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NAME

       pt_export_api - Parser Tools Export API

SYNOPSIS

       package require Tcl  8.5

       CONVERTER reset

       CONVERTER configure

       CONVERTER configure option

       CONVERTER configure option value...

       CONVERTER convert serial

       ::export serial configuration

________________________________________________________________________________________________________________

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

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

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

       IMAGE: arch_core_export

CONVERTER API

       Any (grammar) export 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::to 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 three methods with the given signatures and
              semantics. Converter commands are allowed to provide more methods of their own, but not less,  and
              they may not provide different semantics for the standardized methods.

              CONVERTER reset
                     This  method  has  to reset the configuration of the converter to its default settings. The
                     result of the method has to be the empty string.

              CONVERTER configure
                     This method, in this form, has to return a dictionary containing the current  configuration
                     of the converter.

              CONVERTER configure option
                     This  method,  in this form, has to return the current value of the specified configuration
                     option of the converter.

                     Please read the section Options for the set  of  standard  options  any  converter  has  to
                     accept.   Any  other  options  accepted  by  a  specific converter will be described in its
                     manpage.

              CONVERTER configure option value...
                     This command, in this form, sets the specified  options  of  the  converter  to  the  given
                     values.

                     Please  read the section Options for the set of standard options a converter has to accept.
                     Any other options accepted by a specific converter will be described in its manpage.

              CONVERTER convert serial
                     This method has to accept the canonical serialization of a parsing expression  grammar,  as
                     specified  in section PEG serialization format, and contained in serial.  The result of the
                     method has to be the result of converting the input grammar into whatever the converter  is
                     for, per its configuration.

PLUGIN API

       Any (grammar) export 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::export::FOO, where FOO is the name of the
              format the plugin will generate output for.

       [3]    The plugin can expect that the package pt::peg::export::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]    A plugin has to provide a single command, in the global namespace, with the signature shown below.
              Plugins are allowed to provide more command of their own, but not less, and they may  not  provide
              different semantics for the standardized command.

              ::export serial configuration
                     This  command has to accept the canonical serialization of a parsing expression grammar and
                     the configuration for the converter invoked by the plugin. The result of the command has to
                     be  the  result  of  the  converter  invoked  by  the  plugin  for  th  input  grammar  and
                     configuration.

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

                     dictionary configuration
                            This argument will contain the configuration to configure the converter with  before
                            invoking it, as a dictionary mapping from options to values.

                            Please read the section Options for the set of standard options any converter has to
                            accept, and thus any plugin as well.  Any  other  options  accepted  by  a  specific
                            plugin will be described in its manpage.

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

OPTIONS

       Each export converter and plugin for an export converter  has  to  accept  the  options  below  in  their
       configure  method.  Converters  are  allowed  to  ignore  the contents of these options when performing a
       conversion, but they must not reject them. Plugins are expected to pass the options given to them to  the
       converter they are invoking.

       -file string
              The  value of this option is the name of the file or other entity from which the grammar came, for
              which the command is run. The default value is unknown.

       -name string
              The value of this option is the name of the grammar we  are  processing.   The  default  value  is
              a_pe_grammar.

       -user string
              The  value  of this option is the name of the user for which the command is run. The default value
              is unknown.

USAGE

       To use a converter do

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

                  # Provide a configuration
                  theconverter configure ...

                  # Perform the conversion
                  set result [theconverter convert $thegrammarserial]

                  ... process the result ...

       To use a plugin FOO do

                  # Get an export plugin manager
                  package require pt::peg::export
                  pt::peg::export E

                  # Provide a configuration
                  E configuration set ...

                  # Run the plugin, and the converter inside.
                  set result [E export serial $grammarserial FOO]

                  ... process the result ...

PEG SERIALIZATION FORMAT

       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.

       regular serialization

              [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

                     rules  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

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

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

                                          value  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.

                                          leaf   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.

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

                     start  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.

       canonical serialization
              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.

   EXAMPLE
       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 <- Term (AddOp Term)*                            ;
                  MulOp      <- '*' / '/'                                     ;
                  Term       <- Factor (MulOp Factor)*                        ;
                  AddOp      <- '+'/'-'                                       ;
                  Factor     <- '(' Expression ')' / 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 {n Term} {* {x {n AddOp} {n Term}}}}                                        mode value}
                      Factor     {is {/ {x {t (} {n Expression} {t )}} {n Number}}                                  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 {x {n Factor} {* {x {n MulOp} {n Factor}}}}                                    mode value}
                  }
                  start {n Expression}
              }

PE SERIALIZATION FORMAT

       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.

       Regular serialization

              Atomic Parsing Expressions

                     [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.

              Combined Parsing Expressions

                     [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.

       Canonical serialization
              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).

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

                  Expression <- Term (AddOp Term)*

       then its canonical serialization (except for whitespace) is

                  {x {n Term} {* {x {n AddOp} {n Term}}}}

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 [http://core.tcl.tk/tcllib/reportlist].  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 (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>