lib/interp/arithm.js

const { SpecialForms } = require("./environment.js");

const { Value } = require("./ast.js");

SpecialForms["++"] = (args, env) => {
  if (args.length !== 1) {
    throw new SyntaxError("Bad use of ++");
  }

  return incrementHelper(args[0], env, 1);
};

SpecialForms["--"] = (args, env) => {
  if (args.length !== 1) {
    throw new SyntaxError("Bad use of --");
  }

  return incrementHelper(args[0], env, -1);
};

SpecialForms["+="] = (args, env) => {
  if (args.length !== 2) {
    throw new SyntaxError("Bad use of +=");
  }

  const inc = args[1].evaluate(env);

  return incrementHelper(args[0], env, inc);
};

SpecialForms["-="] = (args, env) => {
  if (args.length !== 2) {
    throw new SyntaxError("Bad use of -=");
  }

  const inc = args[1].evaluate(env);

  return incrementHelper(args[0], env, -inc);
};

const incrementHelper = (variable, env, inc) => {
  const varValue = variable.evaluate(env);

  const token = { type: "value", value: varValue + inc };
  const newValue = new Value(token);

  return SpecialForms["set"]([variable, newValue], env);
};