On Sun, Nov 17, 2013 at 7:47 PM, Robert Krahn
instance Tester Integer where test 0 = False test _ = True
For the Maybe instance I want to delegate to the value of Just and add a class constraint:
instance (Tester m) => Tester (Maybe m) where test Nothing = False test (Just x) = test x
It compiles nicely and works for Just values test (Just 3) -- True test (Just 0) -- False
But test Nothing
gives me No instance for (Tester a0) arising from a use of `test'
Several things are happening here. First is that numbers undergo defaulting; with nothing else to specify the type of `3` or `0`, they default to Integer and all is well. This cannot be said of `Nothing`; it needs to pick an instance, but `Nothing` doesn't give it anything to work from. "But I only have an instance for Integer!" Typeclasses work under the "open world assumption": a typeclass could be added at any time, so it cannot commit to a given typeclass instance simply because it only happens to know of one valid instance. In particular, it cannot conclude that `Nothing` is of type `Maybe Integer` solely because it happens to only be aware of a `Tester Integer` instance. (Instances are program global and cannot be hidden, so not making the open world assumption could easily cause programs to completely change their meaning with the addition of a single additional instance *anywhere* in that program, even if not imported to that module.) -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net