pt::peg(n) Parser Tools pt::peg(n) 







pt::peg - Parsing Expression Grammar Serialization








package require Tcl 8.5


package require pt::peg ?1?


package require pt::pe 


::pt::peg verify serial ?canonvar?


::pt::peg verify-as-canonical serial


::pt::peg canonicalize serial


::pt::peg print serial


::pt::peg merge seriala serialb


::pt::peg equal seriala serialb












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 commands to work with the serializations of
    parsing expression grammars as managed by the Parser Tools, and specified in
    section PEG serialization format.


This is a supporting package in the Core Layer of Parser
  Tools.


IMAGE: arch_core_support









  
::pt::peg verify serial ?canonvar?

  
This command verifies that the content of serial is a valid
      serialization of a parsing expression and will throw an error if that is
      not the case. The result of the command is the empty string.
    
If the argument canonvar is specified it is interpreted
        as the name of a variable in the calling context. This variable will be
        written to if and only if serial is a valid regular
        serialization. Its value will be a boolean, with True indicating
        that the serialization is not only valid, but also canonical.
        False will be written for a valid, but non-canonical
        serialization.

    
For the specification of serializations see the section PE
        serialization format.

  

  
::pt::peg verify-as-canonical serial

  
This command verifies that the content of serial is a valid
      canonical serialization of a PEG and will throw an error if that is
      not the case. The result of the command is the empty string.
    
For the specification of canonical serializations see the
        section PEG serialization format.

  

  
::pt::peg canonicalize serial

  
This command assumes that the content of serial is a valid
      regular serialization of a PEG and will throw an error if that is
      not the case.
    
It will then convert the input into the canonical
        serialization of the contained PEG and return it as its result. If the
        input is already canonical it will be returned unchanged.

    
For the specification of regular and canonical serializations
        see the section PEG serialization format.

  

  
::pt::peg print serial

  
This command assumes that the argument serial contains a valid
      serialization of a parsing expression and returns a string containing that
      PE in a human readable form.
    
The exact format of this form is not specified and cannot be
        relied on for parsing or other machine-based activities.

    
For the specification of serializations see the section PEG
        serialization format.

  

  
::pt::peg merge seriala serialb

  
This command accepts the regular serializations of two grammars and uses
      them to create their union. The result of the command is the canonical
      serialization of this unified grammar.
    
A merge errors occurs if for any nonterminal symbol S occuring
        in both input grammars the two input grammars specify different semantic
        modes.

    
The semantic mode of each nonterminal symbol S is the semantic
        mode of S in any of its input grammars. The previous rule made sure that
        for symbols occuring in both grammars these values are identical.

    
The right-hand side of each nonterminal symbol S occuring in
        both input grammars is the choice between the right-hand sides of S in
        the input grammars, with the parsing expression of S in seriala
        coming first, except if both expressions are identical. In that case the
        first expression is taken.

    
The right-hand side of each nonterminal symbol S occuring in
        only one of the input grammars is the right-hand side of S in its input
        grammar.

    
The start expression of the unified grammar is the choice
        between the start expressions of the input grammars, with the start
        expression of seriala coming first, except if both expressions
        are identical. In that case the first expression is taken

  

  
::pt::peg equal seriala serialb

  
This command tests the two grammars seriala and serialb for
      structural equality. The result of the command is a boolean value. It will
      be set to true if the expressions are identical, and false
      otherwise.
    
String equality is usable only if we can assume that the two
        grammars are pure Tcl lists and dictionaries.

  










Here we specify the format used by the Parser Tools to serialize
    Parsing Expression Grammars as immutable values for transport, comparison,
    etc.


We distinguish between regular and canonical
    serializations. While a PEG may have more than one regular serialization
    only exactly one of them will be canonical.



  

  







  
[1]

  
The serialization of any PEG is a nested Tcl dictionary.

  
[2]

  
This dictionary holds a single key, pt::grammar::peg, and its
      value. This value holds the contents of the grammar.

  
[3]

  
The contents of the grammar are a Tcl dictionary holding the set of
      nonterminal symbols and the starting expression. The relevant keys and
      their values are







  

  
The value is a Tcl dictionary whose keys are the names of the nonterminal
      symbols known to the grammar.







