Space Station Lambda

The CLHS says the following about DEFPACKAGE:

If the new definition is at variance with the current state of that package, the consequences are undefined; an implementation might choose to modify the existing package to reflect the new definition.

SBCL opts to modify the package but throws a warning, most often in the form of “PACKAGE also exports the following symbols: …”.

Luckily the warning is a specific one, i.e. SB-INT:PACKAGE-AT-VARIANCE.

So to get rid of the warning the following macro helps:

(defmacro without-package-variance-warnings (&body body)
  `(eval-when (:compile-toplevel :load-toplevel :execute)
     (handler-bind (#+sbcl(sb-int:package-at-variance #'muffle-warning))
       ,@body)))

Apply sparsely.

We need the EVAL-WHEN to ensure that the non-toplevel definition of the package is available at compile time.

There’s a minor catch here. One might be tempted to put the EVAL-WHEN clause inside the HANDLER-BIND. But this leads to the package not being available at compile time (therefore rendering the EVAL-WHEN ineffective) because “the compile-time side effects described in Section 3.2 (Compilation) only take place when eval-when appears as a top level form.” (CLHS on EVAL-WHEN).

Does anyone know how to muffle these redefinition warnings in other implementations?

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