pt_import_api(i) Parser Tools pt_import_api(i) 







pt_import_api - Parser Tools Import API








package require Tcl 8.5


CONVERTER convert text


IncludeFile currentfile path


::import text












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 document describes two APIs. First the API shared by all
    packages for the conversion of some other format into Parsing Expression
    Grammars , and then the API shared by the packages which implement the
    import plugins sitting on top of the conversion packages.


Its intended audience are people who wish to create their own
    converter for some type of input, and/or an import plugin for their or some
    other converter.


It resides in the Import section of the Core Layer of Parser
    Tools.


IMAGE: arch_core_import








Any (grammar) import converter has to follow the rules set out
    below:



  
[1]

  
A converter is a package. Its name is arbitrary, however it is recommended
      to put it under the ::pt::peg::from namespace.

  
[2]

  
The package provides either a single Tcl command following the API
      outlined below, or a class command whose instances follow the same API.
      The commands which follow the API are called converter
    commands.

  
[3]

  
A converter command has to provide the following single method with the
      given signature and semantic. Converter commands are allowed to provide
      more methods of their own, but not less, and they may not provide
      different semantics for the standardized method.







  

  
This method has to accept some text, a parsing expression grammar
      in some format. The result of the method has to be the canonical
      serialization of a parsing expression grammar, as specified in section
      PEG serialization format, the result of reading and converting the
      input text.












Any (grammar) import plugin has to follow the rules set out
  below:



  
[1]

  
A plugin is a package.

  
[2]

  
The name of a plugin package has the form pt::peg::import::FOO,
      where FOO is the name of the format the plugin will accept input
      for.

  
[3]

  
The plugin can expect that the package pt::peg::import::plugin is
      present, as indicator that it was invoked from a genuine plugin manager.
    
It is recommended that a plugin does check for the presence of
        this package.

  

  
[4]

  
The plugin can expect that a command named IncludeFile is present,
      with the signature







  

  
This command has to be invoked by the plugin when it has to process an
      included file, if the format has the concept of such.
    
The plugin has to supply the following arguments

  







  

  
The path of the file it is currently processing. This may be the empty
      string if no such is known.

  

  
The path of the include file as specified in the include directive being
      processed.





The result of the command will be a 5-element list containing




  
[1]

  
A boolean flag indicating the success (True) or failure
      (False) of the operation.

  
[2]

  
In case of success the contents of the included file, and the empty string
      otherwise.

  
[3]

  
The resolved, i.e. absolute path of the included file, if possible, or the
      unchanged path argument. This is for display in an error message,
      or as the currentfile argument of another call to
      IncludeFile should this file contain more files.

  
[4]

  
In case of success an empty string, and for failure a code indicating the
      reason for it, one of







  

  
The specified file could not be found.

  

  
The specified file was found, but not be read into memory.







  
[5]

  
An empty string in case of success of a notfound failure, and an
      additional error message describing the reason for a notread error
      in more detail.









  
[5]

  
A plugin has to provide a single command, in the global namespace, with
      the signature shown below. Plugins are allowed to provide more commands of
      their own, but not less, and they may not provide different semantics for
      the standardized command.







  
::import text

  
This command has to accept the a text containing a parsing expression
      grammar in some format. The result of the command has to be the result of
      the converter invoked by the plugin for the input grammar, the canonical
      serialization of the parsing expression grammar contained in the
    input.







  

  
This argument will contain the parsing expression grammar for which to
      generate the serialization. The specification of what a canonical
      serialization is can be found in the section PEG serialization
      format.









  
[6]

  
A single usage cycle of a plugin consists of an invokation of the command
      import. This call has to leave the plugin in a state where another
      usage cycle can be run without problems.










To use a converter do




    # Get the converter (single command here, not class)


    package require the-converter-package


    # Perform the conversion


    set serial [theconverter convert $thegrammartext]


    ... process the result ...



To use a plugin FOO do




    # Get an import plugin manager


    package require pt::peg::import


    pt::peg::import I


    # Run the plugin, and the converter inside.


    set serial [I import serial $thegrammartext FOO]


    ... process the result ...









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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