Partial application in C

[David Chase]

(Replies, in no particular order)

> With rare exception, I think we can all say that self-modifying
> code is horrid, and anyone who writes it should be left alone.

You should look a little harder about the example I provided.  I was
implementing an interesting (to some people) abstraction; the use of
executed data was just one way to make this happen.  The code, once
written, does not later modify itself.  Partial application is
certainly less horrid than static data.  If nothing else, it is
re-entrant (unlike the Unix library).

> [comments on pipelines]

As a practical matter, since the "make_p_app" subroutine is
machine-dependent, one can always write code that does the
appropriate number of nops before returning the partially applied
function.

> [comments on caches]

The first time around you can count on that code NOT being in the
instruction cache.  One hopes, for the sake of dynamic loading of code
and similar things, that it is possible to get some sort of handle on
the caches.  Assuming that this is in fact true, then the
machine-dependent make_p_app flushes the appropriate caches in the
appropriate ways.

> [comments on split I&D]

I was aware of this, but wanted to know how pervasive this technique
is.  I'm not a real big fan of the technique precisely because it
makes this sort of thing difficult, but then that appears to be a
matter of taste.  So far it appears that the following machines use
separate I and D segments:

some PDP-11s
some 68ks (not Suns)
     HP3000
some 80386s (depending on segment registers?)

> It's nice to have one's code segment protected from out-of-control
> writes.

I agree, and that is not incompatible with my implementation of
partial application (it can't be, since my code segment is protected
from out-of-control writes).  (In fact, I agree so much that I have
become a fan of garbage collection.  As I said before, this is
possible in C, and though it is very easy to write code that breaks
the collector it is also very easy to write useful programs that do
not.).

I hope this covers all the various questions.  I was sort of hoping
that somebody out there would get excited about the idea of partial
application, or maybe send details on how to do it on their machine.

Someone also proposed a special widget for doing the rest of a partial
application--I'm sorry, but that is totally uninteresting.  I want a
result that looks exactly like any other function, so I can pass it
around, assign it to pointer-to-function variables, etc.

David