JS-MACROS

A handful of useful macros as babel-plugin.

Install

npm install --save-dev babel-plugin-transform-js-macros

.babelrc <- { "plugins": ["babel-plugin-transform-js-macros"] }

SymbolicExpression

Detect free variables in an expression and return a lambda that takes those as arguments.

(symbolic, x + y);

gives

({ x, y }) => x + y;

DoNotation

Mimics haskell do-notation. Takes an additional function to flatten the structure.

(join,
  x = 1,
  y = 2,
  3
);

gives

_joiner => _joiner(1, x => _joiner(2, y => 3));

examples:

// Promise chain
const then = (promise, callback) => Promise.resolve(promise).then(callback);
const six = (join,
  x = Promise.resolve(4), // binding a Promise
  y = x * 2, // Promise.resolve will take care non-promise values
  Promise.resolve(y - 2)
);
six(then).then(console.log); // 6

// Maybe
class Nothing {
  mbind(f) {
    return new Nothing();
  }
}
class Just {
  constructor(x) {
    this.x = x;
  }
  mbind(f) {
    return f(this.x);
  }
}

const bind = (monad, binder) => monad.mbind(binder);
// here the magic
const plus3 = n => (join,
  a = n, // n must be a monad
  b = new Just(3),
  new Just(a + b)
)(bind);

expect(plus3(new Nothing())).toEqual(new Nothing());
expect(plus3(new Just(5))).toEqual(new Just(8));

const toArray = (item, next) => [item].concat(next(item));
expect((join,
  a = 1,
  b = 2,
  a + b
)(toArray)).toEqual([1, 2, 3]);

const assign = (item, next) => next(item);
expect((join,
  a = 1,
  b = 2,
  { a, b }
)(assign)).toEqual({ a: 1, b: 2 });

TODO

• fix name collision
• allow nested macros