Functor instance for MinLen breaks abstraction

[snoyberg]

Simple demonstration of the problem:

import Data.MinLen

len1 :: MinLen (Succ Zero) [()]
len1 = mlcons () (toMinLenZero [])

len0 :: MinLen (Succ Zero) [()]
len0 = fmap (drop 1) len1

main :: IO ()
main = print $ Data.MinLen.head len0

Results in:

Data.MonoTraversable.headEx: empty

I see no option but to release a new major version with the Functor instance removed. Slightly tweaked: we can make the Functor instance apply only to MinLen Zero.

Pinging @gregwebs, in case you have another idea.

[gregwebs]

Functor instance should act on what is inside the newtype, not the newtype itself

[snoyberg]

If we try to implement that, we'd have to change the kind of MinLen, and most likely lose support for monomorphic containers in the process. What you're describing is handled by the MonoFunctor instance right now.

[gregwebs]

Ok