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# pt::param(n) 1 tcllib "Parser Tools"

## Name

pt::param - PackRat Machine Specification

## Synopsis

- package require Tcl 8.5

## Description

Are you lost ? Do you have trouble understanding this document ? In that case please read the overview provided by the Introduction to Parser Tools. This document is the entrypoint to the whole system the current package is a part of.

Welcome to the PackRat Machine (short: PARAM), a virtual machine geared towards the support of recursive descent parsers, especially packrat parsers. Towards this end it has features like the caching and reuse of partial results, the caching of the encountered input, and the ability to backtrack in both input and AST creation.

This document specifies the machine in terms of its architectural state and instruction set.

## Architectural State

Any PARAM implementation has to manage at least the following state:

Input (IN)

  This is the channel the characters to process are read from.

  This part of the machine's state is used and modified by the instructions defined in the section Input Handling.

Current Character (CC)

  The character from the input currently tested against its possible alternatives.

  This part of the machine's state is used and modified by the instructions defined in the section Character Processing.

Current Location (CL)

  The location of the current character in the input, as offset relative to the beginning of the input. Character offsets are counted from 0.

  This part of the machine's state is used and modified by the instructions defined in the sections Character Processing, Location Handling, and Nonterminal Execution.

Location Stack (LS)

  A stack of locations in the input, saved for possible backtracking.

  This part of the machine's state is used and modified by the instructions defined in the sections Character Processing, Location Handling, and Nonterminal Execution.

Status (ST)

  The status of the last attempt of testing the input, indicating either success or failure.

  This part of the machine's state is used and modified by the instructions defined in the sections Status Control, Character Processing, and Nonterminal Execution.

Semantic Value (SV)

  The current semantic value, either empty, or a node for AST constructed from the input.

  This part of the machine's state is used and modified by the instructions defined in the sections Value Construction, and AST Construction.

AST Reduction Stack (ARS)

  The stack of partial ASTs constructed during the processing of nonterminal symbols.

  This part of the machine's state is used and modified by the instructions defined in the sections Value Construction, and AST Construction.

AST Stack (AS)

  The stack of reduction stacks, saved for possible backtracking.

  This part of the machine's state is used and modified by the instructions defined in the sections Value Construction, and AST Construction.

Error Status (ER)

  The machine's current knowledge of errors. This is either empty, or set to a pair of location in the input and the set of messages for that location.

  Note that this part of the machine's state can be set even if the last test of the current character was successful. For example, the *-operator (matching a sub-expression zero or more times) in a PEG is always successful, even if it encounters a problem further in the input and has to backtrack. Such problems must not be forgotten when continuing the parsing.

  This part of the machine's state is used and modified by the instructions defined in the sections Error Handling, Character Processing, and Nonterminal Execution.

Error Stack (ES)

  The stack of error stati, saved for backtracking. This enables the machine to merge current and older error stati when performing backtracking in choices after an failed match.

  This part of the machine's state is used and modified by the instructions defined in the sections Error Handling, Character Processing, and Nonterminal Execution.

Nonterminal Cache (NC)

  A cache of machine states keyed by pairs name of nonterminal symbol and location in the input. Each pair (N, L) is associated with a 4-tuple holding the values to use for CL, ST, SV, and ER after the nonterminal N was parsed starting from the location L. It is a performance aid for backtracking parsers, allowing them to avoid an expensive reparsing of complex nonterminal symbols if they have been encountered before at a given location.

  The key location is where machine started the attempt to match the named nonterminal symbol, and the location in the saved 4-tuple is where machine ended up after the attempt completed, independent of the success of the attempt.

  This part of the machine's state is used and modified by the instructions defined in the section Nonterminal Execution.

Terminal Cache (TC)

  A cache of characters read from IN, with their location in IN as pair of line and column, keyed by the location in IN, this time as character offset from the beginning of IN. It is a performance aid for backtracking parsers, allowing them to avoid a possibly expensive rereading of characters from IN, or even enabling backtracking at, i.e. in the case of IN not randomly seekable.

  This part of the machine's state is used and modified by the instructions defined in the section Input Handling.

## Instruction Set

With the machine's architectural state specified it is now possible to specify the instruction set operating on that state and to be implemented by any realization of the PARAM. The 37 instructions are grouped roughly by the state they influence and/or query during their execution.

