Thank you very much for the detailed explanation!

I wrote TCO in the title because with "proper tail calls" the title exceeded the allowed limit on hacker news! Moreover most people will know the term TCO and less the more precise term proper tail calls. You are of course correct that tail calls are a required aspect of the Scheme specification even though many implementations that call themselves "Scheme" don't implement this fully.

Let's be extra super pedantic.

If (let x () (x)) is allowed to take more than a constant amount of space, then for any finite memory there is always some finite runtime after which the program will exhaust the memory.

With proper tail calls, there is always some finite memory above which the program will not consume, even given an arbitrarily long runtime.

So, while it is a problem of degrees, one degree is infinite while the other is finite.

Ribbit achieves a 4K footprint, but that just means the size of the Ribbit VM plus the size of the compacted compiled code. The uncompacted code is stored in RAM in the form of linked "ribs" (3 cell objects, or 6/12/24 bytes depending on the word size) and this is not very memory efficient compared to a bytecode representation. So it takes on the order of 64K RAM to run the REPL, which is about 1000 lines of Scheme code. So a rule of thumb is about 64 bytes of RAM per line of Scheme code. You can use that ratio to determine how large of a Scheme program will fit on your specific device.

The Ribbit design is optimized for a setting where the program is communicated say over the Web before it is executed, so it is important to minimize the footprint = transfer time.

The go implementation of the RVM is not yet finished. I would say the reference implementations are the JavaScript, Python and Scheme ones. They were written in that order and build up on various ways of expressing things compactly. Originally there was no minification step so the code was written with short identifiers and few comments (the size of the source code is the metric for the "footprint" of the system and comments count). To understand the code it helps to read the following paper that explains the Ribbit VM: http://www.iro.umontreal.ca/~feeley/papers/YvonFeeleyVMIL21.... .

Very interesting!

An important design goal for the POSIX shell version of the RVM is to depend on the fewest external tools possible. Each dependency brings with it a portability risk because unix tools can be subtly different between environments (for example macOS vs linux, and even between linuxes, and even between versions of the same distribution of linux).

So the core of the RVM has no dependencies with external tools (i.e. it is in "pure" shell script). The only dependencies are for I/O, namely for the "putchar" RVM primitive and the "getchar" RVM primitive (both of which do byte-at-a-time binary I/O on stdin and stdout). For putchar the shell "printf" (which is usually builtin) command is used, specifically

    printf \\$((byte/64))$((byte/8%8))$((byte%8))

For getchar is is not possible to use the shell "read" command because it does not handle null bytes on stdin (and we want Ribbit to support binary I/O). This is admittedly somewhat of an artificial constraint given that Ribbit will probably be mostly used for text processing, like to write compilers that map source code text to target code text. So getchar is implemented with

    set `sed q | od -v -A n -t u1`

and then the next byte on stdin is available using $1 and "shift" is used to move to the next byte. The call to sed is to read each line separately (useful to implement a REPL where you enter a command which is executed before having to enter the next command). Both "sed" and "od" are standard unix tools. Moreover we checked that those set of options have existed for a long time (at least the early 1990's). It would be interesting to test many POSIX shells to verify Ribbit's portability (we used bash, dash, zsh and ksh for our testing).

The low performance of the POSIX shell RVM is due to two things. First, the shell's interpreter is not fast. The simple loop

    n=10000000; while [ $n -gt 0 ]; do n=$((n-1)); done

runs about 5000 times slower with bash than the equivalent JavaScript code run with nodejs, which is implemented with a JIT. Secondly, the POSIX shell does not have arrays so it is necessary to use the shell "eval" command to do indexing. For example a[i]=0 becomes

    eval a$i=0

As you can imagine the RVM implementation uses arrays all over the place for implementing access to the RVM's stack and Scheme objects (both of which are implemented using a garbage collected heap, which is conceptually an array of 3 field records and implemented in the shell as 3 "arrays").

The POSIX shell version of Ribbit was designed to be as portable as possible, only relying on a POSIX shell to run Scheme code. This can be useful in a context where you only have a POSIX shell at hand and you want to bootstrap a more efficient set of development tools (C compiler, editor, etc) and you don't mind waiting a day or two for the build to complete. It is also useful when you want a reproducible build pipeline and the only tool you trust is the shell.

Wow Marc! I see all your fun projects all the time and I feel sad I have so little time to play with them!

I will make sure to dive into the internals of this during my vacation, though.