  
[1]

  
Each nonterminal symbol may occur only once.

  
[2]

  
The empty string is not a legal nonterminal symbol.

  
[3]

  
The value for each symbol is a Tcl dictionary itself. The relevant keys
      and their values in this dictionary are







  

  
The value is the serialization of the parsing expression describing the
      symbols sentennial structure, as specified in the section PE
      serialization format.

  

  
The value can be one of three values specifying how a parser should handle
      the semantic value produced by the symbol.







  

  
The semantic value of the nonterminal symbol is an abstract syntax tree
      consisting of a single node node for the nonterminal itself, which has the
      ASTs of the symbol's right hand side as its children.

  

  
The semantic value of the nonterminal symbol is an abstract syntax tree
      consisting of a single node node for the nonterminal, without any
      children. Any ASTs generated by the symbol's right hand side are
      discarded.

  

  
The nonterminal has no semantic value. Any ASTs generated by the symbol's
      right hand side are discarded (as well).











  

  
The value is the serialization of the start parsing expression of the
      grammar, as specified in the section PE serialization format.







  
[4]

  
The terminal symbols of the grammar are specified implicitly as the set of
      all terminal symbols used in the start expression and on the RHS of the
      grammar rules.







  

  
The canonical serialization of a grammar has the format as specified in
      the previous item, and then additionally satisfies the constraints below,
      which make it unique among all the possible serializations of this
      grammar.







  
[1]

  
The keys found in all the nested Tcl dictionaries are sorted in ascending
      dictionary order, as generated by Tcl's builtin command lsort
      -increasing -dict.

  
[2]

  
The string representation of the value is the canonical representation of
      a Tcl dictionary. I.e. it does not contain superfluous whitespace.










Assuming the following PEG for simple mathematical expressions




PEG calculator (Expression)
    Digit      <- '0'/'1'/'2'/'3'/'4'/'5'/'6'/'7'/'8'/'9'	;
    Sign       <- '-' / '+'					;
    Number     <- Sign? Digit+					;
    Expression <- '(' Expression ')' / (Factor (MulOp Factor)*)	;
    MulOp      <- '*' / '/'					;
    Factor     <- Term (AddOp Term)*				;
    AddOp      <- '+'/'-'					;
    Term       <- Number					;
END;



then its canonical serialization (except for whitespace) is




pt::grammar::peg {
    rules {
	AddOp      {is {/ {t -} {t +}}                                                                mode value}
	Digit      {is {/ {t 0} {t 1} {t 2} {t 3} {t 4} {t 5} {t 6} {t 7} {t 8} {t 9}}                mode value}
	Expression {is {/ {x {t (} {n Expression} {t )}} {x {n Factor} {* {x {n MulOp} {n Factor}}}}} mode value}
	Factor     {is {x {n Term} {* {x {n AddOp} {n Term}}}}                                        mode value}
	MulOp      {is {/ {t *} {t /}}                                                                mode value}
	Number     {is {x {? {n Sign}} {+ {n Digit}}}                                                 mode value}
	Sign       {is {/ {t -} {t +}}                                                                mode value}
	Term       {is  {n Number}                                                                    mode value}
    }
    start {n Expression}
}











Here we specify the format used by the Parser Tools to serialize
    Parsing Expressions as immutable values for transport, comparison, etc.


We distinguish between regular and canonical
    serializations. While a parsing expression may have more than one regular
    serialization only exactly one of them will be canonical.



  

  







  

  







  
[1]

  
The string epsilon is an atomic parsing expression. It matches the
      empty string.

  
[2]

  
The string dot is an atomic parsing expression. It matches any
      character.

  
[3]

  
The string alnum is an atomic parsing expression. It matches any
      Unicode alphabet or digit character. This is a custom extension of PEs
      based on Tcl's builtin command string is.

  
[4]

  
The string alpha is an atomic parsing expression. It matches any
      Unicode alphabet character. This is a custom extension of PEs based on
      Tcl's builtin command string is.

