From: Clifford Heath <no@...>
Date: 2005-04-29T19:29:30+09:00
Subject: Re: Announcing Reg 0.4.0

Good achievement!

vikkous wrote:
> Lexers, parsers, and pattern matching languages get too short a shrift
> in my opinion.

You're right. As I said in a recent discussion, I think it's because
the people of a sufficient theoretical bent to create the tools don't
seem to be able to make them usable:-).

Talking about substitutions, made me wonder whether you were familiar
with Txl (Tree Transformation Language).

When you come to doing substitutions, read up on BURGs (Bottom Up
Rewriting Grammars) if you aren't already familiar with them. They're
how optimising compilers choose an optimal sequence of code to emit.
They basically match leaf portions of an expression tree, and for each
match, accumulate the cost of the instructions that need to be emitted
to allow that sub-tree to be simplified. Within reason, all possible
paths are explored that allow the tree to be rewritten to the empty
tree, by emitting the optimal instruction sequence. It'd be excellent
if reg could deal with the kind of ambiguity this entails, choosing a
minimum-cost resolution.

Clifford Heath.