path: root/README.txt

~~~~~~~~~~~~~~
~~ Welcome! ~~~~~~
~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~
~~~~~~~~~~~~~~~

  The documentation is a work in progress. It doesn't say most of the things
it needs to, yet.

  Evocation is a dialect of Forth, grown to Irenes' tastes. It is meant to
someday be a platform for experimenting with parse theory, type theory,
databases, and other things Forth is not traditionally known for, as well as
with language design, which it is. It is a self-hosting compiler, meaning the
only thing you need to build it is a copy of itself.

  At present, Evocation targets only one architecture, amd64. It is rare among
compiled Forths in that it targets a 64-bit architecture.

  Someday, Evocation will also be self-bootstrapping, meaning that it will be
able to "compile" itself into a commented hex dump of itself for ease of
auditing. This rests on the insight, from the mescc and guix developers, that
the difference between source code and binary is comments. The efforts in this
direction are described below under "Hexing Evocation for Distribution".


 ~~~~~~~~~~~~
~~ Building ~~
 ~~~~~~~~~~~~

  Since we have chosen not to distribute Evocation in binary form, you don't
have a copy of it yet and cannot take advantage of its self-hosting properties
for your first-ever version. Happily, until the self-bootstrapping properties
are ready, we have maintained compatibility with the original version of
Evocation which was written in a program called flatassembler, which you will
have to acquire.

  To get started, first build the flatassembler version:

  $ fasmg quine.asm quine
  $ chmod 755 quine

  It's called "quine" for historical, sentimental reasons having to do with
the original architecture. It is not a Quine, in the sense that it is not a
program that outputs its own source. When the self-bootstrapping is done, it
will be moved to a historical subdirectory and this misnomer will no longer
matter.

  This "quine" binary is a working Evocation interpreter, but it's incomplete
and will become more so with time. So, next, build Evocation-in-Evocation:

  $ (cat labels.e elf.e transform.e execution.e; echo 's" pyrzqxgl" allocate-string dup 262144 read-to-buffer'; cat core.e linux.e output.e amd64.e execution-support.e log-load.e; echo pyrzqxgl swap 262144 read-to-buffer; cat core.e linux.e output.e amd64.e execution-support.e log-load.e dynamic.e input.e interpret.e flow-control.e linux-dynamic.e ; echo pyrzqxgl; cat evoke.e) | ./quine > evoke
  $ chmod 755 evoke

  Now keep your "evoke" binary somewhere safe, and use it to build new
versions as you modify Evocation.


 ~~~~~~~~~~~~~
~~ Exploring ~~
 ~~~~~~~~~~~~~

  You can now try out Evocation. Type to it interactively:

  $ ./evoke
  ." Hi, Irenes!"
  6 7 * . newline
  bye

  See what it prints!

  Some helpful words to try to get started are list-dictionary and describe.


 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~ Reading Evocation's source code ~~
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  Even if you're only interested in using Evocation, not in modifying it, we
encourage you to at least skim through the source. If you've looked at it,
even a little, it won't be so scary next time. It's heavily commented and
meant for anyone with a little programming knowledge to be able to read, even
if you've never done systems programming before.

  If you find something in it confusing, please don't be afraid to ask! It's
likely other people are confused too, and sharing your questions helps improve
the documentation and lets others learn by watching.

  The top-level source file whose job is to compile Evocation itself is
evoke.e. It's really short, and worth a quick glance right now. It lists all
the other source files and the order they get loaded in and how they're
processed.

  The files that do the work to make Evocation run at all are execution.e and
core.e. It's worth reading through both of them slowly. After you've read
core.e, you'll know a lot of basic words that can be used as commands within
Evocation.

  A lot of Evocation is written in Evocation's version of assembly language.
The file amd64.e is the one that implements all the assembly instructions.
Writing a real program in assembly also requires resolving labels, which are
a special syntax that gives names to addresses. The behavior of labels is all
implemented in labels.e.

  On the assembly language front, there's also linux.e which contains assembly
words for doing things specific to the Linux operating system, such as reading
input, and there's elf.e which contains words for outputting the special file
headers that let the operating system understand that a file is an executable
program.