  
[5]

  
The string ascii is an atomic parsing expression. It matches any
      Unicode character below U0080. This is a custom extension of PEs based on
      Tcl's builtin command string is.

  
[6]

  
The string control is an atomic parsing expression. It matches any
      Unicode control character. This is a custom extension of PEs based on
      Tcl's builtin command string is.

  
[7]

  
The string digit is an atomic parsing expression. It matches any
      Unicode digit character. Note that this includes characters outside of the
      [0..9] range. This is a custom extension of PEs based on Tcl's builtin
      command string is.

  
[8]

  
The string graph is an atomic parsing expression. It matches any
      Unicode printing character, except for space. This is a custom extension
      of PEs based on Tcl's builtin command string is.

  
[9]

  
The string lower is an atomic parsing expression. It matches any
      Unicode lower-case alphabet character. This is a custom extension of PEs
      based on Tcl's builtin command string is.

  
[10]

  
The string print is an atomic parsing expression. It matches any
      Unicode printing character, including space. This is a custom extension of
      PEs based on Tcl's builtin command string is.

  
[11]

  
The string punct is an atomic parsing expression. It matches any
      Unicode punctuation character. This is a custom extension of PEs based on
      Tcl's builtin command string is.

  
[12]

  
The string space is an atomic parsing expression. It matches any
      Unicode space character. This is a custom extension of PEs based on Tcl's
      builtin command string is.

  
[13]

  
The string upper is an atomic parsing expression. It matches any
      Unicode upper-case alphabet character. This is a custom extension of PEs
      based on Tcl's builtin command string is.

  
[14]

  
The string wordchar is an atomic parsing expression. It matches any
      Unicode word character. This is any alphanumeric character (see alnum),
      and any connector punctuation characters (e.g. underscore). This is a
      custom extension of PEs based on Tcl's builtin command string
    is.

  
[15]

  
The string xdigit is an atomic parsing expression. It matches any
      hexadecimal digit character. This is a custom extension of PEs based on
      Tcl's builtin command string is.

  
[16]

  
The string ddigit is an atomic parsing expression. It matches any
      decimal digit character. This is a custom extension of PEs based on Tcl's
      builtin command regexp.

  
[17]

  
The expression [list t x] is an atomic parsing expression. It
      matches the terminal string x.

  
[18]

  
The expression [list n A] is an atomic parsing expression. It
      matches the nonterminal A.







  

  







  
[1]

  
For parsing expressions e1, e2, ... the result of [list /
      e1 e2 ... ] is a parsing expression as well. This is the
      ordered choice, aka prioritized choice.

  
[2]

  
For parsing expressions e1, e2, ... the result of [list x
      e1 e2 ... ] is a parsing expression as well. This is the
      sequence.

  
[3]

  
For a parsing expression e the result of [list * e] is a
      parsing expression as well. This is the kleene closure, describing
      zero or more repetitions.

  
[4]

  
For a parsing expression e the result of [list + e] is a
      parsing expression as well. This is the positive kleene closure,
      describing one or more repetitions.

  
[5]

  
For a parsing expression e the result of [list & e] is a
      parsing expression as well. This is the and lookahead
    predicate.

  
[6]

  
For a parsing expression e the result of [list ! e] is a
      parsing expression as well. This is the not lookahead
    predicate.

  
[7]

  
For a parsing expression e the result of [list ? e] is a
      parsing expression as well. This is the optional input.









  

  
The canonical serialization of a parsing expression has the format as
      specified in the previous item, and then additionally satisfies the
      constraints below, which make it unique among all the possible
      serializations of this parsing expression.







  
[1]

  
The string representation of the value is the canonical representation of
      a pure Tcl list. I.e. it does not contain superfluous whitespace.

  
[2]

  
Terminals are not encoded as ranges (where start and end of the
      range are identical).










Assuming the parsing expression shown on the right-hand side of
    the rule




    Expression <- '(' Expression ')'
                / Factor (MulOp Factor)*



then its canonical serialization (except for whitespace) is




    {/ {x {t (} {n Expression} {t )}} {x {n Factor} {* {x {n MulOp} {n Factor}}}}}











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








Parsing and Grammars








Copyright (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>








  

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