Lesson 15 · Booleans - Build a TOML Parser

Booleans

TOML has a boolean type with exactly two values. Today you parse them, and pin the rule that they are lowercase words only.

The goal

Parse the boolean literals true and false, rejecting other spellings.

Start here - the target

TO DO

Scenario: Boolean literals

Giventhe values true and false

Whenthey are parsed

Thentrue is the boolean true and false is the boolean false

Andbooleans are lowercase only, so True and FALSE are not booleans and are an error

Background

TOML’s boolean type is the simplest value there is: exactly two literals, true and false, mapping to a KindBool value. When the bare token you read is one of those two words, you have a boolean.

The rule worth pinning is that booleans are lowercase always. Unlike some config formats that accept True, yes, on, or 1, TOML recognizes only the exact lowercase spellings. True and FALSE are not booleans - they are bare words that match no value form, so they are an error rather than a quietly-accepted truthy value. This strictness is deliberate: it keeps documents unambiguous. Testing both valid literals and a rejected miscased one keeps the boundary honest.

Make it work

// when the bare token is exactly "true"  -> KindBool, Bool=true
// when it is exactly "false" -> KindBool, Bool=false
// "True", "TRUE", "yes", "1" are NOT booleans
//   (bare words that are not true/false fall through to an error)

CheckpointDONE

The two boolean literals parse and nothing else does. Commit and stop here.