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pt::ast(n)                                      Parser Tools                                      pt::ast(n)



____________________________________________________________________________________________________________

NAME
       pt::ast - Abstract Syntax Tree Serialization

SYNOPSIS
       package require Tcl  8.5

       package require pt::ast  ?1.1?

       ::pt::ast verify serial ?canonvar?

       ::pt::ast verify-as-canonical serial

       ::pt::ast canonicalize serial

       ::pt::ast print serial

       ::pt::ast bottomup cmdprefix ast

       cmdprefix ast

       ::pt::ast topdown cmdprefix pe

       ::pt::ast equal seriala serialb

       ::pt::ast terminal loc

       ::pt::ast nonterminal s start end ?child...?

____________________________________________________________________________________________________________

DESCRIPTION
       Are you lost ?  Do you have trouble understanding this document ?  In that case please read the over-view overview
       view provided by the Introduction to Parser Tools. This document is the entrypoint to the whole  sys-tem system
       tem the current package is a part of.

       This package provides commands to work with the serializations of abstract syntax trees as managed by
       the Parser Tools, and specified in section AST serialization format.

       This is a supporting package in the Core Layer of Parser Tools.

       IMAGE: arch_core_support


API
       ::pt::ast verify serial ?canonvar?
              This command verifies that the content of serial is a valid serialization of an abstract  syn-tax syntax
              tax  tree  and  will  throw an error if that is not the case. The result of the command is the
              empty string.

              If the argument canonvar is specified it is interpreted as the name of a variable in the call-ing calling
              ing context. This variable will be written to if and only if serial is a valid regular serial-ization. serialization.
              ization. Its value will be a boolean, with True indicating that the serialization is not  only
              valid, but also canonical. False will be written for a valid, but non-canonical serialization.

              For the specification of serializations see the section AST serialization format.

       ::pt::ast verify-as-canonical serial
              This command verifies that the content of serial is a  valid  canonical  serialization  of  an
              abstract  syntax  tree and will throw an error if that is not the case. The result of the com-mand command
              mand is the empty string.

              For the specification of canonical serializations see the section AST serialization format.

       ::pt::ast canonicalize serial
              This command assumes that the content of  serial  is  a  valid  regular  serialization  of  an
              abstract syntax and will throw an error if that is not the case.

              It  will  then  convert  the  input into the canonical serialization of the contained tree and
              return it as its result. If the input is already canonical it will be returned unchanged.

              For the specification of regular and canonical serializations see the section  AST  serializa-tion serialization
              tion format.

       ::pt::ast print serial
              This  command  assumes  that the argument serial contains a valid serialization of an abstract
              syntax tree and returns a string containing that tree in a human readable form.

              The exact format of this form is not specified and cannot be relied on for  parsing  or  other
              machine-based activities.

              For the specification of serializations see the section AST serialization format.

       ::pt::ast bottomup cmdprefix ast
              This  command  walks the abstract syntax tree ast from the bottom up to the root, invoking the
              command prefix cmdprefix for each node. This implies that the children of a node N are handled
              before N.

              The command prefix has the signature

              cmdprefix ast
                     I.e. it is invoked with the ast node the walk is currently at.

                     The  result  returned by the command prefix replaces ast in the node it was a child of,
                     allowing transformations of the tree.

                     This also means that for all inner node the contents of the children elements  are  the
                     results of the command prefix invoked for the children of this node.

       ::pt::ast topdown cmdprefix pe
              This command walks the abstract syntax tree ast from the root down to the leaves, invoking the
              command prefix cmdprefix for each node. This implies that the children of a node N are handled
              after N.

              The command prefix has the same signature as for bottomup, see above.

              The result returned by the command prefix is ignored.

       ::pt::ast equal seriala serialb
              This  command tests the two sbstract syntax trees seriala and serialb for structural equality.
              The result of the command is a boolean value. It will be set to true if the trees are  identi-cal, identical,
              cal, and false otherwise.

              String equality is usable only if we can assume that the two trees are pure Tcl lists.

       ::pt::ast terminal loc
              This  command  command  constructs the ast for a terminal node refering to the position loc in
              the input, and returns it as the result of the command.

       ::pt::ast nonterminal s start end ?child...?
              This command command constructs the ast for a nonterminal node refering to the symbol s cover-ing covering
              ing  the  range  of positions start to end in the input, and the set of child nodes child ...,
              from left right. The latter may be empty. The constructed node is returned as  the  result  of
              the command.


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 pre-vious previous
              vious 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


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://source -
       forge.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 pars-ing parsing
       ing languages, transducer

CATEGORY
       Parsing and Grammars

COPYRIGHT
       Copyright (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>




pt                                                   1.1                                          pt::ast(n)