For people working on libraries intended to be portable but using/offering GHC specials when compiled with GHC, it's nice to be able to verify quickly that nothing relies on a GHC extension that shouldn't. For example, I just submitted a pull request to add an IsString instance to Data.Sequence: instance IsString (Seq Char) where fromString = fromList This instance requires FlexibleInstances to compile. Since that option is global, GHC will no longer complain about non-standard instance heads *anywhere* in the (large) module. The obvious long-term solution, I believe, is to add {-# FlexibleInstance #-} (and {-# FlexibleContext #-}) pragmas. Well, that or get some sort of termination checker into Haskell 2015! David