Lesson 22 · Booleans and null - Build a Programming Language

Booleans and null

Alongside integers the language has boolean values and a null. Today you add those objects - and since there are only ever two booleans and one null, you make them shared singletons.

The goal

Evaluate boolean literals into shared true and false singleton objects, and define a single null.

Start here - the target

TO DO

Scenario: Evaluating boolean literals

Giventhe source true

Whenthe evaluator evaluates the program

Thenthe result is the boolean object whose value is true and whose Inspect is true

Andevaluating true twice returns the very same object both times

Background

Integers are unbounded, so each one is its own object. But there are only ever two booleans and one null, so allocating a fresh object for each is wasteful and, more importantly, makes comparison awkward. Instead, create them once as package-level singletons - TRUE, FALSE, NULL - and always return those same instances.

This pays off immediately in the next lessons: because true is always the exact same object, you can later test boolean equality by comparing object identity rather than unwrapping values. Evaluating true twice returning the identical object is the observable proof the singletons are shared, not copied.

Make it work

type Boolean struct { Value bool }
type Null struct {}
var (
  TRUE  = &Boolean{Value: true}
  FALSE = &Boolean{Value: false}
  NULL  = &Null{}
)
// a Boolean literal node -> return TRUE or FALSE (never a fresh object)

CheckpointDONE

Boolean literals evaluate to shared singletons, and null exists. Commit.