Status: Proposal DELETE accepted 12/91
References: CLtL2 p.844-846
Related issues: WITH-ADDED-METHODS
Edit history: 22-Aug-91, Version 1 by Barmar
10-Sep-91, Version 2 by Barmar - minor RPG-suggested
10-Jan-92, Version 3 by Barrett - remove GENERIC-FUNCTION,
per X3J13 ammendment
10-Feb-92, Version 4 by Barrett - put back GENERIC-FUNCTION
symbol mistakenly removed; fix Current practice
Very few, if any, CLOS implemementations implement GENERIC-FLET and
Remove the operators GENERIC-FLET, GENERIC-LABELS, and GENERIC-FUNCTION from
the language. Delete the symbols GENERIC-FLET and GENERIC-LABELS from the
The current form of lexical generic functiona is inadequate for
general use, and therefore there is no existing compelling reason to
retain them. Because they are poorly designed, it would be better to
remove them from the language hoping for research and better designs
than to prejudice the case for them by retaining a bad design.
The symbol GENERIC-FUNCTION is retained because it also names a class.
Apple Macintosh Common Lisp 2.0 provides the listed operators.
Lucid CL 4.0 and Symbolics Genera 8.1 both export GENERIC-FLET and
GENERIC-LABELS from their CLOS packages, and Lucid imports it into
the implementation-dependent package that USER inherits from. Neither
of them defines them, though.
Preliminary Symbolics Genera 8.2 and Chesnut Lisp-to-C Translator 2.0
provided the listed operators at the time of the December meeting. Since
then they have deleted from both implementations, in deference to the
Symbolics CLOE does not provide them.
Cost to Implementors:
Very little. For many implementors, no work at all is involved.
Any implementors that export these symbols from COMMON-LISP will have
to change their package specification, but that's it.
Cost to Users:
Since this is a new feature that not everyone implements, programs
that use them are not currently portable, so there should be little
effect on users.
Cost of non-adoption:
It will probably take longer for complete CLOS implementations to
See Esthetics and Cost of non-adoption.
Since no one really understands what these special forms are for, this
clearly simplifies the language.
The reason that lexical generic functions are relatively less useful
than lexical functions is that with a generic function you are
expecting that someone will extend it when they extend the ontology of
the world, and you want to get a hold of those extensions for free.
The mechanism for the for-freeness is the global namespace for generic
function names. When you make those names lexical you lose this
feature. Presumably this is the primary nice feature of OOP.
The use of lexical generic functions is to create private operations
within a scope, which can be larger than the scope of a function. The
purpose of generic-flet/labels is to allow folks to write these
private functions. The point of with-added-methods was to allow these
local functions to be extended within the larger lexical scope.
With-added-methods had the flaw that it did not allow one to extend a
global generic function within its dynamic extent, which I believe was
the intent of its proposer. That is, it couldn't be used to add a
method to a generic version of print such that that extended version
would be used by all free references to it dynamically within the
extent of the with-added-methods.
Still, it had a use within the scenario I outlined above.
The real flaw with the whole design is that one would like to have
lexical class definitions, which would render lexical generic
functions more useful, because then the lexical environments provide a
possible worlds mechanism. So, one would like to create an environment
with a locally extended hierarchy within which the lexical generic
functions are used.
A minor flaw with lexical generic functions are specified is their
stupid syntax. If I were in a catty mood, I would ascribe the bad
I believe there are a number of other operations one would like to
perform with generic functions that are now not possible, such as
explicitly combining separately defined generic functions. This along
with lexical generic functions would allow people to do
encapsulation. But that's another story.
Right now I would advocate removing lexical generic functions so that
someone can do a proper design.
What about GENERIC-FUNCTION? If lexical generic functions are
useless, maybe anonymous ones are as well.