I have made some strides with Akku.scm since the introductory blog article and the announcement on the Chez Scheme mailing list. The big feature on the horizon is support for translating R7RS libraries to run on R6RS.
But first of all I need to apologize for building version 0.2.3 with
libncurses6. Chez Scheme uses ncurses for its expression editor and
Debian sid, which I use, has just had a migration from libncurses5 to
libncurses6. Most Linux systems do not have that version of ncurses
yet, so many of you could not get the pre-compiled version of Akku to
run (the +src version works). I will have this fixed for the next release.
So what is this about translating R7RS to R6RS? The subset of R7RS
that already exists in R6RS is quite large and the incompatible bits
are manageable. Anyone interested in the details can
read Implementing R7RS on an R6RS Scheme system (Kato,
2014). There are some syntactical differences, so a new reader is
needed and the define-library forms need to be translated into
library forms. Lastly the R7RS standard library needs to be implemented.
For this purpose I’ve written a reader called laesare
that can handle both R6RS and R7RS. The bulk of the reader already
existed and I added support for the R7RS lexical syntax. Next I added
code to Akku to have it understand R7RS libraries and translate them
to R6RS libraries. The final piece of the puzzle is
the akku-r7rs package that provides the standard
library. The latter is based on yuni by okuoku, with my own additions.
Some trickery was needed to support include and cond-expand. The
include form in R7RS is somewhat loosely specified and leaves it up
to the implementation to decide how to search for the files, but in
practice the file paths are relative to the file where the include
form appeared. This is not trivial to get working in straight up R6RS,
but is easy with some help from Akku.
The next release of Akku will install the (akku metadata) library
that describes all the libraries and assets (i.e. included files) that
exist in the project. The include form uses this library to look up
the location of the referenced files:
(define installed-assets
'(((include "match/match.scm")
("chibi/match/match.scm")
(chibi match))
...))
The location is relative to the library search path, which is how all
R6RS include forms already do the job today. The metadata library
also contains a list of installed libraries so that cond-expand
library clauses work. These tricks work because Akku is
project-oriented: it has full knowledge of all files it has installed.
Currently the R7RS support works with Chez Scheme. There is a slight
problem with Racket: it has its own (scheme *) modules. Other
implementations either already have R7RS support (Sagittarius and
Larceny) or they don’t have a compatibility library in akku-r7rs
and/or chez-srfi.
Another fly in the ointment is that many of the packages in Snow require implementation-specific code, which doesn’t exist yet for R6RS implementations. Hopefully that will change if Akku gains some popularity.
And finally, a bit of news for everyone who wants to use the git master version of Akku: you no longer need to manually bootstrap the dependencies. See CONTRIBUTING.md for instructions. Happy hacking!