Polymorphic extensible records with subtyping are already expressible
in Haskell. There is nothing needs to be added:
http://homepages.cwi.nl/~ralf/HList/
http://homepages.cwi.nl/~ralf/OOHaskell/
The full code is available via darcs
http://darcs.haskell.org/OOHaskell/
As to polymorphic variants, they too are implementable in Haskell
right now. No new extensions are required.
http://www.haskell.org/pipermail/haskell/2006-July/018172.html
http://darcs.haskell.org/HList/src/VariantP.hs
The code shows that the expression problem is solved in Haskell as it
is.
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