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[RankNTypes] Prelude.!!: index too large

The following (simplified) program compiles correctly:

{-# LANGUAGE RankNTypes #-}

newtype ParcurryT m a = ParcurryT {
  runP :: forall r . (() -> r) -> m a
}

as soon as we try to use runP, the frontend fails with Prelude.!! index too large. Just add:

test = runP

Note:

  • If we desugar the RecordConstructor to a "normal" one and add an explicit function for runP, the problem still exists.
  • Using data instead of newtype makes no difference.