Rephrase

Rewrite rules for JavaScript.

What?

A rewrite rule is represented as three consecutive block comments in a JavaScript file:

/* forall a f g. */
/* a.map(f).map(g) */
/* a.map(compose(f, g)) */

The above means whenever we see an expression that looks like a.map(f).map(g) (where a, f and g are "holes"), then we can replace that expression with a.map(function(a) { return f(g(a)); }) (with a, f and g filled in with what occured in the first expression).

After we put the above in the header of a JavaScript file, we can write code like so:

function compose(f, g) {
    return function(x) {
        return f(g(x));
    };
}

function add(a) {
    return function(b) {
        return a + b;
    };
}

console.log([1, 2, 3].map(add(1)).map(add(2)));

This original code is mapping over our array twice. Those two passes should be done in one.

If we use the rephrase command, we will get the rewritten JavaScript output:

function compose(f, g) {
    return function (x) {
        return f(g(x));
    };
}
function add(a) {
    return function (b) {
        return a + b;
    };
}
console.log([
    1,
    2,
    3
].map(compose(add(1), add(2))));

There is now only a single map call. Much more efficient!

Usage

Command-line:

npm install -g rephrase
rephrase examples/array-fusion.js

From repository:

node main.js examples/array-fusion.js