David Roundy wrote:
The trouble, of course, is that classes could have rather complicated
"minimum instance" requirements. Still, if someone came up with a
decent syntax such as (but better than)
class Foo f where
foo :: f
foo = bar
bar :: f
bar = foo
requiring ( foo || bar )
it would be very nice. The trouble, of course, is deciding how
expressive to make the requirements.
probably a pragma, since it only affects warnings. And it seems like
each class only has a finite number of functions, so a Boolean
expression should be able to express the full range of possibilities
with ( || , && , not , True , False ); and combinations that are
complicated to express with those, aren't very common, due to the
structure of the problem.
> class Foo f where
> {-# MINIMAL_INSTANCE foo || bar #-}
> foo :: f
> foo = bar
> bar :: f
> bar = foo
I wonder whether it would be safe for the compiler to infer simply by
the default methods mentioning each other in a cycle. It might miss
some cases when (probably involving laziness) the default methods
actually terminate and form an intended set of implemention, and warn
when it shouldn't... which is bad, but does that ever happen?
Isaac
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