Sweet.js DSL for making persistent data structures feel imperative

gistfile1.js

macro := {
  rule infix { $obj $([ $key ] ...) | $rval:expr } => {
    $obj = mori.assoc_in($obj, [$key (,) ...].reverse(), $rval)
  }
}

macro hash_map {
  rule {{ $($key : $value) (,) ... }} => {
    mori.hash_map($($key, $value) (,) ...)
  }
}

// You can use expressions as keys, like Python.
var myName = 'Pete';

var lastNames = hash_map {
  myName: 'Hunt',
  'Christopher': 'Chedeau'
};

// Save a reference to the original version
var originalLastNames = lastNames;

// Add some new friends
lastNames['Jordan'] := 'Walke';
lastNames['Sebastian'] := 'Markbage';

// This will only have the first two
console.log(originalLastNames);

// This will have all four.
console.log(lastNames);