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page_util_norm_peg(n)                      Parser generator tools                      page_util_norm_peg(n)



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NAME
       page_util_norm_peg - page AST normalization, PEG

SYNOPSIS
       package require page::util::norm_peg  ?0.1?

       package require snit

       ::page::util::norm::peg tree

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DESCRIPTION
       This  package  provides  a single utility command which takes an AST for a parsing expression grammar
       and normalizes it in various ways. The result is called a Normalized PE Grammar Tree.

       Note that this package can only be used from within a plugin managed by the package  page::pluginmgr.

API
       ::page::util::norm::peg tree
              This  command assumes the tree object contains for a parsing expression grammar. It normalizes
              this tree in place.  The result is called a  Normalized PE Grammar Tree.

              The following operations are performd

              [1]    The data for all terminals is stored in their grandparental nodes. The  terminal  nodes
                     and their parents are removed. Type information is dropped.

              [2]    All  nodes which have exactly one child are irrelevant and are removed, with the excep-tion exception
                     tion of the root node. The immediate child of the  root  is  irrelevant  as  well,  and
                     removed as well.

              [3]    The  name of the grammar is moved from the tree node it is stored in to an attribute of
                     the root node, and the tree node removed.

                     The node keeping the start expression separate is removed as irrelevant  and  the  root
                     node of the start expression tagged with a marker attribute, and its handle saved in an
                     attribute of the root node for quick access.

              [4]    Nonterminal hint information is moved from nodes into attributes, and the  now  irrele-vant irrelevant
                     vant nodes are deleted.

                     Note:  This  transformation  is  dependent on the removal of all nodes with exactly one
                     child, as it removes the all 'Attribute' nodes already. Otherwise  this  transformation
                     would have to put the information into the grandparental node.

                     The default mode given to the nonterminals is value.

                     Like with the global metadata definition specific information is moved out out of nodes
                     into attributes, the now irrelevant nodes are deleted, and the root nodes of all  defi-nitions definitions
                     nitions are tagged with marker attributes. This provides us with a mapping from nonter-minal nonterminal
                     minal names to their defining nodes as well, which is saved in an attribute of the root
                     node for quick reference.

                     At  last  the  range  in the input covered by a definition is computed. The left extent
                     comes from the terminal for the nonterminal symbol it defines. The right  extent  comes
                     from  the  rightmost  child under the definition. While this not an expression tree yet
                     the location data is sound already.

              [5]    The remaining nodes under all definitions are transformed into proper expression trees.
                     First  character ranges, followed by unary operations, characters, and nonterminals. At
                     last the tree is flattened by the removal of superfluous inner nodes.

                     The order matters, to shed as much nodes as possible early, and  to  avoid  unnecessary
                     work later.


BUGS, IDEAS, FEEDBACK
       This  document, will undoubtedly contain bugs and other problems.  Please report such in the category
       page of the Tcllib SF Trackers [http://sourceforge.net/tracker/?group_id=12883].  Please also  report
       any ideas for enhancements you may have.

KEYWORDS
       PEG, graph walking, normalization, page, parser generator, text processing, tree walking

CATEGORY
       Page Parser Generator

COPYRIGHT
       Copyright (c) 2007 Andreas Kupries <andreas_kupries@users.sourceforge.net>




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