[Grammatica-users] Poll: Analyzers rebuilding the parse tree - bug or feature?

[Oliver Gramberg]

Dear Grammatica users,

in the current implementation of Grammatica,
Analyzers that are run on an existing parse tree (using Analyzer's Analyze(Node
node)) completely rebuild the parse tree; meaning: new nodes are created,
the old ones discarded, all values attached to the nodes (using Node's
AddValue(object value)) are removed.

--- Is there anybody out there who relies
on this behaviour?
--- Would it be of help to anybody if
this were different?

I am using Grammatica to build an application
where I need at least two extra passes on the parse tree: one to propagate
information top-down, one to collect information bottom-up again. I have
a patch that makes my Analyzers keep the tree and any values attached to
the nodes.

--- Is this something somebody could
use/have used?

Some theory: I see three different applications
for Analyzers.
- Building an initial tree during parsing.
The current Analyzer supports this by adding to the tree whatever Exit...()
returns.
- Re-building the tree in a later phase
as part of a tree transformation. This could be a complete rebuild as it
currently is; one may want values attached to nodes to be retained, then
the traversal algorithm must be changed to replace each node by what Exit...()
returns.
- Analyzing a tree without modifying
it. Ideally, the Exit...() methods would be defined to return void, resulting
in cleaner code in overridden methods in Analyzer subclasses. Alternatively,
the traversal algorithm must be changed to ignore the value returned from
Exit...().

--- Does anybody see a need for any
of the latter two?
--- What kind of interface would you
like to have for these alternatives?
- Constructor arguments that tell the
Analyzer to rebuild/replace/ignore, retain/remove values attached.
- Public members that let you configure
the Analyzer to rebuild/replace/ignore, retain/remove values attached.
- Three different (sub)classes.

I suggest the following:
- Make clearing the values attached
to nodes configurable.
- Keep the current implementation of
Analyzer (rebuilds the tree from scratch, clears the values by default),
maybe rename it to TreeBuilder (because that's what it actually does).

- Introduce a rebuilding analyzer (replaces
nodes with what is returned from Exit...(), does not clear the values by
default), named TreeTransformer.
- Introduce a non-rebuilding analyzer
(leaves the tree untouched, does not clear the values by default), possibly
named Analyzer (because that's what it is for). Define the grammar-specific
Exit...() callback methods of this Analyzer to return void.
- Introduce a common superclass for
the three styles.

Please let me know your opinion on this!

Regards
Oliver

---

Oliver Gramberg
ABB AG
Forschungszentrum Deutschland
DECRC/I2
Wallstadter Str. 59
D-68526 Ladenburg

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