pt_parser_api(i)                 Parser Tools                 pt_parser_api(i)



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NAME
       pt_parser_api - Parser API

SYNOPSIS
       package require Tcl  8.5

       className ?objectName?

       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 document describes the API shared by the grammar interpreter
       provided by the package pt::peg::interp and the parsers generated by
       the pt application for the result formats critcl, snit, and oo
       regarding access to the actual parsing functionality.

       Its intended audience are people who wish to create a parser for some
       language of theirs and then use that parser within a Tcl-based package
       or application.

       It resides in the User Layer of Parser Tools.

       IMAGE: arch_user_pkg

CLASS API
       className ?objectName?
              The class command constructs parser instances, i.e. objects. The
              result of the command is the fully-qualified name of the
              instance command.

              If no objectName is specified the class will generate and use an
              automatic name. If the objectName was specified, but is not
              fully qualified the command will be created in the curent
              namespace.

INSTANCE API
       All parser instances provide at least the methods shown below:

       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
              manner it will be necessary to go to Tcl 8.6+ and use the
              coroutine::auto to wrap it into a coroutine where read is
              properly changed for push-operation.

              Upon successful completion the command returns an abstract
              syntax 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,
              doing transformations, semantic checks, etc.  To help in this
              the package pt::ast provides a set of convenience commands for
              validation 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
                     error.

              [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
                     containing line-number and column in the line.

              [4]    A set of atomic parsing expressions indicating encoding
                     the characters and/or nonterminal symbols the parser
                     expected to see at the location of the parse error, but
                     did not get.  For the specification of atomic parsing
                     expressions 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.

USAGE
       A generated parser is used like this


                  package require the-parser-package ;# Generated by result-formats 'critcl', 'snit' or 'oo' of 'pt'.
                  set parser [the-parser-class]

                  set ast [$parser parse $channel]
                  ... process the abstract syntax tree ...

       When using a grammar interpreter for parsing some differences creep in


                  package require the-grammar-package ;# Generated by result-format 'container' of 'pt'.
                  set grammar [the-grammar-class]

                  package require pt::peg::interp
                  set parser [pt::peg::interp]

                  $parser use $grammar

                  set ast [$parser parse $channel]
                  $parser destroy

                  ... process the abstract syntax tree ...


AST SERIALIZATION FORMAT
       Here we specify the format used by the Parser Tools to serialize
       Abstract Syntax Trees (ASTs) as immutable values for transport,
       comparison, 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
       recovered 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
                                   integer offsets from the beginning of the
                                   token 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
                            without 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
                     contain 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

       IMAGE: expr_ast

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 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.

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>




pt                                     1                      pt_parser_api(i)
