Relaxed version dependency for mtl

[konn]

Git HEAD version dependency for mtl requires >= 2.2.1. Haskell Platform 2014.2.0.0 is shipped with mtl-2.1.1 so we can't use with it.

[goldfirere]

This dependency is because a test case imports Control.Monad.Except, which was introduced in mtl-2.2.1. It seems wrong to have different dependencies for test cases than for the main build, and the way to get the exception monad in mtl-2.1.x is deprecated in 2.2.1.

Is there a reason you (or your users) can't upgrade mtl? Is the issue cabal hell?

[konn]

I got it. There are no deps hell so far, but it's likely to happen as users and library writers switch to new version of Haskell Platform. We can probably avoid cabal hell by sandboxing, but I think it's not quite convenient.

[goldfirere]

I'm somewhat inexperienced when it comes to managing versions, etc., to make things easier downstream. If you have suggestions for a better way to do this, please speak up!