package-renaming

2012-04-07
PACKAGE-RENAMING

This library allows you to locally rename CL packages.


==== Exported Functionality ====

The package-renaming library creates a package PACKAGE-RENAMING,
that exports the following macros and functions:

check-same-package-names
check-package-candidate-names
package-names
find-package-from-names
effect-package-renaming
effect-package-renamings
call-with-effective-package-renamings
with-effective-package-renamings

NB: any kind of package renaming is only safe when package operations
(including any use of the Lisp reader) are single-threaded.


==== Compile-time renaming via ASDF ====

with-effective-package-renamings and call-with-effective-package-renamings
are pretty low-level primitives that let you do what you need,
but require you to do a lot of the groundwork.

First, an important constraint to understand about building CL software.
By intentional design, ASDF only has an :around-compile hook,
but no :around-load hook around load,
or :around-build hook around both compile and load;
indeed, some implementations that rely on linking, such as ECL or GCL,
might not even allow any kind of useful hook around loading,
and some things one might want to do with fasls
(such as linking or concatenating them together into a big fasl)
might preclude any such hook.
Therefore, whatever renamings you do at compile-time
will NOT be done available at load-time.
When the fasl is loaded, only the non-renamed names are available,
and symbols, stored with the principal name of their package at compile-time,
will be resolved against the set of packages at load-time.
This imposes this important constraint on renamings that are admissible
in ASDF's :around-compile hook:
	THE (PRINCIPAL) NAME OF A PACKAGE AS RENAMED AT COMPILE-TIME
	*MUST* BE ONE OF THE NAMES OF THAT PACKAGE AT LOAD-TIME.

For instance, if your target package is COM.FOO.BLAH
and has nicknames CFBLAH and BLAH,
then during your rename, the main name of the package must be one of
COM.FOO.BLAH, CFBLAH or BLAH,
or the symbols will fail to resolve properly at load-time.
You can add as many new local nicknames as you want, and use them locally
in your files, you cannot delete all the original names,
and you must use the original names in any operation that will happen
at runtime or load-time, such as defpackage or find-symbol.

Now, if you want to use package-renaming
to locally override a package A with another B,
it is still possible; but you have to use further tricks.
The trick is to somehow ADD a new unique non-clashing nickname
to each of the packages that are involved in a clash,
at some point after they are defined and before they are used by your code.
For instance, give A the nickname A.at.runtime and B the nickname B.at.runtime.
Then, you can make B.at.runtime the principal name
of whatever renaming you make that targets B,
even with local nickname A,
whereas A is previously renamed away to A.at.runtime.

With this constraint in mind,
a typical way to use package-renaming would be
to use the following in your my-system.asd file:

(defsystem my-system
  :depends-on (:my-system-meta)
  :around-compile "my-system-meta::call-with-renamings"
  ...)

And in B-renaming.asd:
  
(defsystem my-system-meta
  :depends-on (:package-renaming)
  :components ((:file "renamings")))

And in renamings.lisp:

(defpackage :my-system-meta (:use :cl :package-renaming))

(effect-package-renaming
  '((:A (:A :A.regular.nick :A.at.runtime))
    (:B (:B :B.regular.nick :B.at.runtime))))

(defun with-renamings (thunk)
  (call-with-package-renamings
    '((:A :A.at.runtime)
      (:B :B.at.runtime :A :B.other.nick))
    thunk))

That's not all that simple, but at least it's possible.
If you implement an easier all-in-one solution based on this,
I'll happily merge it into this package-renaming.


==== TO DO ====

1- document it

2- write tests

3- document how to use it with ASDF

4- make it work when there are non-trivial overlaps
  between old names and new names

5- write more tests
License
MIT
Categories
library