evaluate.js

(function(global) {
  function evaluate(exp, env) {
    switch (exp.type) {
      case "num":
      case "str":
      case "bool":
        return exp.value;
      case "var":
        return env.get(exp.value);
      case "assign":
        if (exp.left.type != "var")
          throw new Error("Cannot assign to " + JSON.stringify(exp.left));
        return env.set(exp.left.value, evaluate(exp.right, env));
      case "binary":
        return apply_op(exp.operator,
                evaluate(exp.left, env),
                evaluate(exp.right, env));
      case "func":
        return make_func(env, exp);
      case "if":
        var cond = evaluate(exp.cond, env);
        if (cond !== false) return evaluate(exp.then, env);
        return exp.else ? evaluate(exp.else, env) : false;
      case "prog":
        var val = false;
        exp.prog.forEach(function(exp){ val = evaluate(exp, env) });
        return val;
      case "call":
        var func = evaluate(exp.func, env);
        return func.apply(null, exp.args.map(function(arg){
          return evaluate(arg, env);
        }));
      default:
        throw new Error("I don't know how to evaluate " + exp.type);
    }
  }

  function apply_op(op, a, b) {
    function num(x) {
      if (typeof x != "number")
        throw new Error("Expected number but got " + x);
      return x;
    }
    function div(x) {
      if (num(x) == 0)
        throw new Error("Divide by zero");
      return x;
    }
    switch (op) {
      case "+": return num(a) + num(b);
      case "-": return num(a) - num(b);
      case "*": return num(a) * num(b);
      case "/": return num(a) / div(b);
      case "%": return num(a) % div(b);
      case "&&": return a !== false && b;
      case "||": return a !== false ? a : b;
      case "<": return num(a) < num(b);
      case ">": return num(a) > num(b);
      case "<=": return num(a) <= num(b);
      case ">=": return num(a) >= num(b);
      case "==": return a === b;
      case "!=": return a !== b;
    }
    throw new Error("Can't apply operator " + op);
  }

  function make_func(env, exp) {
    function func() {
      var names = exp.vars;
      var scope = env.extend();
      for (var i = 0; i < names.length; ++i)
        scope.def(names[i], i < arguments.length ? arguments[i] : false);
      return evaluate(exp.body, scope);
    }
    return func;
  }

  global.Mason = global.Mason || {};
  global.Mason.Evaluate = evaluate;
})(window);