Lesson 13 · A value tree and parsing a scalar

A value tree and parsing a scalar

With tokens in hand, the parser's job is to build a tree of values. Today you define that value type and parse the four scalar tokens - null, boolean, number, string - into it, giving the library its public Parse entry point.

The goal

Define the Value type and parse a single scalar token into the matching value.

Start here - the target

TO DO

Scenario: Parsing a lone scalar

Givenan input holding a single scalar token

WhenParse is called

ThenParse("null") is a Null value, Parse("true") is Bool true, Parse("42") is Number 42, and Parse a quoted hi is String hi

AndParse returns a value and a nil error for each of these

Background

The scanner produced a flat stream of tokens; the parser turns that stream into a tree. The tree’s node type is Value - a tagged union whose Kind says which of the six JSON types it holds, with a field for each payload: a bool, a number, a string, a slice of child values for arrays, and a slice of key/value members for objects. Every JSON document parses into exactly one Value.

Start at the leaves. The four scalar tokens each map straight to a value: a Null token to a Null value, True/False to a Bool, a Number token to a Number value carrying its Num, a String token to a String value carrying its Str. This is also where the public Parse function is born: it scans the input, reads one value, and returns it. Give it a ParseError type carrying a position now, even though the scalars all succeed - the error chapter will lean on it, and arrays and objects, which recurse back into this same value-reading step, come next.

Make it work

// name the value's kind apart from the scanner's token Kind
// (same package), or the two collide
type ValueKind int
const ( KindNull ValueKind = iota; KindBool; KindNumber
        KindString; KindArray; KindObject )
type Member struct { Key string; Value Value }
type Value struct {
  Kind ValueKind
  Bool bool; Num float64; Str string
  Arr []Value; Obj []Member
}
// ParseError carries position; Parse scans then reads one value
type ParseError struct { Offset, Line, Col int; Msg string }
func Parse(input string) (Value, error) { /* scan, parseValue */ }

CheckpointDONE

Parse turns a single scalar into a typed Value. Commit and stop here.