[Haskell-cafe] Are conditional changes in library interfaces problematic?

[Adam Bergmark]

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
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