pt_peg_interp(3)



pt::peg::interp(3tcl)            Parser Tools            pt::peg::interp(3tcl)

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NAME
       pt::peg::interp - Interpreter for parsing expression grammars

SYNOPSIS
       package require Tcl  8.5

       package require pt::peg::interp  ?1.0.1?

       package require pt::rde  ?1?

       package require snit

       ::pt::peg::interp objectName grammar

       objectName use grammar

       objectName destroy

       objectName parse chan

       objectName parset text

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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 class whose instances are Packrat parsers  con-
       figurable  with  a  parsing expression grammar. The grammar is executed
       directly, i.e. interpreted, with the underlying runtime provided by the
       package pt::rde, basing everything on the PARAM.

       Like  the supporting runtime this package resides in the Execution sec-
       tion of the Core Layer of Parser Tools.

       IMAGE: arch_core_transform

       The interpreted grammar is copied from  an  instance  of  pt::peg::con-
       tainer,  or anything providing the same API, like the container classes
       created by  pt::peg::to::container  or  the  associated  export  plugin
       pt::peg::export::container.

   CLASS API
       The package exports the API described here.

       ::pt::peg::interp objectName grammar
              The  command  creates  a new parser object and returns the fully
              qualified name of the object command as its result. The  API  of
              this  object  command is described in the section Object API. It
              may be used to invoke various operations on the object.

              This new parser is configured for the execution of an empty PEG.
              To  configure the object for any other PEG use the method use of
              the Object API.

   OBJECT API
       All objects created by this package provide the following methods.

       objectName use grammar
              This method configures the grammar interpreter / parser for  the
              execution  of the PEG stored in grammar, an object which is API-
              compatible to instances of pt::peg::container. The parser copies
              the  relevant information of the grammar, and does not take own-
              ership of the object.

              The information of any previously used grammar is overwritten.

              The result of the method the empty string.

       objectName destroy
              This method destroys the parser instance, releasing all  claimed
              memory and other resources, and deleting the instance command.

              The result of the command is the empty string.

       objectName parse chan
              This  method runs the parser using the contents of chan as input
              (starting at the current location in the channel), until parsing
              is  not  possible anymore, either because parsing has completed,
              or run into a syntax error.

              Note here that the Parser Tools are based on Tcl 8.5+. In  other
              words, the channel argument is not restricted to files, sockets,
              etc. We have the full power of reflected channels available.

              It should also be noted that the  parser  pulls  the  characters
              from  the  input stream as it needs them. If a parser created by
              this package has to be operated in a push aka event-driven  man-
              ner  it  will  be necessary to go to Tcl 8.6+ and use the corou-
              tine::auto to wrap it into a coroutine where  read  is  properly
              changed for push-operation.

              Upon  successful completion the command returns an abstract syn-
              tax tree as its result.  This AST is in the  form  specified  in
              section AST serialization format.  As a plain nested Tcl-list it
              can then be processed with any Tcl commands the user likes,  do-
              ing  transformations, semantic checks, etc.  To help in this the
              package pt::ast provides a set of convenience commands for vali-
              dation of the tree's basic structure, printing it for debugging,
              and walking it either from the bottom up, or top down.

              When encountering a syntax error the command will throw an error
              instead.   This  error will be a 4-element Tcl-list, containing,
              in the order listed below:

              [1]    The string pt::rde identifying it as parser  runtime  er-
                     ror.

              [2]    The location of the parse error, as character offset from
                     the beginning of the parsed input.

              [3]    The location of parse error, now as a 2-element list con-
                     taining line-number and column in the line.

              [4]    A  set  of atomic parsing expressions indicating encoding
                     the characters and/or nonterminal symbols the parser  ex-
                     pected to see at the location of the parse error, but did
                     not get.  For the specification of atomic parsing expres-
                     sions please see the section PE serialization format.

       objectName parset text
              This  method  runs the parser using the string in text as input.
              In all other ways it behaves like the method parse, shown above.

AST SERIALIZATION FORMAT
       Here we specify the format used by the Parser Tools  to  serialize  Ab-
       stract  Syntax Trees (ASTs) as immutable values for transport, compari-
       son, etc.

       Each node in an AST represents a nonterminal symbol of a  grammar,  and
       the  range of tokens/characters in the input covered by it. ASTs do not
       contain terminal symbols, i.e. tokens/characters. These can  be  recov-
       ered from the input given a symbol's location.

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

       Regular serialization

              [1]    The  serialization of any AST is the serialization of its
                     root node.

              [2]    The serialization of any node is a Tcl list containing at
                     least three elements.

                     [1]    The  first  element is the name of the nonterminal
                            symbol stored in the node.

                     [2]    The second and third element are the locations  of
                            the  first  and last token in the token stream the
                            node represents (covers).

                            [1]    Locations are provided as non-negative  in-
                                   teger offsets from the beginning of the to-
                                   ken stream, with the first token  found  in
                                   the stream located at offset 0 (zero).

                            [2]    The  end  location  has  to  be equal to or
                                   larger than the start location.

                     [3]    All elements after the first three  represent  the
                            children  of the node, which are themselves nodes.
                            This means that the serializations of nodes  with-
                            out  children, i.e. leaf nodes, have exactly three
                            elements.  The children are  stored  in  the  list
                            with  the  leftmost child first, and the rightmost
                            child last.

       Canonical serialization
              The canonical serialization of an abstract syntax tree  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 tree.

              [1]    The  string  representation of the value is the canonical
                     representation of a pure Tcl list. I.e. it does not  con-
                     tain superfluous whitespace.

   EXAMPLE
       Assuming the parsing expression grammar below

              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;

       and the input string

               120+5
       then a parser should deliver the abstract syntax tree below (except for
       whitespace)

              set ast {Expression 0 4
                  {Factor 0 4
                      {Term 0 2
                          {Number 0 2
                              {Digit 0 0}
                              {Digit 1 1}
                              {Digit 2 2}
                          }
                      }
                      {AddOp 3 3}
                      {Term 4 4
                          {Number 4 4
                              {Digit 4 4}
                          }
                      }
                  }
              }

       Or, more graphical

       .nf +- Digit 0 0 | 1 |            | +- Term 0 2  ---  Number  0  2  -+-
       Digit   1   1   |   2   |                            |             |  |
       +- Digit 2 2 | 0 |                                        |  Expression
       0  4  ---  Factor  0  4 -+----------------------------- AddOp 3 3 | + |
       | +- Term 4 4 --- Number 4 4 --- Digit 4 4 | 5 .fi

PE SERIALIZATION FORMAT
       Here we specify the format used by the Parser Tools to serialize  Pars-
       ing 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 expres-
                            sion. 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  charac-
                            ter.  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  expres-
                            sion.  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 charac-
                            ter. 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, includ-
                            ing 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 charac-
                            ter. This is a custom extension of  PEs  based  on
                            Tcl's builtin command string is.

                     [14]   The  string  wordchar is an atomic parsing expres-
                            sion. It matches any Unicode word character.  This
                            is any alphanumeric character (see alnum), and any
                            connector  punctuation  characters  (e.g.   under-
                            score). 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 ex-
                            pression. It matches the terminal string x.

                     [18]   The expression [list n A] is an atomic parsing ex-
                            pression. 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 repeti-
                            tions.

                     [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 for-
              mat 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 con-
                     tain 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 out-
       put 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>

tcllib                               1.0.1               pt::peg::interp(3tcl)

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