[Haskell-cafe] Existential quantification problem

Marco Túlio Gontijo e Silva marcot at riseup.net
Thu Jul 10 13:53:20 EDT 2008


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


Marco Túlio Gontijo e Silva
Página: http://marcotmarcot.googlepages.com/
Blog: http://marcotmarcot.blogspot.com/
Correio: marcot at riseup.net
XMPP: marcot at jabber.org
IRC: marcot at irc.freenode.net
Telefone: 25151920
Celular: 98116720
 Rua Turfa, 639/701
 Prado 30410-370
 Belo Horizonte/MG Brasil

More information about the Haskell-Cafe mailing list