Sunday, December 2, 2007

Genera and Packages

As I alluded to in an earlier post, Symbolics Genera takes a slightly different approach to packages than most current environments (such as SLIME under Emacs).

Part of that is because, unlike GNU Emacs today, the editor and all its intelligence is running inside the Lisp environment; another part is due to Genera supporting multiple dialects of Lisp, each slightly compatible with the other.

One consequence is that Zmacs pays close attention to the "file attributes line" in your Lisp source files. (That's the line at the beginning of the file which contains the marker "-*-"). Other Lisp environments use a heuristic, like looking for the first IN-PACKAGE form.

A chicken-and-egg problem arises when loading a file that expects to be read in a particular package. What if the package doesn't exist? Well, it isn't possible to use that package to read the file. But the package itself is defined in a file.

For me this caused a bit of confusion, because if you try to load a file with a non-existent package defined in the attributes, and you have not given Genera permission to create the package, it complains that "<package name> is not meaningful as a package name in <Lisp dialect>".

You can press Resume to allow it to be created, and then the problem goes away until you try to bootstrap again without the package existing.

The answer is found in the documentation section "Specifying Packages in Programs." If the package attribute is enclosed in parentheses, it is automatically created if it does not exist. Furthermore, you can include one package name, or a list of package names, as the second element of the list in order to specify packages which this package uses.

The other way to approach this is to create a system definition file which defines the packages up front.

(Another aspect that made me more confused is that the package name was "6502", which if in the package attribute without quotes will read as an integer, which cannot designate a package. I had to muck around a bit trying double-quotes and vertical-bar escapes, in an effort to make the problem go away, before I realized the "not meaningful" was not referring to violating some non-existent rule about package names. From time to time, I would create the package and not realize that made the message go away.)

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