"Simon Marlow"
Given the new evidence that it's actually rather hard to demonstrate any performance loss in the absence of the M-R with GHC, I'm attracted to the option of removing it in favour of a warning.
As another data point, today for the first time I received an error (not a warning) from ghc about the M-R: Ambiguous type variable `a' in the constraint: `Ord a' arising from use of `Data.Set.insert' at Pretty.hs:28:11-20 Possible cause: the monomorphism restriction applied to the following: addToSet :: a -> Data.Set.Set a -> Data.Set.Set a (bound at Pretty.hs:28:0) Probable fix: give these definition(s) an explicit type signature or use -fno-monomorphism-restriction So, without the M-R or a type signature, my code is OK. The proposal to accept this code but produce an optional warning is (I think) better than the current error. Regards, Malcolm