pt::param(3tcl) Parser Tools pt::param(3tcl)
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NAME
pt::param - PackRat Machine Specification
SYNOPSIS
package require Tcl 8.5
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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 pack-
rat 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 Process-
ing, 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 Con-
struction.
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 Con-
struction.
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 Con-
struction.
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 Process-
ing, 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 Process-
ing, 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 imple-
mented by any realization of the PARAM. The 37 instructions are grouped roughly by the state they influence and/or query during their exe-
cution.
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 loca-
tion, 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
InstructionInputsOutputs
======================= ===========================================
ast_pop_discardAS->AS
ast_pop_rewindAS->AS, ARS
ast_push ARS, AS->AS
ast_value_pushSV, ARS->ARS
======================= ===========================================
error_clear-->ER
error_nonterminal symER, LS->ER
error_pop_merge ES, ER->ER
error_pushES, ER->ES
======================= ===========================================
input_next msgIN->TC, CL, CC, ST, ER
======================= ===========================================
loc_pop_discardLS->LS
loc_pop_rewindLS->LS, CL
loc_push CL, LS->LS
======================= ===========================================
status_fail-->ST
status_negateST->ST
status_ok -->ST
======================= ===========================================
symbol_restore symNC->CL, ST, ER, SV
symbol_save symCL, ST, ER, SV LS->NC
======================= ===========================================
test_alnum CC->ST, ER
test_alphaCC->ST, ER
test_asciiCC->ST, ER
test_char charCC->ST, ER
test_ddigitCC->ST, ER
test_digitCC->ST, ER
test_graphCC->ST, ER
test_lowerCC->ST, ER
test_printCC->ST, ER
test_punctCC->ST, ER
test_range chars chareCC->ST, ER
test_spaceCC->ST, ER
test_upperCC->ST, ER
test_wordcharCC->ST, ER
test_xdigitCC->ST, ER
======================= ===========================================
value_clear-->SV
value_leaf symbolLS, CL->SV
value_reduce symbolARS, 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 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.
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 (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>
pt 1 pt::param(3tcl)