Hello Guilers,
I thought I’d share a bit about the “fancy” record constructs in
Guix [0], which provide syntactic constructors, default field values,
and “functional” setters.
Guix is about manipulating packages, which are essentially records with
many fields. Thus, I wanted the ability to create new record instances
using field names (à la SRFI-35), and to have field name checks and
name-to-offset mappings happen at compile-time, so one could type things
like:
(package
(name "hello")
(version "2.8")
(source (origin
(method http-fetch)
(uri (string-append "http://ftp.gnu.org/gnu/hello/hello-";
version ; refers to the “2.8” above
".tar.gz"))
(sha256
(base32
"0wqd8sjmxfskrflaxywc7gqw7sfawrfvdxd9skxawzfgyy0pzdz6"))))
(build-system gnu-build-system)
(description "GNU Hello")
(long-description "Yeah...")
(home-page "http://www.gnu.org/software/hello/";)
(license "GPLv3+"))
and have it directly expand to a single ‘make-struct’ call, with missing
or unknown fields reported at compile-time.
In addition, there may be default field values. For instance, the
‘location’ field should default to (current-source-location).
So I ended up with a ‘define-record-type*’ macro, based on SRFI-9, which
is used like this:
(define-record-type* <package>
package ; the “syntactic” constructor used above
make-package ; the procedural constructor
package?
(name package-name)
(version package-version)
(source package-source)
;; ...
(location package-location
(default (and=> (current-source-location)
source-properties->location))))
Finally, these syntactic record constructors support “functional
setters”, where a new record instance is created with some fields copied
from an existing instance, and others set to a new value:
(package (inherit coreutils)
(arguments
`(#:configure-flags
'("--disable-nls"
"--disable-silent-rules"
"--enable-no-install-program=stdbuf,libstdbuf.so"
"LDFLAGS=-static -pthread")
,@(package-arguments coreutils))))
Here field values are those of the ‘coreutils’ variable, except for the
‘arguments’ field.
The code for this macro magic is at
<https://gitorious.org/guix/guix/blobs/master/guix/utils.scm#line484>.
Happy hacking!
Ludo’.
[0] https://gitorious.org/guix
