Lesson 14 · Parsing arrays - Build a JSON Parser

Parsing arrays

An array is a value that contains other values, which is where recursive descent earns its name. Today you parse bracketed, comma-separated lists by calling the value parser on each element.

The goal

Parse a bracketed, comma-separated list of values into an Array value.

Start here - the target

TO DO

Scenario: Parsing arrays of values

Givena bracketed list

WhenParse is called

ThenParse("[]") is an empty Array, Parse("[1,2,3]") is an Array of Number 1, 2, 3, and Parse("[true,null]") is an Array of Bool true then Null

AndParse("[[1],2]") is an Array whose first element is an Array containing Number 1

Background

An array is a value that holds an ordered list of other values, written [ ... ] with commas between elements. Parsing it is the first taste of recursive descent: to read an element you call the very same value parser you are already inside, so an array of arrays falls out for free. When the parser sees a left bracket, it reads values in a loop, expecting a comma between each and a right bracket to finish.

The one special case is the empty array: right after the opening bracket, a closing bracket means zero elements, with no value to read. Otherwise each iteration parses one value and then looks at the next token - a comma means “another element follows”, a closing bracket means “done”. Because the element parser is the general one, [[1],2] nests naturally: the first element recurses into another array. Keep the happy path clean today; malformed lists like a trailing comma get precise errors in the errors chapter.

Make it work

// parseValue sees an LBracket:
//   consume it; if the next token is RBracket, it is the empty array
//   else loop: parseValue (recursion!), then expect ',' to continue
//   or ']' to stop
// append each parsed element to Arr

CheckpointDONE

Parse builds Array values, including nested arrays. Commit and stop here.