### Input Handling

The instructions in this section mainly access IN, pulling the characters to process into the machine.

input_next msg

  This method reads the next character, i.e. the character after CL, from IN. If successful this character becomes CC, CL is advanced by one, ES is cleared, and the operation is recorded as a success in ST.

  The operation may read the character from IN if the next character is not yet known to TC. If successful the new character is stored in TC, with its location (line, column), and the operation otherwise behaves as specified above. Future reads from the same location, possible due to backtracking, will then be satisfied from TC instead of IN.

  If, on the other hand, the end of IN was reached, the operation is recorded as failed in ST, CL is left unchanged, and the pair of CL and msg becomes the new ES.

### Character Processing

The instructions in this section mainly access CC, testing it against character classes, ranges, and individual characters.

test_alnum

  This instruction implements the special PE operator "alnum", which checks if CC falls into the character class of the same name, or not.

  Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_alpha

  This instruction implements the special PE operator "alpha", which checks if CC falls into the character class of the same name, or not.

  Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_ascii

  This instruction implements the special PE operator "ascii", which checks if CC falls into the character class of the same name, or not.

  Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_char char

  This instruction implements the character matching operator, i.e. it checks if CC is char.

  Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_ddigit

  This instruction implements the special PE operator "ddigit", which checks if CC falls into the character class of the same name, or not.

  Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_digit

  This instruction implements the special PE operator "digit", which checks if CC falls into the character class of the same name, or not.

  Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_graph

  This instruction implements the special PE operator "graph", which checks if CC falls into the character class of the same name, or not.

  Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_lower

  This instruction implements the special PE operator "lower", which checks if CC falls into the character class of the same name, or not.

  Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_print

  This instruction implements the special PE operator "print", which checks if CC falls into the character class of the same name, or not.

  Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_punct

  This instruction implements the special PE operator "punct", which checks if CC falls into the character class of the same name, or not.

  Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_range chars chare

  This instruction implements the range matching operator, i.e. it checks if CC falls into the interval of characters spanned up by the two characters from chars to chare, both inclusive.

  Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_space

  This instruction implements the special PE operator "space", which checks if CC falls into the character class of the same name, or not.

  Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_upper

  This instruction implements the special PE operator "upper", which checks if CC falls into the character class of the same name, or not.

  Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_wordchar

  This instruction implements the special PE operator "wordchar", which checks if CC falls into the character class of the same name, or not.

  Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_xdigit

  This instruction implements the special PE operator "xdigit", which checks if CC falls into the character class of the same name, or not.

  Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

### Error Handling

The instructions in this section mainly access ER and ES.

error_clear

  This instruction clears ER.

error_push

  This instruction makes a copy of ER and pushes it on ES.

error_pop_merge

  This instruction takes the topmost entry of ES and merges the error status it contains with ES, making the result the new ES.

  The merge is governed by four rules, with the merge result

  1. Empty if both states are empty.

  2. The non-empty state if only one of the two states is non-empty.

  3. The state with the larger location, if the two states specify different locations.

  4. The pair of the location shared by the two states, and the set-union of their messages for states at the same location.

error_nonterminal symbol

  This is a guarded instruction. It does nothing if either ES is empty, or if the location in ES is not just past the last location saved in LS. Otherwise it sets the pair of that location and the nonterminal symbol as the new ES.

  Note: In the above "just past" means "that location plus one", or also "the location of the next character after that location".

### Status Control

The instructions in this section directly manipulate ST.

status_ok

  This instruction sets ST to true, recording a success.

status_fail

  This instruction sets ST to false, recording a failure.

status_negate

  This instruction negates ST, turning a failure into a success and vice versa.

### Location Handling

The instructions in this section access CL and LS.

loc_push

  This instruction makes a copy of CL and pushes it on LS.

loc_pop_discard

  This instructions pops the last saved location from LS.

loc_pop_rewind

  This instruction pops the last saved location from LS and restores it as CL.

### Nonterminal Execution

The instructions in this section access and manipulate NC.

symbol_restore symbol

  This instruction checks if NC contains data for the nonterminal symbol at CL, or not. The result of the instruction is a boolean flag, with True indicating that data was found in the cache. In that case the instruction has further updated the architectural state of the machine with the cached information, namely CL, ST, ER, and SV.

