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# pt_export_api(i) 1.1 tcllib "Parser Tools"

## Name

pt_export_api - Parser Tools Export API

## Synopsis

- package require Tcl 8.5 9

- CONVERTER reset
- CONVERTER configure
- CONVERTER configure option
- CONVERTER configure option value...
- CONVERTER convert serial
- ::export serial configuration

## 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 two APIs. First the API shared by all packages for the conversion of Parsing Expression Grammars into some other format, and then the API shared by the packages which implement the export plugins sitting on top of the conversion packages.

Its intended audience are people who wish to create their own converter for some type of output, and/or an export plugin for their or some other converter.

It resides in the Export section of the Core Layer of Parser Tools.

## Converter API

Any (grammar) export converter has to follow the rules set out below:

1. A converter is a package. Its name is arbitrary, however it is recommended to put it under the ::pt::peg::to namespace.

2. The package provides either a single Tcl command following the API outlined below, or a class command whose instances follow the same API. The commands which follow the API are called converter commands.

3. A converter command has to provide the following three methods with the given signatures and semantics. Converter commands are allowed to provide more methods of their own, but not less, and they may not provide different semantics for the standardized methods.

   CONVERTER reset

     This method has to reset the configuration of the converter to its default settings. The result of the method has to be the empty string.

   CONVERTER configure

     This method, in this form, has to return a dictionary containing the current configuration of the converter.

   CONVERTER configure option

     This method, in this form, has to return the current value of the specified configuration option of the converter.

     Please read the section Options for the set of standard options any converter has to accept. Any other options accepted by a specific converter will be described in its manpage.

   CONVERTER configure option value...

     This command, in this form, sets the specified options of the converter to the given values.

     Please read the section Options for the set of standard options a converter has to accept. Any other options accepted by a specific converter will be described in its manpage.

   CONVERTER convert serial

     This method has to accept the canonical serialization of a parsing expression grammar, as specified in section PEG serialization format, and contained in serial. The result of the method has to be the result of converting the input grammar into whatever the converter is for, per its configuration.

## Plugin API

Any (grammar) export plugin has to follow the rules set out below:

1. A plugin is a package.

2. The name of a plugin package has the form pt::peg::export::FOO, where FOO is the name of the format the plugin will generate output for.

3. The plugin can expect that the package pt::peg::export::plugin is present, as indicator that it was invoked from a genuine plugin manager.

   It is recommended that a plugin does check for the presence of this package.

4. A plugin has to provide a single command, in the global namespace, with the signature shown below. Plugins are allowed to provide more command of their own, but not less, and they may not provide different semantics for the standardized command.

   ::export serial configuration

     This command has to accept the canonical serialization of a parsing expression grammar and the configuration for the converter invoked by the plugin. The result of the command has to be the result of the converter invoked by the plugin for th input grammar and configuration.

     string serial

       This argument will contain the canonical serialization of the parsing expression grammar for which to generate the output. The specification of what a canonical serialization is can be found in the section PEG serialization format.

     dictionary configuration

       This argument will contain the configuration to configure the converter with before invoking it, as a dictionary mapping from options to values.

       Please read the section Options for the set of standard options any converter has to accept, and thus any plugin as well. Any other options accepted by a specific plugin will be described in its manpage.

5. A single usage cycle of a plugin consists of an invokation of the command export. This call has to leave the plugin in a state where another usage cycle can be run without problems.

## Options

Each export converter and plugin for an export converter has to accept the options below in their configure method. Converters are allowed to ignore the contents of these options when performing a conversion, but they must not reject them. Plugins are expected to pass the options given to them to the converter they are invoking.

-file string

  The value of this option is the name of the file or other entity from which the grammar came, for which the command is run. The default value is unknown.

-name string

  The value of this option is the name of the grammar we are processing. The default value is a_pe_grammar.

-user string

  The value of this option is the name of the user for which the command is run. The default value is unknown.

