oriented. This includes statement processing -- for example, if you type ;;; in a function, then there are two empty statements. Those statements must exist in some form in an expression-oriented parse tree, but they don't actually get rendered into the output. So the values representing those statements would be undefined.