[asdf-devel] An interesting ASDF conundrum

[Faré]

Unhappily, ASDF as it stands follows a "plan then perform" model
that does not allow for interleaving dependency detection and computation.
The best it can provide is to have two stages, with
a second-stage system that :defsystem-depends-on the first-stage system
for each "step" that requires such interleaving, with the
loading of the first stage causing all the effects needed for the second stage.
Unwieldy, but that's all you can do with ASDF.

NB: in the new ASDF 2.27, OPERATION-DONE-P will be a boolean that
complements the builtin stamp computations. Make it T normally,
NIL if there's a reason *besides* timestamps to invalidate the
previous computation.

—♯ƒ • François-René ÐVB Rideau •Reflection&Cybernethics• http://fare.tunes.org
It may be bad manners to talk with your mouth full, but it isn't too
good either if you speak when your head is empty.


On Sun, Jan 6, 2013 at 5:24 PM, Robert Goldman <rpgoldman at sift.info> wrote:
>
>>> 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