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# pt::rde(n) 1.2 tcllib "Parser Tools"

## Name

pt::rde - Parsing Runtime Support, PARAM based

## Synopsis

- package require Tcl 8.5 9
- package require pt::rde ?1.2?
- package require snit
- package require struct::stack 1.6
- package require pt::ast 1.2

- ::pt::rde objectName
- objectName destroy
- objectName reset chan
- objectName complete
- objectName chan
- objectName line
- objectName column
- objectName current
- objectName location
- objectName locations
- objectName ok
- objectName value
- objectName error
- objectName errors
- objectName tokens ?from ?to??
- objectName symbols
- objectName known
- objectName reducible
- objectName asts
- objectName ast
- objectName position loc
- objectName i_input_next msg
- objectName i_test_alnum
- objectName i_test_alpha
- objectName i_test_ascii
- objectName i_test_char char
- objectName i_test_ddigit
- objectName i_test_digit
- objectName i_test_graph
- objectName i_test_lower
- objectName i_test_print
- objectName i_test_punct
- objectName i_test_range chars chare
- objectName i_test_space
- objectName i_test_upper
- objectName i_test_wordchar
- objectName i_test_xdigit
- objectName i_error_clear
- objectName i_error_push
- objectName i_error_pop_merge
- objectName i_error_nonterminal symbol
- objectName i_status_ok
- objectName i_status_fail
- objectName i_status_negate
- objectName i_loc_push
- objectName i_loc_pop_discard
- objectName i_loc_pop_rewind
- objectName i:ok_loc_pop_rewind
- objectName i_loc_pop_rewind/discard
- objectName i_symbol_restore symbol
- objectName i_symbol_save symbol
- objectName i_value_clear
- objectName i_value_clear/leaf
- objectName i_value_clear/reduce
- objectName i:ok_ast_value_push
- objectName i_ast_push
- objectName i_ast_pop_rewind
- objectName i:fail_ast_pop_rewind
- objectName i_ast_pop_rewind/discard
- objectName i_ast_pop_discard
- objectName i_ast_pop_discard/rewind
- objectName i:ok_continue
- objectName i:fail_continue
- objectName i:fail_return
- objectName i:ok_return
- objectName si:void_state_push
- objectName si:void2_state_push
- objectName si:value_state_push
- objectName si:void_state_merge
- objectName si:void_state_merge_ok
- objectName si:value_state_merge
- objectName si:value_notahead_start
- objectName si:void_notahead_exit
- objectName si:value_notahead_exit
- objectName si:kleene_abort
- objectName si:kleene_close
- objectName si:voidvoid_branch
- objectName si:voidvalue_branch
- objectName si:valuevoid_branch
- objectName si:valuevalue_branch
- objectName si:voidvoid_part
- objectName si:voidvalue_part
- objectName si:valuevalue_part
- objectName si:value_symbol_start symbol
- objectName si:value_void_symbol_start symbol
- objectName si:void_symbol_start symbol
- objectName si:void_void_symbol_start symbol
- objectName si:reduce_symbol_end symbol
- objectName si:void_leaf_symbol_end symbol
- objectName si:value_leaf_symbol_end symbol
- objectName si:value_clear_symbol_end symbol
- objectName si:void_clear_symbol_end symbol
- objectName si:next_char tok
- objectName si:next_range toks toke
- objectName si:next_alnum
- objectName si:next_alpha
- objectName si:next_ascii
- objectName si:next_ddigit
- objectName si:next_digit
- objectName si:next_graph
- objectName si:next_lower
- objectName si:next_print
- objectName si:next_punct
- objectName si:next_space
- objectName si:next_upper
- objectName si:next_wordchar
- objectName si:next_xdigit

## 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.

This package provides a class whose instances provide the runtime support for recursive descent parsers with backtracking, as is needed for the execution of, for example, parsing expression grammars. It implements the PackRat Machine Specification, as such that document is required reading to understand both this manpage, and the package itself. The description below does make numerous shorthand references to the PARAM's instructions and the various parts of its architectural state.

The package resides in the Execution section of the Core Layer of Parser Tools.

