Hi everybody, I try to understand existential quantification. I have two questions. 1/ I have just read the answer of yairchu at http://stackoverflow.com/questions/3071136/what-does-the-forall-keyword-in-h... He writes: """ So with Existential-Quantification, foralls in data definitions mean that, the value contained *can* be of *any* suitable type, not that it *must* be of *all* suitable types. """ This made me think to an error I obtained with the code: --------------- test :: Show s => s test = "foobar" --------------- The error is: Could not deduce (s ~ [Char]) from the context (Show s) bound by the type signature for test :: Show s => s [...] `s' is a rigid type variable bound by the type signature for test :: Show s => s Indeed, `test :: Show s => s` means "for any type s which is an instance of Show, test is a value of that type s". But for example "foobar" can't be an Int that is an instance of Show, so it yields an error. By comparison, --------------- test :: Num a => a test = 42 --------------- works because 42 can be a value of type Int or Integer or Float or anything else that is an instance of Num. So I thought that by using existential quantification, the first example could work: --------------- {-# LANGUAGE ExistentialQuantification #-} test :: forall s . Show s => s test = "asd" --------------- But I obtain the same error, why? 2/ Is the notion of existential type related in some way to the classical mathematical quantifier "∃" (Unicode symbol U+2203: "There exists")? If yes, then why using "forall" for an "existential type"? At the following address http://www.haskell.org/ghc/docs/6.12.1/html/users_guide/data-type-extensions... we read """ 7.4.4.1. Why existential? What has this to do with existential quantification? Simply that MkFoo has the (nearly) isomorphic type MkFoo :: (exists a . (a, a -> Bool)) -> Foo But Haskell programmers can safely think of the ordinary universally quantified type given above, thereby avoiding adding a new existential quantification construct. """ But I don't understand the explanation. Thanks in advance, TP