At Tue, 31 May 2011 21:30:01 -0500, austin seipp wrote:
The short story is thus: when you turn on GADTs, it also now turns on another extension implicitly (MonoLocalBinds) which restricts let generalization...
You can find a little more info about the change here:
http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7
Thanks for the precise response I needed. It definitely felt like I was running up against something like the monomorphism restriction, but my bindings were function and not pattern bindings, so I couldn't understand what was going on. I had even gone and re-read the GHC documentation (http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/data-type-extens...), which says that -XGADTs enables -XRelaxedPolyRec, but makes no mention of -XMonoLocalBinds. It might save users some frustration if the GHC manual and/or the error message mentioned something about let bindings being monomorphic by default. On a related note, I already started fixing this in my code by enabling ScopedTypeVariables, as it's too much of a pain to do this without that extension. I usually try to use the minimum number of extensions possible to future-proof my code. However, is it reasonable to conclude that if I'm going to use GADTs anyway, then additionally enabling ScopedTypeVariables doesn't really make my code any less future-proof? Thanks, David