Units of measure with sweet.js

sweet-units.sjs

// Some minimal class for values with units
// (nothing exciting going on...)

function Unit(name,val) {
  this.name = name;
  this.val = val;
}
Unit.prototype.toString = function() {
  return this.val + " " + this.name;
}
Unit.prototype.add = function(other) {
  if ("number" == typeof other) return new Unit(this.name, this.val + other);
  if (other.name != this.name) throw new Error("does not work");
  return new Unit(this.name, this.val + other.val);
}
Unit.prototype.times = function(other) {
  if ("number" == typeof other) return new Unit(this.name, this.val * other);
  if (other.name != this.name) throw new Error("does not work");
  return new Unit(this.name, this.val * other.val);
}
Unit.prototype.lessThan = function(other) {
  if ("number" == typeof other) return this.val < other;
  if (other.name != this.name) throw new Error("does not work");
  return this.val < other.val;
}

// How to define new units

// You can have basic SI units or derived units.
macro unit {
  rule { $name  = $r:expr } => {
    macro $name {
      rule infix {$x:expr |} => { ($r).times($x) }
    }
  }
  rule { $name } => {
    macro $name {
      case infix {$x:expr | $name} => {
        var name = #{$name};
        var nom = name[0].token.value;
        return withSyntax($nlit = [makeValue(nom,name)]) {
          return #{new Unit($nlit, $x)}
        }
      }
    }
  }
}

// Adding some syntactic sugar
// (Using "++" so we don't lose native "+")
macro ++ { rule infix { $x:expr | $y:expr } => { $x.add($y) } }
macro << { rule infix { $x:expr | $y:expr } => { $x.lessThan($y) } }

// Now, let's define some units!

unit m;
unit km = 1000 m;
unit mile = 1609 m;
unit miles = 1 mile;

unit s;
unit min = 60 s;
unit h = 60 min;
unit day = 24 h;

// And here you can use them:

alert(2 miles);
alert(1.5 km ++ 100 m);
alert(2 miles << 10 km);
alert(1 day << 23 h ++ 70 min);

var todos = [30 min, 2 h, 45 min, 2.5 h, 30 min];
var done = 0 min;
while (done << 5 h) {
  done = done ++ todos.shift();
}
alert("Not enough time to finish: " + todos);

// Hint: Paste this file into http://sweetjs.org/browser/editor.html to try it out!