Source §1 Typed

Source §1 Typed is a small programming language, designed for the first chapter of the textbook Structure and Interpretation of Computer Programs, JavaScript Adaptation (SICP JS). Source §1 Typed introduces type syntax and type checking.

What is static type checking?

Programming languages handle types in two main ways: dynamically typed languages, only check types when the code is run (at runtime), while statically typed languages check types before the code is run (at compile time). Though Source is a dynamically typed language, we can still introduce type checks that check the code for errors at compile time.

What can you do in Source §1 Typed?

You can use all features of Source §1, but with the added option of type checking your code at compile time.

This can be done by annotating functions and variables with type annotations:

const x = 1; // no type annotation
const x: number = 1; // with type annotation

// no type annotation
function f(x) {
    return x;

// with type annotation
function f(x: number): number {
    return x;

The types available in Source §1 Typed are:

  • Basic types: number, string, boolean, undefined, void (used to annotate the return type of functions that do not return), any (skips all typechecks)
  • Literal types: specific integer, string or boolean values (e.g. 1, '1', true)
  • Function types: used to annotate types of functions (e.g. (x: number) => number)
  • Union types: used to combine types (e.g. string | number)

Additionally, support for the following is added:

  • Type alias declarations: used to declare new types to be used elsewhere in the program (e.g. type x = string | number;)
  • As expressions: used to cast a variable to a specific type (e.g. const x: number = y as number;)
  • typeof operations: used to get the type of a variable

The addition of type annotations is optional, i.e. if there are no type annotations, type checks will be skipped (the type is assumed to be any).

You want the definitive specs?

For our development team, we are maintaining a definitive description of the language, called the Specification of Source §1 Typed. Feel free to take a peek!