Note: This package not only has the standard Tcl implementation, but also an accelerator, i.e. a C implementation, based on Critcl.

### Class API

The package exports the API described here.

::pt::rde objectName

  The command creates a new runtime object for a recursive descent parser with backtracking and returns the fully qualified name of the object command as its result. The API of this object command is described in the section Object API. It may be used to invoke various operations on the object.

### Object API

All objects created by this package provide the following 63 methods for the manipulation and querying of their state, which is, in essence the architectural state of a PARAM.

First some general methods and the state accessors.

objectName destroy

  This method destroys the object, releasing all claimed memory, and deleting the associated object command.

objectName reset chan

  This method resets the state of the runtme to its defaults, preparing it for the parsing of the character in the channel chan, which becomes IN.

  Note here that the Parser Tools are based on Tcl 8.5+. In other words, the channel argument is not restricted to files, sockets, etc. We have the full power of reflected channels available.

  It should also be noted that the parser pulls the characters from the input stream as it needs them. If a parser created by this package has to be operated in a push aka event-driven manner it will be necessary to go to Tcl 8.6+ and use the coroutine::auto to wrap it into a coroutine where read is properly changed for push-operation.

objectName complete

  This method completes parsing, either returning the AST made from the elements of ARS, or throwing an error containing the current ER.

objectName chan

  This method returns the handle of the channel which is IN.

objectName line

  This method returns the line number for the position IN is currently at. Note that this may not match with the line number for CL, due to backtracking.

objectName column

  This method returns the column for the position IN is currently at. Note that this may not match with the column for CL, due to backtracking.

objectName current

  This method returns CC.

objectName location

  This method returns CL.

objectName locations

  This method returns the LS. The topmost entry of the stack will be the first element of the returned list.

objectName ok

  This method returns ST.

objectName value

  This method returns SV.

objectName error

  This method returns ER. This is either the empty string for an empty ER, or a list of 2 elements, the location the error is for, and a set of messages which specify which symbols were expected at the location. The messages are encoded as one of the possible atomic parsing expressions (special operators, terminal, range, and nonterminal operator).

objectName errors

  This method returns ES. The topmost entry of the stack will be the first element of the returned list. Each entry is encoded as described for error.

objectName tokens ?from ?to??

  This method returns the part of TC for the range of locations of IN starting at from and ending at to. If to is not specified it is taken as identical to from. If neither argument is specified the whole of TC is returned.

  Each token in the returned list is a list of three elements itself, containing the character at the location, and the associated line and column numbers, in this order.

objectName symbols

  This method returns a dictionary containing NC. Keys are two-element lists containing nonterminal symbol and location, in this order. The values are 4-tuples containing CL, ST, ER, and SV, in this order. ER is encoded as specified for the method error.

objectName known

  This method returns a list containing the keys of SC. They are encoded in the same manner as is done by method symbols.

objectName reducible

  This method returns ARS. The topmost entry of the stack will be the first element of the returned list

objectName asts

  This method returns AS. The topmost entry of the stack will be the first element of the returned list

objectName ast

  This is a convenience method returning the topmost element of ARS.

objectName position loc

  This method returns the line and column numbers for the specified location of IN, assuming that this location has already been reached during the parsing process.

The following methods implement all PARAM instructions. They all have the prefix "i_".

The control flow is mainly provided by Tcl's builtin commands, like if, while, etc., plus a few guarded variants of PARAM instructions and Tcl commands.. That means that these instruction variants will do nothing if their guard condition is not fulfilled. They can be recognized by the prefix "i:ok_" and "i:fail_", which denote the value ST has to have for the instruction to execute.