  TODO output.e interpret.e dynamic.e flow-control.e execution-support.e

  TODO talk about transformations

  If you want examples of programs that are smaller than Evocation itself,
quine.e is a tiny program written in proper Evocation that outputs its own
source code; hello.e is a hello-world written in Evocation-assembly, and hex.e
is another small Evocation-assembly program that might make a good example of
how to do slightly more complex things that way. All three of these are
self-contained, consisting of just that one file plus calls to Evocation's
built-in library.


 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~ Modifying Evocation's Internals ~~
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  There may come a point in your explorations when you wish to make changes to
the compiler. When doing so, please always make sure to build
Evocation-in-Evocation both via Evocation-from-flatassembler, as described in
"Building" above, and with itself, like this:

  $ (cat labels.e elf.e transform.e execution.e; echo 's" pyrzqxgl" allocate-string dup 262144 read-to-buffer'; cat core.e linux.e output.e amd64.e execution-support.e log-load.e; echo pyrzqxgl swap 262144 read-to-buffer; cat core.e linux.e output.e amd64.e execution-support.e log-load.e dynamic.e input.e interpret.e flow-control.e linux-dynamic.e ; echo pyrzqxgl; cat evoke.e) | ./evoke > evoke2
  $ chmod 755 evoke2

  The two versions evoke and evoke2 should be bytewise identical; if they are
not, please fix that. This is an important property which would be very
difficult to get back if we ever lose it, it's easier to maintain it
in-the-moment.


 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~ Hexing Evocation for Distribution ~~
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  The long-term strategy for Evocation's binary bootstrapping is not yet
ready, but it's described here anyway because this is where the explanation
should eventually go, and it's easier to write about the pieces as they're
created.

  The binary bootstrapping strategy rests on something called the
"hex transform", the most complex of the transformations provided as part of
Evocation's transformation facility in transform.e. The hex transform has the
task of transforming an entire compilation process, which would otherwise
produce an executable binary, and instead output a commented hex dump of that
binary which describes its internals and their purpose, byte by byte, in
sufficient detail to allow a human reader to audit their correctnes. It will
do this by passing through comments and call-stack information from the
compilation process to the resulting output.

  In order to turn this commented hex dump into a binary, there is a tiny
program called "hex" which handles comments in Evocations ~ syntax, and
converts ASCII hexadecimal to raw binary. This program is in hex.e and is
written in Evocation-assembly. When compiled it is only 480 bytes, which is
small enough to fully audit in its raw, binary form. This is slightly larger
than necessary; many of those bytes are used for error message strings, on the
principle that it's very important that it be easy to distinguish a successful
invocation of "hex" from a failed one.

  When the hex transform is fully working, a copy of the compiled "hex" will
be checked into source control so that it can serve as the root of trust for
all Evocation builds. Meanwhile, you can compile it as follows:

  $ cat labels.e elf.e hex.e | ./evoke > hex
  $ chmod 755 hex

  Although the hex transform doesn't work, if you intend to play around with
this you may wish to know how to attempt to run it on things. You can run it
on hex.e as follows. (We heard you liked metacircularity, so we put some
metacircularity in your metacircularity so you can be metacircular while
you're metacircular.)

  $ (cat labels.e elf.e transform.e; echo 's" xyzzy" allocate-string dup 262144 read-to-buffer'; cat core.e linux.e output.e amd64.e execution-support.e log-load.e dynamic.e input.e interpret.e flow-control.e linux-dynamic.e labels.e elf.e hex.e; echo 'xyzzy s" hex-source" variable 1024 1024 * allocate s" hex-binary" variable 1024 1024 * allocate s" hex-metadata" variable hex-metadata hex-binary dup hex-source 5 roll hex-transform hex-metadata hexdump bye ' ) | ./evoke > hex.hex

  There is not yet a recommended way to run the hex transform on evoke.e, but
when there is it will look like appending most of the Evocation-in-Evocation
build steps to the hex.hex build steps, swapping them in where hex.e is now.