Header lexy/dsl.hpp

The rule DSL for specifying the grammar.

template <typename T>
concept rule = ;

The grammar in lexy is specified in several productions, where each one defines an associated rule. This rule is an object built from the objects and functions of namespace lexy::dsl that defines some (implementation-defined) parsing function. Parsing a rule takes the reader, which remembers the current position of the input, and the context, which stores information about the current production and whitespace rules, and is responsible for handling errors and values; it can have one of the following results:

  • Parsing can succeed. Then it consumes some input by advancing the reader position and produces zero or more values.

  • Parsing can fail. Then it reports an error, potentially after having consumed some input but without producing values. The parent rule can react to the failure by recovering from it or they fail itself.

  • Parsing can fail, but then recover. Then it has reported an error, but now it has consumed enough input to be in a known good state and parsing continues normally. See error recovery for details.

A branch rule is a special kind of rule that has an easy to check condition. They are used to guide decisions in the parsing algorithm. Every branch rule defines some (implementation defined) branch parsing function. It mostly behaves the same as the normal parse rule, but can have one additional result: branch parsing can backtrack. If it backtracks, it hasn’t consumed any input, raised errors or produced values. The parsing algorithm is then free to try another branch.

Note
The idea is that a branch rule can relatively quickly decide whether or not it should backtrack. If a branch rule does not backtrack, but fails instead, this failure is propagated and the parsing algorithm does not try another branch.

A token rule is a special kind of rule that describes the atomic elements. Parsing them never produces any values and can happen easily, as such they’re also branch rules where the entire rule is used as the condition. Because they’re atomic elements of the input, they also participate in automatic whitespace skipping: after every token, lexy will automatically skip whitespace, if one has been defined.

How to read the DSL documentation [1]

The behavior of a rule is described by the following sections.

Matching/parsing

This section describes what input is matched for the rule to succeed, and what is consumed. For token rules it is called "matching", otherwise "parsing".

It often delegates to the behavior of other rules: Here, the term "parsing" refers to the parsing operation of a rule, "branch parsing" or "try to parse" refers to the special parsing operation of a branch rule, which can backtrack, "matching" refers to the parsing operation of a token rule, which cannot produce values, and "try matching" refers to the branch parsing operation of a token rule, which cannot produce values or raise errors.

Branch parsing

This section describes what input is matched, consumed, and leads to a backtracking for a branch rule. Note that a rule can parse something different here than during non-branch parsing.

Errors

This section describes what errors are raised, when, and where. It also describes whether the rule can recover after the error.

Values

This section describes what values are produced during a successful parsing operation. It is omitted for token rules, which never produce values.

Parse tree

This section describes what nodes are created in the lexy::parse_tree. If omitted, a token rule creates a single token node covering everything consumed, and a rule produces no extra nodes besides the ones created by the other rules it parses.

The rule DSL

Primitive tokens
lexy::dsl::lit and lexy::dsl::lit_c

match character sequences

lexy::dsl::any

match anything

lexy::dsl::eof

match EOF

lexy::dsl::newline and lexy::dsl::eol

match the end of a line

lexy::dsl::token

turn a rule into a token

Character classes
lexy::dsl::ascii

match ASCII character classes

lexy::dsl::code_point

match Unicode code points

lexy::dsl::operator-

exclude some characters

lexy::dsl::operator/

combine character classes

lexy::dsl::bom

parse a byte-order mark (BOM)

Branch conditions
lexy::dsl::operator>>

add a branch condition to a rule

lexy::dsl::else_

branch condition that is always taken

lexy::dsl::peek and lexy::dsl::peek_not

check whether something matches without consuming it

lexy::dsl::lookahead

check whether something matches somewhere in the input without consuming it

Combinators
lexy::dsl::operator+

parse a sequence of rules

lexy::dsl::operator|

parse one of the specified (branch) rules

lexy::dsl::combination and lexy::dsl::partial_combination

parse all (some) of the (branch) rules in arbitrary order

lexy::dsl::if_ and lexy::dsl::opt

parse a branch rule if its condition matches

lexy::dsl::loop

parse a rule repeatedly

lexy::dsl::while_ and lexy::dsl::while_one

parse a branch rule while its condition matches

lexy::dsl::times

parse a rule N times

lexy::dsl::list

parse a list of things

lexy::dsl::until

skip everything until a rule matches

Brackets and delimited
lexy::dsl::terminator

parse something that ends with a terminator

lexy::dsl::brackets

parse something surrounded by brackets

lexy::dsl::delimited and lexy::dsl::escape

parse everything between two delimiters, with optional escape sequences

Productions
lexy::dsl::p and lexy::dsl::recurse

parse another production

lexy::dsl::inline_

parse another production’s rule inline

lexy::dsl::return_

exit early from parsing a production

Values
lexy::dsl::capture and lexy::dsl::capture_token

capture everything consumed by a rule

lexy::dsl::position

produce the current input position

lexy::dsl::nullopt

produce an empty placeholder value

lexy::dsl::member

parse something into a member variable

Errors and error recovery
lexy::dsl::error

explicitly raise an error

lexy::dsl::must

raise an error if a branch backtracks

lexy::dsl::try_

recover from a failed rule

lexy::dsl::recover

recover by looking and then continuing with some other rule

lexy::dsl::find

recover by looking for synchronization tokens

Whitespace
lexy::dsl::whitespace

explicitly skip whitespace

lexy::dsl::no_whitespace

do not skip whitespace

Identifiers
lexy::dsl::identifier

parse an identifier

lexy::dsl::keyword

parse a keyword

lexy::dsl::symbol

parse one of the specified symbols and produce their value

Numbers
lexy::dsl::zero

parse zero

lexy::dsl::digit

parse a digit

lexy::dsl::digits

parse one or more digits

lexy::dsl::n_digits

parse N digits

lexy::dsl::integer

convert digits to an integer

lexy::dsl::sign, lexy::dsl::plus_sign and lexy::dsl::minus_sign

parse a sign

lexy::dsl::code_point_id

convert N digits into a code point

Context-sensitive parsing
lexy::dsl::context_flag

a boolean flag

lexy::dsl::context_counter

an integer counter

lexy::dsl::context_identifier

an identifier variable

Input and action specific rules
lexy::dsl::argv_separator

match the argument separator of a lexy::argv_input

lexy::dsl::debug

generate a debug event that is visualized by lexy::trace