[Haskell-cafe] TypeFamillies and UndecidableInstances - why?

[Christopher Lane Hinson]

On Wed, 23 Jun 2010, Maciej Piechotka wrote:

> When I tried to do something like:
>
>> {-# LANGUAGE FlexibleContexts #-}
>> {-# LANGUAGE TypeFamilies #-}
>>
>> class Test a where
>> 	type TestMonad a :: * -> *
>> 	from :: a b -> TestMonad a b
>> 	to :: TestMonad a b -> a b
>>
>> data Testable a b = Testable (a b)
>>
>> instance (Test a, Functor (TestMonad a)) => Functor (Testable a) where
>> 	f `fmap` Testable v = Testable $! (to . fmap f . from) v
>>
>
> It asks for adding UndecidableInstances as:
>
> test.hs:11:0:
>    Constraint is no smaller than the instance head
>      in the constraint: Functor (TestMonad a)
>    (Use -XUndecidableInstances to permit this)
>    In the instance declaration for `Functor (Testable a)'
>

Basically, the compiler starts with "Is `Testable a` a Functor?" and
ends with "Is `a` a Test and (figure out what `TestMonad a`) a Functor?"
The second question is more work to do than it started with.

The `Test a` constraint is fine, because you're at least narrowing
down the type in question.  But `TestMonad a` is a type function
that could be literally anything, including `Testable a` itself,
which would leave us at:

instance (Functor (Testable a)) => Functor (Testable a)

Which is obviously problematic.

Friendly,
--Christopher Lane Hinson