[asdf-devel] An interesting ASDF conundrum

[Robert Goldman]

>> This would be coupled (here's the icky part) with a PERFORM method that
>> does nothing, because by the time it's invoked the make will already
>> have run.
>>
> That's not the icky part. The icky part is OPERATION-DONE-P lying to ASDF.

I have been mulling this over, and here's what is interesting to me
about this case:

The ASDF protocol has OPERATION-DONE-P, which is used as
NOT-OPERATION-NEEDS-DOING-P.  I.e., if it returns a non-NIL value, when
applied to operation O and component C, we may skip doing O on C.

For my MADE-FILE, I cannot[*] compute the answer to this question:  all
I can do is compute a conservative bound on OPERATION-NEEDS-DOING-P.  In
fact, all I can compute is a degenerate bound: I assume that
OPERATION-NEEDS-DOING-P is always true.

Now, let's assume we have another operation, O', and component C', and
we have to do O'(C') if O(C) did something.

The problem here arises because ASDF assumes DID-SOMETHING-P(O,C) ==
OPERATION-NEEDS-DOING-P(O,C).  That is, ASDF assumes that if it BELIEVED
it needed to do O to C, then something will have changed AFTER it does O
to C.

In the case of a MADE-FILE (and some other possible ASDF components?),
this is not the case.  Since we are conservative about whether or not
O(C) needs doing, sometimes we are wrong and after we apply O to C,
nothing has changed.

Furthermore, for MADE-FILE, it is possible to examine the filesystem
*AFTER* O(C) and determine a *better answer* to the DID-SOMETHING-P
query than we can for OPERATION-NEEDS-DOING.

Unfortunately, there's no room in the ASDF protocol to do this.  But it
would not be impossible to modify the protocol  to make it happen, and
this could be done in a backwards-compatible way, by having
OEPRATION-DID-SOMETHING return the value of OPERATION-DONE-P before the
operation by default...

I'm not sure that this is worth doing:  it would depend on whether there
are enough interesting cases where OPERATION-DONE-P (approximate!) and
OPERATION-DID-SOMETHING diverge.

cheers,
r