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







pt::ast - Abstract Syntax Tree Serialization








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...?












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 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









  
::pt::ast verify serial ?canonvar?

  
This command verifies that the content of serial is a valid
      serialization of an abstract syntax 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 calling context. This variable will be
        written to if and only if serial is a valid regular
        serialization. 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 command 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 serialization 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

  







  

  
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 identical,
      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 covering 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.










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.



  

  







  
[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.









  

  
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.










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










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.








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








Parsing and Grammars








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








  

    1.1
    pt