Consider using Data.Functor.Identity about containers HOT 4 CLOSED

Containers should work with older GHCs (Data.Functor.Identity will be added only to base-4.8, if I look correctly), and also with other Haskell compilers (it used to be Haskell-98 with CPP and compiler-specific stuff, today probably Haskell-2010 with CPP is the right target). Therefore, the Sequence-specific Id has to stay for older GHC and non-GHC compilers.

I am not sure whether it would be worth it to use type Id = Data.Functor.Identity for GHC base-4.8 -- is there any advantage in using Data.Functor.Identity then the Functor/Monad/Applicative instances?

from containers.

It looks like base-4.8 is actually stealing Data.Functor.Identity away from transformers. If containers is allowed to depend on transformers for older versions, then we can use Data.Functor.Identity regardless, and get all that code out of Data.Sequence. The version in base-4.8 will have some coercion-based optimizations (e.g., fmap = coerce, (<*>) = coerce, foldMap = coerce, foldl = coerce), but I don't know if any of those will help anything in Data.Sequence—they only matter when certain sorts of polymorphism are in play.

from containers.

As containers are a boot package (https://ghc.haskell.org/trac/ghc/wiki/Commentary/Libraries), any dependency is a big burden, so adding transformers for saving 10 lines of code is definitely not going to happen.

Nevertheless, if using Data.Functor.Identity on new GHCs should bring some advantage (some speedup?), we could use it, while providing Data.Sequence.Id for other compilers.

from containers.

Fixed by #76

from containers.