Re: [Haskell-beginners] understanding type classes and class constraints

Thanks a lot for the thorough and quick explanations! On Sun, Nov 17, 2013 at 6:57 PM, Brandon Allbery wrote:

On Sun, Nov 17, 2013 at 7:47 PM, Robert Krahn wrote:

...

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

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