Lesson 45 · Array built-ins - Build a Programming Language

Array built-ins

Arrays need operations to be worth having. Today you add a family of built-ins - first, last, rest, and push - all following the same pattern and all producing new arrays rather than mutating.

The goal

Add the first, last, rest, and push built-ins, each returning a new value without mutating its input.

Start here - the target

TO DO

Scenario: Array built-in functions

Givenarray built-in calls

Whenthe evaluator evaluates first([1, 2, 3]), last([1, 2, 3]), rest([1, 2, 3]), and push([1, 2], 3)

Thenthe results are 1, 3, [2, 3], and [1, 2, 3]

Andpush returns a new array, leaving its argument [1, 2] unchanged; first([]) yields null

Background

With indexing in place, a handful of built-ins make arrays genuinely useful, and they all follow the builtin pattern you set up for len: validate the argument is an array, then compute. first and last return the end elements (or null when empty), rest returns everything after the first, and push returns the array with one element added.

The important design rule is immutability: rest and push build and return new arrays rather than changing their input, so push([1,2], 3) gives [1,2,3] while the original [1,2] is untouched. That keeps values predictable and is exactly what lets you write recursive list algorithms - like the map you will build in the final lesson - without one call clobbering another’s data.

Make it work

// add to the builtins registry, each validating it got an *Array:
//   first -> element 0 (or NULL if empty)
//   last  -> last element (or NULL if empty)
//   rest  -> a NEW array of all but the first element
//   push  -> a NEW array with one element appended (copy, don't mutate)

CheckpointDONE

The array built-ins first, last, rest, and push all work immutably. Commit.