  The method with which the instruction's result is transformed into control flow is left undefined and the responsibility of the implementation.

symbol_save symbol

  This instructions saves the current settings of CL, ST, ER, and SV in NC, using the pair of nonterminal symbol and the last location saved in LS as key.

### Value Construction

The instructions in this section manipulate SV.

value_clear

  This instruction clears SV.

value_leaf symbol

  This instruction constructs an AST node for symbol covering the range of IN from one character after the last location saved on LS to CL and stores it in SV. ...

value_reduce symbol

  This instruction generally behaves like value_nonterminal_leaf, except that it takes all AST nodes on ARS, if any, and makes them the children of the new node, with the last node saved on ARS becoming the right-most / last child. Note that ARS is not modfied by this operation.

### AST Construction

The instructions in this section manipulate ARS and AS.

ast_value_push

  This instruction makes a copy of SV and pushes it on ARS.

ast_push

  This instruction pushes the current state of ARS on AS and then clears ARS.

ast_pop_rewind

  This instruction pops the last entry saved on AS and restores it as the new state of ARS.

ast_pop_discard

  This instruction pops the last entry saved on AS.

### Control Flow

Normally this section would contain the specifications of the control flow instructions of the PARAM, i.e. (un)conditional jumps and the like. However, this part of the PARAM is intentionally left unspecified. This allows the implementations to freely choose how to implement control flow.

The implementation of this machine in Parser Tools, i.e the package pt::rde, is not only coded in Tcl, but also relies on Tcl commands to provide it with control flow (instructions).

## Interaction of the Instructions with the Architectural State

    Instruction		Inputs				Outputs
    ======================= =======================		====================
    ast_pop_discard		AS			->	AS
    ast_pop_rewind		AS			->	AS, ARS
    ast_push		ARS, AS			->	AS
    ast_value_push		SV, ARS			->	ARS
    ======================= =======================		====================
    error_clear		-			->	ER
    error_nonterminal sym	ER, LS			->	ER
    error_pop_merge   	ES, ER			->	ER
    error_push		ES, ER			->	ES
    ======================= =======================		====================
    input_next msg		IN			->	TC, CL, CC, ST, ER
    ======================= =======================		====================
    loc_pop_discard		LS			->	LS
    loc_pop_rewind		LS			->	LS, CL
    loc_push		CL, LS			->	LS
    ======================= =======================		====================
    status_fail		-			->	ST
    status_negate		ST			->	ST
    status_ok		-			->	ST
    ======================= =======================		====================
    symbol_restore sym	NC			->	CL, ST, ER, SV
    symbol_save    sym	CL, ST, ER, SV LS	->	NC
    ======================= =======================		====================
    test_alnum  		CC			->	ST, ER
    test_alpha		CC			->	ST, ER
    test_ascii		CC			->	ST, ER
    test_char char		CC			->	ST, ER
    test_ddigit		CC			->	ST, ER
    test_digit		CC			->	ST, ER
    test_graph		CC			->	ST, ER
    test_lower		CC			->	ST, ER
    test_print		CC			->	ST, ER
    test_punct		CC			->	ST, ER
    test_range chars chare	CC			->	ST, ER
    test_space		CC			->	ST, ER
    test_upper		CC			->	ST, ER
    test_wordchar		CC			->	ST, ER
    test_xdigit		CC			->	ST, ER
    ======================= =======================		====================
    value_clear		-			->	SV
    value_leaf symbol	LS, CL			->	SV
    value_reduce symbol	ARS, LS, CL		->	SV
    ======================= =======================		====================

## Bugs, Ideas, Feedback

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category pt of the Tcllib Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.

When proposing code changes, please provide unified diffs, i.e the output of diff -u.

Note further that attachments are strongly preferred over inlined patches. Attachments can be made by going to the Edit form of the ticket immediately after its creation, and then using the left-most button in the secondary navigation bar.

## Keywords

EBNF, LL(k), PEG, TDPL, context-free languages, expression, grammar, matching, parser, parsing expression, parsing expression grammar, push down automaton, recursive descent, state, top-down parsing languages, transducer, virtual machine

## Category

Parsing and Grammars

## Copyright

Copyright © 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>