The instructions are listed in the same order they occur in the PackRat Machine Specification, with the guard variants listed after their regular implementation, if any, or in their place.

objectName i_input_next msg

  This method implements the PARAM instruction input_next.

objectName i_test_alnum

  This method implements the PARAM instruction test_alnum.

objectName i_test_alpha

  This method implements the PARAM instruction test_alpha.

objectName i_test_ascii

  This method implements the PARAM instruction test_ascii.

objectName i_test_char char

  This method implements the PARAM instruction test_char.

objectName i_test_ddigit

  This method implements the PARAM instruction test_ddigit.

objectName i_test_digit

  This method implements the PARAM instruction test_digit.

objectName i_test_graph

  This method implements the PARAM instruction test_graph.

objectName i_test_lower

  This method implements the PARAM instruction test_lower.

objectName i_test_print

  This method implements the PARAM instruction test_print.

objectName i_test_punct

  This method implements the PARAM instruction test_punct.

objectName i_test_range chars chare

  This method implements the PARAM instruction test_range.

objectName i_test_space

  This method implements the PARAM instruction test_space.

objectName i_test_upper

  This method implements the PARAM instruction test_upper.

objectName i_test_wordchar

  This method implements the PARAM instruction test_wordchar.

objectName i_test_xdigit

  This method implements the PARAM instruction test_xdigit.

objectName i_error_clear

  This method implements the PARAM instruction error_clear.

objectName i_error_push

  This method implements the PARAM instruction error_push.

objectName i_error_pop_merge

  This method implements the PARAM instruction error_pop_merge.

objectName i_error_nonterminal symbol

  This method implements the PARAM instruction error_nonterminal.

objectName i_status_ok

  This method implements the PARAM instruction status_ok.

objectName i_status_fail

  This method implements the PARAM instruction status_fail.

objectName i_status_negate

  This method implements the PARAM instruction status_negate.

objectName i_loc_push

  This method implements the PARAM instruction loc_push.

objectName i_loc_pop_discard

  This method implements the PARAM instruction loc_pop_discard.

objectName i_loc_pop_rewind

  This method implements the PARAM instruction loc_pop_rewind.

objectName i:ok_loc_pop_rewind

  This guarded method, a variant of i_loc_pop_rewind, executes only for "ST == ok".

objectName i_loc_pop_rewind/discard

  This method is a convenient combination of control flow and the two PARAM instructions loc_pop_rewind and loc_pop_discard. The former is executed for "ST == fail", the latter for "ST == ok".

objectName i_symbol_restore symbol

  This method implements the PARAM instruction symbol_restore.

  The boolean result of the check is returned as the result of the method and can be used with standard Tcl control flow commands.

objectName i_symbol_save symbol

  This method implements the PARAM instruction symbol_save.

objectName i_value_clear

  This method implements the PARAM instruction value_clear.

objectName i_value_clear/leaf

  This method is a convenient combination of control flow and the two PARAM instructions value_clear and value_leaf. The former is executed for "ST == fail", the latter for "ST == ok".

objectName i_value_clear/reduce

  This method is a convenient combination of control flow and the two PARAM instructions value_clear and value_reduce. The former is executed for "ST == fail", the latter for "ST == ok".

objectName i:ok_ast_value_push

  This method implements a guarded variant of the the PARAM instruction ast_value_push, which executes only for "ST == ok".

objectName i_ast_push

  This method implements the PARAM instruction ast_push.

objectName i_ast_pop_rewind

  This method implements the PARAM instruction ast_pop_rewind.

objectName i:fail_ast_pop_rewind

  This guarded method, a variant of i_ast_pop_rewind, executes only for "ST == fail".

objectName i_ast_pop_rewind/discard

  This method is a convenient combination of control flow and the two PARAM instructions ast_pop_rewind and ast_pop_discard. The former is executed for "ST == fail", the latter for "ST == ok".

objectName i_ast_pop_discard

  This method implements the PARAM instruction ast_pop_discard.

objectName i_ast_pop_discard/rewind

  This method is a convenient combination of control flow and the two PARAM instructions ast_pop_discard and ast_pop_rewind. The former is executed for "ST == fail", the latter for "ST == ok".

objectName i:ok_continue

  This guarded method executes only for "ST == ok". Then it aborts the current iteration of the innermost loop in the calling Tcl procedure.

objectName i:fail_continue

  This guarded method executes only for "ST == fail". Then it aborts the current iteration of the innermost loop in the calling Tcl procedure.

objectName i:fail_return

  This guarded method executes only for "ST == fail". Then it aborts the calling Tcl procedure.

objectName i:ok_return

  This guarded method executes only for "ST == ok". Then it aborts the calling Tcl procedure.

