Lesson 44 · Index expressions - Build a Programming Language

Index expressions

An array is only useful if you can read from it. Today you parse and evaluate the index operator arr[i], treating the opening bracket as an infix operator - and decide what an out-of-range index produces.

The goal

Parse and evaluate array indexing, returning null for an out-of-range index.

Start here - the target

TO DO

Scenario: Indexing into an array

Givenarray index expressions

Whenthe evaluator evaluates [1, 2, 3][0], then [1, 2, 3][2], then [1, 2, 3][5]

Thenthe results are 1, 3, and null

Anda negative index like [1, 2, 3][-1] also yields null

Background

Reading an element uses the index operator arr[i], which - like a call - is really an infix use of a bracket: the expression on the left is the array, and the expression inside the brackets is the position. Register [ as an infix parse function at INDEX precedence, the tightest of all so arr[0] binds before any surrounding operator.

The design decision is out-of-bounds behavior: rather than crash or error, an index past the end - or a negative one - evaluates to null. That keeps indexing total (it always produces a value) and composes nicely with the truthiness rules, so callers can test the result. Evaluate the array and the index, bounds-check, and return the element or null.

Make it work

type IndexExpression struct { Left Expression; Index Expression }
const ( /* ...levels... */ INDEX ) // even tighter than CALL
precedences["["] = INDEX
// register '[' as an INFIX parse fn too; eval:
//   evaluate Left (array) and Index (integer); bounds-check before reading

CheckpointDONE

Indexing reads array elements, with out-of-range indices returning null. Commit.