I've been working on a new macro expander for Racket, and I'm starting
to think that it will work. The new expander is not completely
compatible with the current expander --- and that will be an issue if
we eventually go forward with the change --- but most existing code
still works.

Here's a report on my current experiment:


The goals for a new expander are

 1. to replace a complicated representation of syntax objects with a
    simpler one (and, as a result, avoid some performance and
    submodule-re-expansion problems that have been too difficult to fix
    with the current expander);

 2. to find a simpler model of binding than the current one, so that
    it's easier to explain and reason about scope and macros; and

 3. to implement the new expander in Racket instead of C.

I have possibly succeeded on 1, possibly succeeded to some degree on 2,
and temporarily given up on 3.

(I know it sounds crazy to continue with a C implementation of macro
expansion. Goals 1 and 2 were impossible for me without being able to
try ideas at scale, though, and it was too daunting to tackle a full
reimplementation of the expander and a restructuring of the VM at the
same time. Continuing in the comfortable-for-only-me C environment was
the way to make progress for now, and I look forward to tackling 3 as a
next step.)

I've pushed my current implementation to the "scope" branches of

 https://github.com/mflatt/compiler [for `raco decompile` updates]

I don't recommend bothering with the implementation, for now, unless
you're especially interested in trying some examples. The current
version builds well enough to run DrRacket, but there some build
errors. I have a lot of work to do, still.

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