transformers 0.4 deprecated ErrorT in favor of ExceptT. I've noticed that several packages use CPP to conditionally add ExceptT instances, like this: ``` #if MIN_VERSION_transformers(0,4,0) instance C m => C (ExceptT e m) where #endif ``` The problem I've found with this is that I will get a different interface depending on which transformers version is picked. There is a transformers-compat[1] library that provides ExceptT when using older versions of transformers, when combining these I end up with a compilation failure since the ExceptT instances are no longer included due to CPP. The alternatives I see are: 1. Use transformers-compat (and as of today mtl-compat[2]) 2. Drop support of older versions of dependencies 3. Advise against compat packages The first alternative seems preferable from a user's standpoint. For transformers and mtl I argue it's the most pragmatic option. I think the compat packages are a good thing, especially for core packages, since they let us maintain backwards compatibility. Do you agree with this line of thought? I started working on pull requests to "fix" this but I'd appreciate feedback on the issue first. Along these lines, does conditionally altering the interface of a package like this violate the PVP? Cheers, Adam [1] http://hackage.haskell.org/package/transformers-compat [2] http://hackage.haskell.org/package/mtl-compat