Controls LispWorks' response when defpackage is used on an existing package that is different from the definition given.
The standard explicitly declines to define what defpackage does if the named package already exists and is in a different state to that described by the defpackage form. The variable
is an extension to Common Lisp which allows you to select between alternative behaviors that are known to be useful.
The two alternatives are to modify the package to conform exactly to the definition, removing features if necessary, or to merely add features specified in the defpackage but missing from the package. You can also control whether a condition is signalled.
Signal an error.
Signal a warning.
Add the new symbols to the externals, imports, and so on.
Modify the package to have only these externals.
The signalled errors or warnings also contain details of the differences.
cannot be specified at the same time. One of
must be specified. Undistinguished internals (that is, internal symbols that are not imported or shadowed),
options and sizes are ignored when deciding whether to signal.
Note that when you use
some symbols can be uninterned if defpackage imports another symbol with the same name from another package through
. This happens whether the symbol has a definition as a function, a variable, or nay other Lisp construct, so after making such a change in the package, you should re-execute the definitions that were (presumably erroneously) attached to the uninterned symbols.
LispWorks User Guide and Reference Manual - 21 Dec 2011