Ralf Laemmel wrote:
You did not say anything that's imprecise about "mentioning each other
in a cycle", just the well-known fact that it's not equivalent to total
termination checking (in fact, it's neither fully an overestimate nor
underestimate of termination -- it's just an estimate that's likely to
be right when used in the context of default method definitions).
It's imprecise also in so far that you would need to define what you mean by it.
Does it mean that we focus on the "pattern"
f ... = g ...
g ... = f ...
... or does it include the case
f ... = h g ...
g ... = f ...
yes it does. It also probably includes
f ... = ...
where _ignored1 = g
g ... = ...
where _ignored2 = f
Predictability is a good thing, I think
although
class where
f ... = g ...
g ... = h ...
h ... = f ...
gets more complicated, but 'h' does have to be defined in the same
module or module-cycle, because it refers to the class and the class
refers to it
... and that's still very imprecise because the dots don't mean anything proper.
Are you willing to look at the pattern *after* overloading resolution.
good point. I think GHC can know when it refers to the same instance.
Let's have a proper termination checker!
I think GHC already does, in its strictness analyzer? An incomplete
checker of course, because termination checking of Haskell, like of most
languages, is undecidable in general.
Btw, obviously a class by itself would not be checked (in terms of the
default methods), but only an instance (with the defaults pulled in).
the class would at most be analyzed to see which were the minimal
methods to implement -- it's warnings about instances that we've been
talking about, anyway
Isaac
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