On 16/10/16 11:21, Andrew Martin wrote:
The advantage this offers is that Free and Cofree would be able to enjoy a greater number of typeclass instances provided libraries across the ecosystem. As it stands, adding the somewhat heavy `free` dependency is not a good choice for libraries like `aeson`, `mustache`, and `hashable`. In the case of Fix, the ecosystem currently lacks a canonical library that provides it (recursion-schemes and data-fix both offer the same definition though, and various tutorials all define it the same way). It could benefit from the new instances as well.
This is a problem I see a lot, having to pull a "heavy" dependency to implement an instance and avoid orphans. I don't favor making a monolith in `base` for this, it goes against modularity and is a maintainer risk for base. Can cabal flags + CPP be abused to implement the instances on demand if the constrains are meet?. That would solve this problem at the cost of having to remember package-flags. -- Ruben Astudillo