# Module Parsing

    module Parsing: sig .. end

The run-time library for parsers generated by `ocamlyacc`.

    val symbol_start : unit -> int

`symbol_start` and `Parsing.symbol_end` are to be called in the action part of a grammar rule only. They return the offset of the string that matches the left-hand side of the rule: `symbol_start()` returns the offset of the first character; `symbol_end()` returns the offset after the last character. The first character in a file is at offset 0.

    val symbol_end : unit -> int

See `Parsing.symbol_start`.

    val rhs_start : int -> int

Same as `Parsing.symbol_start` and `Parsing.symbol_end`, but return the offset of the string matching the `n`th item on the right-hand side of the rule, where `n` is the integer parameter to `rhs_start` and `rhs_end`. `n` is 1 for the leftmost item.

    val rhs_end : int -> int

See `Parsing.rhs_start`.

    val symbol_start_pos : unit -> Lexing.position

Same as `symbol_start`, but return a `position` instead of an offset.

    val symbol_end_pos : unit -> Lexing.position

Same as `symbol_end`, but return a `position` instead of an offset.

    val rhs_start_pos : int -> Lexing.position

Same as `rhs_start`, but return a `position` instead of an offset.

    val rhs_end_pos : int -> Lexing.position

Same as `rhs_end`, but return a `position` instead of an offset.

    val clear_parser : unit -> unit

Empty the parser stack. Call it just after a parsing function has returned, to remove all pointers from the parser stack to structures that were built by semantic actions during parsing. This is optional, but lowers the memory requirements of the programs.

    exception Parse_error

Raised when a parser encounters a syntax error. Can also be raised from the action part of a grammar rule, to initiate error recovery.

    val set_trace : bool -> bool

Control debugging support for `ocamlyacc`-generated parsers. After `Parsing.set_trace true`, the pushdown automaton that executes the parsers prints a trace of its actions (reading a token, shifting a state, reducing by a rule) on standard output. `Parsing.set_trace false` turns this debugging trace off. The boolean returned is the previous state of the trace flag.

- Since 3.11.0