Hi Mark,

Mark H Weaver <mhw@netris.org> writes:

> Hi Alex,
>
> Alex Vong <alexvong1995@gmail.com> writes:
>
>> Recently, I've read the chapter on Macros in Guile's manual. The manual
>> says that Guile's expander originated from that of Chez Scheme's and
>> that version was portable to other schemes as well. So I search the
>> Internet and find it[0]. The website states that the expander needs an
>> expanded version of itself for bootstrapping, i.e. psyntax.ss needs
>> psyntax.pp for bootstrapping.
>>
>> After playing for awhile, I realize the relationship of Guile's
>> psyntax.scm and psyntax-pp.scm is exactly analogus. One needs an
>> expanded version of psyntax.scm, i.e. psyntax-pp.scm for bootstrapping.
>>
>> Does this mean Guile is not bootstrappable from source only?
>
> That's correct.  psyntax-pp.scm is not source code, and it is needed to
> bootstrap Guile.  However, I made an effort some years ago to make
> psyntax-pp.scm far smaller and more readable than the corresponding file
> from upstream psyntax.  See:
>
>   https://git.savannah.gnu.org/cgit/guile.git/commit/?h=stable-2.0&id=72ee0ef71b9a0514874976cdcf3ea9d5333db4b1
>   https://git.savannah.gnu.org/cgit/guile.git/commit/?h=stable-2.0&id=1af6d2a717f499564fbbc297c79e00ac14b0dcf9
>
I see. So it used to be worse --- several millions LOC. Now it is only
~3000, which is shorter than Emacs's bytecomp.el

> At this point, I believe it would be quite feasible for a single hacker
> to audit our psyntax-pp.scm and compare it to psyntax.scm within a
> reasonable time frame.  Furthermore, when we make local changes to
> psyntax.scm, the corresponding changes to psyntax-pp.scm are localized
> and quite easy to audit as well, so the full audit need not be repeated.
>
It is really good that small change in source --> small change in
binary. A fancy way of saying it: the map between source and binary is
in some sense "continuious".

> Having said this, I agree that it would be better if psyntax.scm were
> written in such a way that it could be bootstrapped without the use of
> itself.  Maybe some day we'll rewrite it to make it so.
>
Agree.

For future interested readers:

I can imagine one way of doing it: First implement a low-level hygienic
macro expander without using macros and then write the syntax-case
expander using low-level macros.

To get started, you can watch the "Let's Build a Hygienic Macro Expander
— Strange Loop 2016, Matthew Flatt" video.

>       Mark

Cheers,
Alex