pt::param - PackRat Machine Specification
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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.
Any PARAM implementation has to manage at least the following state:
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The instructions in this section mainly access IN, pulling the characters to process into the machine.
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.
The instructions in this section mainly access CC, testing it against character classes, ranges, and individual characters.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The instructions in this section mainly access ER and ES.
This instruction clears ER.
This instruction makes a copy of ER and pushes it on ES.
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
Empty if both states are empty.
The non-empty state if only one of the two states is non-empty.
The state with the larger location, if the two states specify different locations.
The pair of the location shared by the two states, and the set-union of their messages for states at the same location.
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".
The instructions in this section directly manipulate ST.
This instruction sets ST to true, recording a success.
This instruction sets ST to false, recording a failure.
This instruction negates ST, turning a failure into a success and vice versa.
The instructions in this section access CL and LS.
This instruction makes a copy of CL and pushes it on LS.
This instructions pops the last saved location from LS.
This instruction pops the last saved location from LS and restores it as CL.
The instructions in this section access and manipulate NC.
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.
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.
The instructions in this section manipulate SV.
This instruction clears SV.
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. ...
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.
The instructions in this section manipulate ARS and AS.
This instruction makes a copy of SV and pushes it on ARS.
This instruction pushes the current state of ARS on AS and then clears ARS.
This instruction pops the last entry saved on AS and restores it as the new state of ARS.
This instruction pops the last entry saved on AS.
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).
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 ======================= ======================= ====================
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.
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
Parsing and Grammars
Copyright © 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>