[Haskell-cafe] Sudden monomorphism with -XTypeFamilies

[Antoine Latter]

On Thu, Jan 5, 2012 at 3:32 PM, Rian Hunter <rian at thelig.ht> 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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