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NAME

       pt::peg::import - PEG Import

SYNOPSIS

       package require Tcl  8.5

       package require snit

       package require configuration

       package require pt::peg

       package require pluginmgr

       package require pt::peg::import  ?1?

       ::pt::peg::import objectName

       objectName method ?arg arg ...?

       objectName destroy

       objectName import text text ?format?

       objectName import file path ?format?

       objectName import object text object text ?format?

       objectName import object file object path ?format?

       objectName includes

       objectName include add path

       objectName include remove path

       objectName include clear

_________________________________________________________________________________________________

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  manager  for  parsing  expression grammars, with each instance
       handling a set of plugins for the import of them from other formats, i.e. their conversion
       from, for example peg, container, json, etc.

       It  resides  in  the  Import  section of the Core Layer of Parser Tools, and is one of the
       three pillars the management of parsing expression grammars resides on.

       IMAGE: arch_core_import

       The other two pillars are, as shown above

       [1]    PEG Export, and

       [2]    PEG Storage

       For information about the data structure which is the major output of the manager  objects
       provided by this package see the section PEG serialization format.

       The  plugin  system of our class is based on the package pluginmgr, and configured to look
       for plugins using

       [1]    the environment variable GRAMMAR_PEG_IMPORT_PLUGINS,

       [2]    the environment variable GRAMMAR_PEG_PLUGINS,

       [3]    the environment variable GRAMMAR_PLUGINS,

       [4]    the path "~/.grammar/peg/import/plugin"

       [5]    the path "~/.grammar/peg/plugin"

       [6]    the path "~/.grammar/plugin"

       [7]    the path "~/.grammar/peg/import/plugins"

       [8]    the path "~/.grammar/peg/plugins"

       [9]    the path "~/.grammar/plugins"

       [10]   the registry entry "HKEY_CURRENT_USER\SOFTWARE\GRAMMAR\PEG\IMPORT\PLUGINS"

       [11]   the registry entry "HKEY_CURRENT_USER\SOFTWARE\GRAMMAR\PEG\PLUGINS"

       [12]   the registry entry "HKEY_CURRENT_USER\SOFTWARE\GRAMMAR\PLUGINS"

       The last three are used only when the package is run on a machine  using  the  Windows(tm)
       operating system.

       The whole system is delivered with three predefined import plugins, namely

       container
              See PEG Import Plugin. From CONTAINER format for details.

       json   See PEG Import Plugin. From JSON format for details.

       peg    See PEG Import Plugin. From PEG format for details.

       For readers wishing to write their own import plugin for some format, i.e. plugin writers,
       reading and understanding the Parser  Tools  Impport  API  specification  is  an  absolute
       necessity, as it documents the interaction between this package and its plugins in detail.

API

   PACKAGE COMMANDS
       ::pt::peg::import objectName
              This  command  creates  a  new import manager object with an associated Tcl command
              whose name is objectName. This object command is explained in full  detail  in  the
              sections  Object  command  and  Object  methods. The object command will be created
              under the current namespace if the objectName is not fully qualified,  and  in  the
              specified namespace otherwise.

   OBJECT COMMAND
       All objects created by the ::pt::peg::import command have the following general form:

       objectName method ?arg arg ...?
              The  method  method and its arg'uments determine the exact behavior of the command.
              See section Object methods for the detailed specifications.

   OBJECT METHODS
       objectName destroy
              This method destroys the object it is invoked for.

       objectName import text text ?format?
              This method takes the text and  converts  it  from  the  specified  format  to  the
              canonical serialization of a parsing expression grammar using the import plugin for
              the format. An error is thrown if no plugin could be found  for  the  format.   The
              serialization generated by the conversion process is returned as the result of this
              method.

              If no format is specified the method defaults to text.

              The specification of what a canonical serialization is can be found in the  section
              PEG serialization format.

              The  plugin  has  to conform to the interface documented in the Parser Tools Import
              API specification.

       objectName import file path ?format?
              This method is a convenient wrapper around the import text method described by  the
              previous  item.  It reads the contents of the specified file into memory, feeds the
              result into import text and returns the resulting serialization as its own result.

       objectName import object text object text ?format?
              This method is a convenient wrapper around the import text method described by  the
              previous  item.   It  expects  that  object  is  an  object  command  supporting  a
              deserialize method expecting the canonical serialization of  a  parsing  expression
              grammar.   It  imports  the  text  using  import  text and then feeds the resulting
              serialization into the object via  deserialize.   This  method  returns  the  empty
              string as it result.

       objectName import object file object path ?format?
              This  method  behaves  like  import  object  text, except that it reads the text to
              convert from the specified file instead of being given it as argument.

       objectName includes
              This method returns a list containing the  currently  specified  paths  to  use  to
              search  for  include  files  when processing input.  The order of paths in the list
              corresponds to the order in which they are used,  from  first  to  last,  and  also
              corresponds to the order in which they were added to the object.

       objectName include add path
              This  methods  adds  the  specified  path to the list of paths to use to search for
              include files when processing input. The path is added to  the  end  of  the  list,
              causing  it  to  be  searched  after  all previously added paths. The result of the
              command is the empty string.

              The method does nothing if the path is already known.

       objectName include remove path
              This methods removes the specified path from the list of paths to use to search for
              include files when processing input. The result of the command is the empty string.

              The method does nothing if the path is not known.

       objectName include clear
              This  method  clears  the  list  of  paths  to use to search for include files when
              processing input. The result of the command is the empty string.

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>