pt_parser_api(i) Parser Tools pt_parser_api(i) 







pt_parser_api - Parser API








package require Tcl 8.5


className ?objectName?


objectName destroy


objectName parse chan


objectName parset text












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









  

  
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.

  










All parser instances provide at least the methods shown below:



  

  
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.

  

  

  
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.







  

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










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 ...









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










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.



  

  







  

  







  
[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.







  

  







  
[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.









  

  
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).










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}}}}}











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>








  

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    pt