Source §1

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

What names are predeclared in Source §1?

On the right, you see all predeclared names of Source §1, in alphabetical order. Click on a name to see how it is defined and used. They come in these groups:

  • MISC: Miscellaneous constants and functions
  • MATH: Mathematical constants and functions

What can you do in Source §1?

You can use all features that are introduced in chapter 1 of the textbook. Below is the list of features, each with a link to the textbook section that introduces it and a small example.

Literal values

Literal values are simple expressions that directly evaluate to values. These include numbers in the usual decimal notation, the two boolean values true and false, and the predeclared names NaN, Infinity and undefined. More on literal values in section 1.1 The Elements of Programming of the textbook.

Constant declarations

Constant declarations are done in Source with

const my_name = x + 2;
Here the name my_name gets declared within the surrounding block, and refers to the result of evaluating x + 2 in the rest of the block. You can read more about the scope of names in section 1.1.8 Functions as Black-Box Abstractions.

Conditional statements and conditional expressions

Within expressions, you can let a predicate determine whether a consequent expression gets evaluated or an alternative expression. This is done by writing, for example

return p(x) ? 7 : f(y);

Read more on conditional expressions in section 1.1.6 Conditional Expressions and Predicates. Conditional evaluation is also possible within statements, for example the body of a function declaration. For that, you can use conditional statements, for example:

if (p(x)) {
return 7;
} else {
return f(y);
Read about conditional statements in section 1.3.2 Function Definition Expressions.

Function declarations and function definitions

A function declaration is a statement that declares a name and binds it to a function. For example

function square(x) {
    return x * x;

declares the name square and binds it to a squaring function, so that it can be applied as in square(5);. You can read about function declaration statements in textbook section 1.1.4 Functions.

Sometimes, it's not necessary to give a name to a function: You may want to create a function only to pass it to some other function as argument. For that, Source supports function definition expressions. For example

(x => x * x)(3); // returns 9

creates a square function just like the function declaration above, but does not give it a name. Its only purpose it to be applied to the number 3. See also textbook section 1.3.2 Function Definition Expressions.


Blocks make up the bodies of functions and the consequent and alternative statements of conditional statements. You can use blocks also elsewhere in your program, if you want to declare constants local to a specific scope. For example in this program

const a = 1;
   const a = 2;

the first application of display shows the value 2, because the declaration const a = 2; re-declares the constant a. However, the second application of display shows the value 1, because the declaration const a = 2; is limited in scope by its surrounding block. You can read more about blocks in section 1.1.8 Functions as Black-Box Abstractions.

Boolean operators

Boolean operators in Source have a special meaning. Usually, an operator combination evaluates all its arguments and then applies the operation to which the operator refers. For example, (2 * 3) + (4 * 5) evaluates 2 * 3 and 4 * 5 first, before the addition is carried out. However, the operator && works differently. An expression e1 && e2 should be seen as an abbreviation for e1 ? e2 : false. The expression e2 only gets evaluated if e1 evaluates to true. The behaviour of || is similar: e1 || e2 should be seen as an abbreviation for e1 ? true : e2. More on these two boolean operators in textbook section 1.1.6 Conditional Expressions and Predicates.


A program or the body of a block does not need to consist of a single statement. You can write multiple statements in a row. In the REPL ("Read-Eval-Print-Loop") of a Source implementation, you can write


The statements in such a sequence are evaluated in the given order. The result of evaluating the sequence is the result of evaluating the last statement in the sequence, in this case square(5);. Read more about sequences in section 1.1.2 Naming and the Environment of the textbook.

You want the definitive specs?

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