The help of all was very useful. But since Jake gave me an example I prefer to follow this up. Although I 'm not so familiar generally with datatypes I have understood you. It seems in this example that with existential types we can put in the same list different types although generally this is not allowed. I have tried to write this example with the ghc compiler 6.8.2 but there was an error pare error in data/newtype declaration. I have also tried to find the etymology of the word existential, since some times somebody can easily find a better answer, but I can't figure out why this is so. (really why "existential" types?). Maybe a completed example would be more helpful. existence Look up existence at Dictionary.com c.1384, from O.Fr. existence, from L.L. existentem "existent," prp. of L. existere "stand forth, appear," and, as a secondary meaning, "exist;" from ex- "forth" + sistere "cause to stand" (see assist). Existential as a term in logic is from 1819. Existentialism is 1941 from Ger. Existentialismus (1919), ult. from Dan. writer Søren Kierkegaard (1813-55), who wrote (1846) of Existents-Forhold "condition of existence," existentielle Pathos, etc. (I hope this way of questioning is not so strange)