On Thu, Jan 5, 2012 at 3:32 PM, Rian Hunter wrote:

hello

i'm getting inconsistent monomorphism behavior with the same code only depending on whether or not -XTypeFamilies is enabled:

Which version of GHC are you using? Starting with GHC 7.0, the TypeFamilies extension implies the MonoLocalBinds language feature, which looks like what you're running into. Here's a blog post from the GHC folks about it: http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7 Antoine

---- {-# LANGUAGE TypeFamilies #-}

snda :: a -> b -> b snda ba = id

main = do  let ma = (return () :: IO ())      mBox bb = snda ma bb

 mBox $ return (4 :: Int)  mBox $ return "G"

 return () ----

in the preceding example, if -XTypeFamilies is enabled then "mBox" is monomorphic and the program will terminate early, otherwise, it's polymorphic and the program will complete successfully.

i think i understand why this is the case but i couldn't find documentation on this inconsistency anywhere. is this expected behavior or is this a bug in GHC? thanks!

rian

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