## Usage

To use a converter do

        # Get the converter (single command here, not class)
        package require the-converter-package
        # Provide a configuration
        theconverter configure ...
        # Perform the conversion
        set result [theconverter convert $thegrammarserial]
        ... process the result ...

To use a plugin FOO do

        # Get an export plugin manager
        package require pt::peg::export
        pt::peg::export E
        # Provide a configuration
        E configuration set ...
        # Run the plugin, and the converter inside.
        set result [E export serial $grammarserial FOO]
        ... process the result ...

## PEG serialization format

Here we specify the format used by the Parser Tools to serialize Parsing Expression Grammars as immutable values for transport, comparison, etc.

We distinguish between regular and canonical serializations. While a PEG may have more than one regular serialization only exactly one of them will be canonical.

regular serialization

  1. The serialization of any PEG is a nested Tcl dictionary.

  2. This dictionary holds a single key, pt::grammar::peg, and its value. This value holds the contents of the grammar.

  3. The contents of the grammar are a Tcl dictionary holding the set of nonterminal symbols and the starting expression. The relevant keys and their values are

     rules

       The value is a Tcl dictionary whose keys are the names of the nonterminal symbols known to the grammar.

       1. Each nonterminal symbol may occur only once.

       2. The empty string is not a legal nonterminal symbol.

       3. The value for each symbol is a Tcl dictionary itself. The relevant keys and their values in this dictionary are

          is

            The value is the serialization of the parsing expression describing the symbols sentennial structure, as specified in the section PE serialization format.

          mode

            The value can be one of three values specifying how a parser should handle the semantic value produced by the symbol.

            value

              The semantic value of the nonterminal symbol is an abstract syntax tree consisting of a single node node for the nonterminal itself, which has the ASTs of the symbol's right hand side as its children.

            leaf

              The semantic value of the nonterminal symbol is an abstract syntax tree consisting of a single node node for the nonterminal, without any children. Any ASTs generated by the symbol's right hand side are discarded.

            void

              The nonterminal has no semantic value. Any ASTs generated by the symbol's right hand side are discarded (as well).

     start

       The value is the serialization of the start parsing expression of the grammar, as specified in the section PE serialization format.

  4. The terminal symbols of the grammar are specified implicitly as the set of all terminal symbols used in the start expression and on the RHS of the grammar rules.

canonical serialization

  The canonical serialization of a grammar 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 grammar.

  1. The keys found in all the nested Tcl dictionaries are sorted in ascending dictionary order, as generated by Tcl's builtin command lsort -increasing -dict.

  2. The string representation of the value is the canonical representation of a Tcl dictionary. I.e. it does not contain superfluous whitespace.

### Example

Assuming the following PEG for simple mathematical expressions

    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;

then its canonical serialization (except for whitespace) is

    pt::grammar::peg {
        rules {
            AddOp      {is {/ {t -} {t +}}                                                                mode value}
            Digit      {is {/ {t 0} {t 1} {t 2} {t 3} {t 4} {t 5} {t 6} {t 7} {t 8} {t 9}}                mode value}
            Expression {is {x {n Term} {* {x {n AddOp} {n Term}}}}                                        mode value}
            Factor     {is {/ {x {t (} {n Expression} {t )}} {n Number}}                                  mode value}
            MulOp      {is {/ {t *} {t /}}                                                                mode value}
            Number     {is {x {? {n Sign}} {+ {n Digit}}}                                                 mode value}
            Sign       {is {/ {t -} {t +}}                                                                mode value}
            Term       {is {x {n Factor} {* {x {n MulOp} {n Factor}}}}                                    mode value}
        }
        start {n Expression}
    }

## 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 Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.

When proposing code changes, please provide unified diffs, i.e the output of diff -u.

Note further that attachments are strongly preferred over inlined patches. Attachments can be made by going to the Edit form of the ticket immediately after its creation, and then using the left-most button in the secondary navigation bar.

## 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 © 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>