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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>