[Haskell-cafe] Existential quantification problem

[Marco Túlio Gontijo e Silva]

Hello,

how do I unbox a existential quantificated data type?

> {-# LANGUAGE ExistentialQuantification #-}
> data L a = forall l. L (l a)
> unboxL (L l) = l

is giving me, in GHC:

    Inferred type is less polymorphic than expected
      Quantified type variable `l' escapes
    When checking an existential match that binds
        l :: l t
    The pattern(s) have type(s): L t
    The body has type: l t
    In the definition of `unboxL': unboxL (L l) = l

Thanks.

-- 
Marco Túlio Gontijo e Silva
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