pt::peg::interp(n)               Parser Tools               pt::peg::interp(n)



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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?

       package require pt::rde  ?1?

       package require snit

       ::pt::peg::interpreter 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
       configurable 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
       section of the Core Layer of Parser Tools.

       IMAGE: arch_core_transform

       The interpreted grammar is copied from an instance of
       pt::peg::container, 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::interpreter 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
              ownership 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
              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.

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 <- '(' Expression ')' / (Factor (MulOp Factor)*);
                  MulOp      <- '*' / '/';
                  Factor     <- Term (AddOp Term)*;
                  AddOp      <- '+'/'-';
                  Term       <- 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 <- '(' Expression ')'
                              / Factor (MulOp Factor)*


       then its canonical serialization (except for whitespace) is

                  {/ {x {t (} {n Expression} {t )}} {x {n Factor} {* {x {n MulOp} {n Factor}}}}}


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::peg::interp(n)
