Yielding models using the SolveHandle produces different results in synchronous and asynchronous mode.

[rkaminsk]

Would it be possible to make the two programs below produce the same output? I am asking because this is one of those hard to document discrepancies. I believe we had a discussion once that it would be difficult in the synchronous mode to yield models before passing them to the solve event handler. It would fine to first trigger the model callback in the asynchronous mode.

>>> from clingo.control import Control
>>> 
>>> ctl = Control()
>>> ctl.add('base', [], 'a.')
>>> ctl.ground([('base', [])])
>>> 
>>> def cb(prefix):
...     return lambda m: print('{}: {}'.format(prefix, m))
... 
>>> print('synchronous yielding')
synchronous yielding
>>> with ctl.solve(on_model=cb('on_model'), yield_=True, async_=False) as hnd:
...     for m in hnd:
...         cb('on_yield')(m)
...     _ = hnd.get()
... 
on_model: a
on_yield: a
>>> print('asynchronous yielding')
asynchronous yielding
>>> with ctl.solve(on_model=cb('on_model'), yield_=True, async_=True) as hnd:
...     for m in hnd:
...         cb('on_yield')(m)
...     _ = hnd.get()
... 
on_yield: a
on_model: a

[BenKaufmann]

From clasp's perspective there are two event handlers. There is the global event handler stored in the SharedContext (this one is set and used by the clasp app to print all kind of progress and it also seems to be used by clingo) as well as an (optional) model handler that can be passed to ClaspFacade::solve(). This second event handler is always called before a yield. For the synchronous mode, the order is:

• model handler
• global handler
• yield

For the asynchronous mode, the order is:

• model handler
• yield
• global handler

Anyhow, while clingo could switch to using the model handler, I agree that this inconsistency feels wrong and is at least annoying.

The major problem with yielding first before calling event handlers is that the user might break the yield loop early so that event handlers are not called at all. This would break certain statistics and/or output and might also be otherwise unexpected.

So, if you are fine with consistently having 1) model handler, 2) global handler, 3) yield, I'll implement it this way.

[rkaminsk]

So, if you are fine with consistently having 1) model handler, 2) global handler, 3) yield, I'll implement it this way.

We stumbled upon this because we once added an interface to extend a model with additional symbols. Now, we need a well-defined place to do this. The above solution seems good to me because:

• I can tell people to always use the model callback to extend a model, and
• the added symbols will be available when the model is yielded and when the model is printed by clasp.

Let's go for yielding at the end and thanks for looking at this!

[BenKaufmann]

Should be fixed in dev.

[rkaminsk]

Seems to work. Thanks.