pt::param(n) Parser Tools pt::param(n) 







pt::param - PackRat Machine Specification








package require Tcl 8.5












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.








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

  







  
[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.







  

  
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 SF Trackers
    [http://sourceforge.net/tracker/?group_id=12883]. 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 (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>








  

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