The next set of methods are super instructions, meaning that each implements a longer sequence of instructions commonly used in parsers. The combinated instructions of the previous set, i.e. those with names matching the pattern "i_*/*", are actually super instructions as well, albeit with limited scope, handling 2 instructions with their control flow. The upcoming set is much broader in scope, folding as much as six or more PARAM instructions into a single method call.

In this we can see the reasoning behind their use well:

1. By using less instructions the generated parsers become smaller, as the common parts are now truly part of the common runtime, and not explicitly written in the parser's code over and over again.

2. Using less instructions additionally reduces the overhead associated with calls into the runtime, i.e. the cost of method dispatch and of setting up the variable context.

3. Another effect of the super instructions is that their internals can be optimized as well, especially regarding control flow, and stack use, as the runtime internals are accessible to all instructions folded into the sequence.

objectName si:void_state_push

  This method combines

      i_loc_push
      i_error_clear
      i_error_push

  Parsers use it at the beginning of void sequences and choices with a void initial branch.

objectName si:void2_state_push

  This method combines

      i_loc_push
      i_error_clear
      i_error_push

  Parsers use it at the beginning of optional and repeated expressions.

objectName si:value_state_push

  This method combines

      i_ast_push
      i_loc_push
      i_error_clear
      i_error_push

  Parsers use it at the beginning of sequences generating an AST and choices with an initial branch generating an AST.

objectName si:void_state_merge

  This method combines

      i_error_pop_merge
      i_loc_pop_rewind/discard

  Parsers use it at the end of void sequences and choices whose last branch is void.

objectName si:void_state_merge_ok

  This method combines

      i_error_pop_merge
      i_loc_pop_rewind/discard
      i_status_ok

  Parsers use it at the end of optional expressions

objectName si:value_state_merge

  This method combines

      i_error_pop_merge
      i_ast_pop_rewind/discard
      i_loc_pop_rewind/discard

  Parsers use it at the end of sequences generating ASTs and choices whose last branch generates an AST

objectName si:value_notahead_start

  This method combines

      i_loc_push
      i_ast_push

  Parsers use it at the beginning of negative lookahead predicates which generate ASTs.

objectName si:void_notahead_exit

  This method combines

      i_loc_pop_rewind
      i_status_negate

  Parsers use it at the end of void negative lookahead predicates.

objectName si:value_notahead_exit

  This method combines

      i_ast_pop_discard/rewind
      i_loc_pop_rewind
      i_status_negate

  Parsers use it at the end of negative lookahead predicates which generate ASTs.

objectName si:kleene_abort

  This method combines

      i_loc_pop_rewind/discard
      i:fail_return

  Parsers use it to stop a positive repetition when its first, required, expression fails.

objectName si:kleene_close

  This method combines

      i_error_pop_merge
      i_loc_pop_rewind/discard
      i:fail_status_ok
      i:fail_return

  Parsers use it at the end of repetitions.

objectName si:voidvoid_branch

  This method combines

      i_error_pop_merge
      i:ok_loc_pop_discard
      i:ok_return
      i_loc_rewind
      i_error_push

  Parsers use it when transiting between branches of a choice when both are void.

objectName si:voidvalue_branch

  This method combines

      i_error_pop_merge
      i:ok_loc_pop_discard
      i:ok_return
      i_ast_push
      i_loc_rewind
      i_error_push

  Parsers use it when transiting between branches of a choice when the failing branch is void, and the next to test generates an AST.

objectName si:valuevoid_branch

  This method combines

      i_error_pop_merge
      i_ast_pop_rewind/discard
      i:ok_loc_pop_discard
      i:ok_return
      i_loc_rewind
      i_error_push

  Parsers use it when transiting between branches of a choice when the failing branch generates an AST, and the next to test is void.

objectName si:valuevalue_branch

  This method combines

      i_error_pop_merge
      i_ast_pop_discard
      i:ok_loc_pop_discard
      i:ok_return
      i_ast_rewind
      i_loc_rewind
      i_error_push

  Parsers use it when transiting between branches of a choice when both generate ASTs.

