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# pt::ast(n) 1.2 tcllib "Parser Tools"

## Name

pt::ast - Abstract Syntax Tree Serialization

## Synopsis

- package require Tcl 8.5 9
- package require pt::ast ?1.2?

- ::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 new0 s loc ?child...?
- ::pt::ast new s start end ?child...?

## 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 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.

## API

::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

  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 identical, and false otherwise.

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

::pt::ast new0 s loc ?child...?

  This command command constructs the ast for a nonterminal node refering refering to the symbol s at position loc 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. The end position is loc-1, i.e. one character before the start. This type of node is possible for rules containing optional parts.

::pt::ast new 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.

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

## 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>