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guile's reader, in guile

lobste.rs - Mon Apr 12 10:49

Good evening! A brief(ish?) note today about some Guile nargery.

the arc of history

Like many language implementations that started life when you could turn on the radio and expect to hear Def Leppard, Guile has a bottom half and a top half. The bottom half is written in C and exposes a shared library and an executable, and the top half is written in the language itself (Scheme, in the case of Guile) and somehow loaded by the C code when the language implementation starts.

Since 2010 or so we have been working at replacing bits written in C with bits written in Scheme. Last week's missive was about replacing the implementation of dynamic-link from using the libltdl library to using Scheme on top of a low-level dlopen wrapper. I've written about rewriting eval in Scheme, and more recently about how the road to getting the performance of C implementations in Scheme has been sometimes long.

These rewrites have a quixotic aspect to them. I feel something in my gut about rightness and wrongness and I know at a base level that moving from C to Scheme is the right thing. Much of it is completely irrational and can be out of place in a lot of contexts -- like if you have a task to get done for a customer, you need to sit and think about minimal steps from here to the goal and the gut doesn't have much of a role to play in how you get there. But it's nice to have a project where you can do a thing in the way you'd like, and if it takes 10 years, that's fine.

But besides the ineffable motivations, there are concrete advantages to rewriting something in Scheme. I find Scheme code to be more maintainable, yes, and more secure relative to the common pitfalls of C, obviously. It decreases the amount of work I will have when one day I rewrite Guile's garbage collector. But also, Scheme code gets things that C can't have: tail calls, resumable delimited continuations, run-time instrumentation, and so on.

Taking delimited continuations as an example, five years ago or so I wrote a lightweight concurrency facility for Guile, modelled on Parallel Concurrent ML. It lets millions of fibers to exist on a system. When a fiber would need to block on an I/O operation (read or write), instead it suspends its continuation, and arranges to restart it when the operation becomes possible.

A lot had to change in Guile for this to become a reality. Firstly, delimited continuations themselves. Later, a complete rewrite of the top half of the ports facility in Scheme, to allow port operations to suspend and resume. Many of the barriers to resumable fibers were removed, but the Fibers manual still names quite a few.

Scheme read, in Scheme

Which brings us to today's note: I just rewrote Guile's reader in Scheme too! The reader is the bit that takes a stream of characters and parses it into S-expressions. It was in C, and now is in Scheme.

One of the primary motivators for this was to allow read to be suspendable. With this change, read-eval-print loops are now implementable on fibers.

Another motivation was to finally fix a bug in which Guile couldn't record source locations for some kinds of datums. It used to be that Guile would use a weak-key hash table to associate datums returned from read with source locations. But this only works for fresh values, not for immediate values like small integers or characters, nor does it work for globally unique non-immediates like keywords and symbols. So for these, we just wouldn't have any source locations.

A robust solution to that problem is to return annotated objects rather than using a side table. Since Scheme's macro expander is already set to work with annotated objects (syntax objects), a new read-syntax interface would do us a treat.

With read in C, this was hard to do. But with read in Scheme, it was no problem to implement. Adapting the expander to expect source locations inside syntax objects was a bit fiddly, though, and the resulting increase in source location information makes the output files bigger by a few percent -- due somewhat to the increased size of the .debug_lines DWARF data, but also due to serialized source locations for syntax objects in macros.

Speed-wise, switching to read in Scheme is a regression, currently. The old reader could parse around 15 or 16 megabytes per second when recording source locations on this laptop, or around 22 or 23 MB/s with source locations off. The new one parses more like 10.5 MB/s, or 13.5 MB/s with positions off, when in the old mode where it uses a weak-key side table to record source locations. The new read-syntax runs at around 12 MB/s. We'll be noodling at these in the coming months, but unlike when the original reader was written, at least now the reader is mainly used only at compile time. (It still has a role when reading s-expressions as data, so there is still a reason to make it fast.)

As is the case with eval, we still have a C version of the reader available for bootstrapping purposes, before the Scheme version is loaded. Happily, with this rewrite I was able to remove all of the cruft from the C reader related to non-default lexical syntax, which simplifies maintenance going forward.

An interesting aspect of attempting to make a bug-for-bug rewrite is that you find bugs and unexpected behavior. For example, it turns out that since the dawn of time, Guile always read #t and #f without requiring a terminating delimiter, so reading "(#t1)" would result in the list (#t 1). Weird, right? Weirder still, when the #true and #false aliases were added to the language, Guile decided to support them by default, but in an oddly backwards-compatible way... so "(#false1)" reads as (#f 1) but "(#falsa1)" reads as (#f alsa1). Quite a few more things like that.

All in all it would seem to be a successful rewrite, introducing no new behavior, even producing the same errors. However, this is not the case for backtraces, which can expose the guts of read in cases where that previously wouldn't happen because the C stack was opaque to Scheme. Probably we will simply need to add more sensible error handling around callers to read, as a backtrace isn't a good user-facing error anyway.

OK enough rambling for this evening. Happy hacking to all and to all a good night!