Parser — AST

parse(source, dialect?, knownNames?) runs the lexer and then the recursive-descent parser, returning the root AstNode.

Signature

function parse(
  source: string,
  dialect?: 'expression' | 'unary-tests',
  knownNames?: Set<string>,
): AstNode

Parameter	Default	Description
source	—	FEEL source string
dialect	'expression'	Grammar entry point
knownNames	new Set()	Multi-word identifier hints

Throws a ParseSyntaxError (with start and end byte offsets) when the input is malformed. Use safeParse() for non-throwing error recovery.

How it works

The parser is a single-pass recursive descent parser. It reads the token stream produced by tokenize() and builds an AST bottom-up by precedence.

Operator precedence (expression dialect)

From lowest to highest:

Level	Operators
1	or
2	and
3	=, !=, <, >, <=, >=
4	+, -
5	*, /
6	**
7	Unary -, not
8	Filter […], path .name, function call (…)

Multi-word identifiers

FEEL allows names with spaces: Monthly Salary, Order Line Item. The lexer emits these as separate Name tokens. The parser reassembles them into a single Identifier node if the full phrase appears in knownNames.

parse('Monthly Salary + 1', 'expression', new Set(['Monthly Salary']));
// BinaryOp {
//   left: Identifier { name: 'Monthly Salary' },
//   right: NumberLiteral { value: '1' }
// }

Without the hint, the parser sees Monthly as one expression and Salary as a second — a parse error or unexpected structure.

Examples

Arithmetic

parse('(1 + 2) * 3')
// BinaryOp {
//   op: '*',
//   left: BinaryOp { op: '+', left: NumberLiteral{1}, right: NumberLiteral{2} },
//   right: NumberLiteral { value: '3' }
// }

Conditional

parse('if score >= 700 then "approved" else "declined"')
// IfExpression {
//   condition: BinaryOp { op: '>=', left: Identifier{score}, right: NumberLiteral{700} },
//   consequent: StringLiteral { value: 'approved' },
//   alternate:  StringLiteral { value: 'declined' }
// }

Unary tests

parse('[100..999]', 'unary-tests')
// UnaryTestList {
//   negated: false,
//   tests: [ RangeLiteral { startIncluded: true, endIncluded: true, ... } ]
// }