Lesson 20 · Table headers - Build a TOML Parser

Table headers

A flat list of pairs is not enough for real config; you need grouping. Today you parse a table header, a name in square brackets, which starts a new subtable that following pairs belong to.

The goal

Parse a [name] header so that following pairs go into that subtable.

Start here - the target

TO DO

Scenario: A single table header

Giventhe document with a line reading open-bracket server close-bracket, then port = 80, then host = "local"

WhenParse is called

Thenthe root table has one entry server, itself a table with entries port (integer 80) then host (string local)

Andpairs written before any header stay at the root, and a header changes where the following pairs are stored

Background

Configuration is naturally grouped - server settings here, database settings there

and TOML expresses grouping with a table header: a name in square brackets on its own line, like [server]. Everything after that header, up to the next header or the end of the file, belongs to that table. So [server] followed by port = 80 puts port inside server, not at the top level.

The mechanism is a current-table pointer. It starts at the root, and every key/value pair is appended to whatever table it currently points at. A header does one thing: create the named subtable under the root and move the pointer to it. Pairs written before any header stay at the root, since that is where the pointer begins. This single piece of state - “which table are we filling?” - is what turns a flat parser into one that builds a shallow tree, and the next lessons deepen it with dotted names.

Make it work

// keep a `current *Table` pointer, starting at the root
// a line matching [ key ] is a table header:
//   make a new child Table under the root at that key
//   set current = that child
// a key/value pair is appended to `current`, not always the root

CheckpointDONE

A [name] header routes following pairs into a subtable. Commit and stop here.