On Thu, Jun 4, 2015 at 11:43 PM, Edward Z. Yang
GHC used to always generalize let-bindings, but our experience with GADTs lead us to decide that let should not be generalized with GADTs. So, it's not like we /wanted/ MonoLocalBinds, but that having them makes the GADT machinery simpler.
This blog post gives more details on the matter: https://ghc.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7
The fact that -XGADTs (in isolation) implies -XMonoLocalBinds isn't the problem. The problem is, the order in which language pragma are offered should not matter. Whether I say {-# LANGUAGE GADTs, NoMonoLocalBinds #-} or {-# LANGUAGE NoMonoLocalBinds, GADTs #-} shouldn't matter. Both should mean the same thing, regardless of how annoying it may be to work in that language. -- Live well, ~wren