[PEAK] Rules clarification

[Phillip J. Eby]

Sorry I didn't reply to this sooner; I meant to think about it before 
replying and then it scrolled out of my immediate view and I forgot about it.


At 06:47 PM 2/15/2008 +0200, Sergey Schetinin wrote:
>I've noticed that "when" function accepts anything as it's second
>parameter (not just tuples),

Yes; all that is required is that it be something which can "imply" a 
type tuple (i.e. using the peak.rules.implies() function).


>  what does this declaration would mean
>then (maybe nothing and it should throw an exception?)
>
> >>> from peak.rules import *
> >>> @abstract
>... def test(foo):
>...     pass
>...
> >>> @when(test, int) # <--------- this is what I'm talking about
>... def test_int(foo):
>...     print foo
>...

It indeed means nothing.  Whether it should throw an exception is a 
more complex problem.  Right now, the default for testing implication 
between two different types is to say that neither implies the 
other.  This is a sensible default, except for where it's misleading 
in this case.  So I could add a specific rule to catch it and throw 
an exception, but I'm wondering how many such special cases would need adding.