objectName si:voidvoid_part

  This method combines

      i_error_pop_merge
      i:fail_loc_pop_rewind
      i:fail_return
      i_error_push

  Parsers use it when transiting between parts of a sequence and both are void.

objectName si:voidvalue_part

  This method combines

      i_error_pop_merge
      i:fail_loc_pop_rewind
      i:fail_return
      i_ast_push
      i_error_push

  Parsers use it when transiting between parts of a sequence and the sucessfully matched part is void, and after it an AST is generated.

objectName si:valuevalue_part

  This method combines

      i_error_pop_merge
      i:fail_ast_pop_rewind
      i:fail_loc_pop_rewind
      i:fail_return
      i_error_push

  Parsers use it when transiting between parts of a sequence and both parts generate ASTs.

objectName si:value_symbol_start symbol

  This method combines

      if/found? i_symbol_restore $symbol
      i:found:ok_ast_value_push
      i:found_return
      i_loc_push
      i_ast_push

  Parsers use it at the beginning of a nonterminal symbol generating an AST, whose right-hand side may have generated an AST as well.

objectName si:value_void_symbol_start symbol

  This method combines

      if/found? i_symbol_restore $symbol
      i:found:ok_ast_value_push
      i:found_return
      i_loc_push
      i_ast_push

  Parsers use it at the beginning of a void nonterminal symbol whose right-hand side may generate an AST.

objectName si:void_symbol_start symbol

  This method combines

      if/found? i_symbol_restore $symbol
      i:found_return
      i_loc_push
      i_ast_push

  Parsers use it at the beginning of a nonterminal symbol generating an AST whose right-hand side is void.

objectName si:void_void_symbol_start symbol

  This method combines

      if/found? i_symbol_restore $symbol
      i:found_return
      i_loc_push

  Parsers use it at the beginning of a void nonterminal symbol whose right-hand side is void as well.

objectName si:reduce_symbol_end symbol

  This method combines

      i_value_clear/reduce $symbol
      i_symbol_save        $symbol
      i_error_nonterminal  $symbol
      i_ast_pop_rewind
      i_loc_pop_discard
      i:ok_ast_value_push

  Parsers use it at the end of a non-terminal symbol generating an AST using the AST generated by the right-hand side as child.

objectName si:void_leaf_symbol_end symbol

  This method combines

      i_value_clear/leaf  $symbol
      i_symbol_save       $symbol
      i_error_nonterminal $symbol
      i_loc_pop_discard
      i:ok_ast_value_push

  Parsers use it at the end of a non-terminal symbol generating an AST whose right-hand side is void.

objectName si:value_leaf_symbol_end symbol

  This method combines

      i_value_clear/leaf  $symbol
      i_symbol_save       $symbol
      i_error_nonterminal $symbol
      i_loc_pop_discard
      i_ast_pop_rewind
      i:ok_ast_value_push

  Parsers use it at the end of a non-terminal symbol generating an AST discarding the AST generated by the right-hand side.

objectName si:value_clear_symbol_end symbol

  This method combines

      i_value_clear
      i_symbol_save       $symbol
      i_error_nonterminal $symbol
      i_loc_pop_discard
      i_ast_pop_rewind

  Parsers use it at the end of a void non-terminal symbol, discarding the AST generated by the right-hand side.

objectName si:void_clear_symbol_end symbol

  This method combines

      i_value_clear
      i_symbol_save       $symbol
      i_error_nonterminal $symbol
      i_loc_pop_discard

  Parsers use it at the end of a void non-terminal symbol with a void right-hand side.

objectName si:next_char tok
objectName si:next_range toks toke
objectName si:next_alnum
objectName si:next_alpha
objectName si:next_ascii
objectName si:next_ddigit
objectName si:next_digit
objectName si:next_graph
objectName si:next_lower
objectName si:next_print
objectName si:next_punct
objectName si:next_space
objectName si:next_upper
objectName si:next_wordchar
objectName si:next_xdigit

  These methods all combine

      i_input_next $msg
      i:fail_return

  with the appropriate i_test_xxx instruction. Parsers use them for handling atomic expressions.

## 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

## Category

Parsing and Grammars

## Copyright

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