[antlr-interest] De-emphasizing tree grammars?

[Jason Osgood]

Hi Gavin.


Read your Hibernate book. Respect.

re: Syntax trees.

What's a "typed syntax tree" is. From looking at your code, it's a
Java class hierarchy representing the parts of your language and you
build an object graph representing a program. Verse a parse tree, AST,
or DOM. Using your objects for "nodes" instead of untyped DOM or parse
nodes.

Right?

Just skimmed your Ceylon.g. I totally forgot about that strategy
(using ANTLR rules with parameters and return values).

I've done that. I didn't like it. Instead of stitching an object graph
together with inlined Java, I opted instead to use a Builder. Method
calls and an internal stack instead of "new" and assignment. Wasn't
much better.

I very much dislike hybrid languages. Stuff like inlining Java in
one's grammar, C#'s LINQ, template languages. I have a hard enough
time understanding one language at a time.

Any way.

Next time, for my ARON project, I'll subclass DebugEventListener and
move all that bookkeeping there. Builders are still complicated, true.
That's just the nature of Builders. But at least the Builder and
grammar are distinct.

One benefit I anticipate is reduced cost of change and testability. I
plan to use gunit (or something). Which (I think) would be more
difficult with inlined Java code gumming up the works.

I DON'T build a "typed syntax tree" in my fado project for a couple
reasons. My pathetic efforts to build a generic SQL object model were
easily defeated. What I really needed to do was find and replace the
interesting bits. By leaving the parse tree in place, I was able to
reemit the input stream with just the interesting bits changes,
preserving formatting, comments, etc.

The Ceylon and fado use cases are very different. But I think moving
inlined Java code out of our grammars is a positive step forward.

Which is why I posted my original question. Debug event listener hooks
are only generated with a command switch. Terence didn't anticipate
they'd be used like this.

Thinking about it, I should probably mention:

ANTLR's DebugEventListener is kinda like a SAX EventListener. But much
better. Partially because all things XML, especially SAX (and
namespaces, and angle brackets, and...), are a terrible bight on the
Earth and should be eradicated with prejudice. But mostly because it's
direct and concise. If some lost soul wants a SAX event source, let
them write an adapter.


Cheers, Jason