Hi,
thing. However, the warnings (and the names of the flags - "undecidable instances" doesn't sound good) make me nervous, and confused. Why is there a problem with saying "every instance of Foo is also an instance of Bar, and here's how"?
From what I understand, this is basically only to prevent cyclic instances, i.e.,
instance Bar a => Foo a where ... instance Foo a => Bar a where ... "undecidable" certainly does sound scary, but I asked Simon PJ about this a few weeks ago and to the best of his recollection at the time, this was the only case it made a difference and, so long as you program compiles with undecidable instances, there should be no runtime errors. That is, undecidable is simply a compile-time property and as long as you don't have any cyclic declarations like the above, you're fine. - Hal