[Haskell-cafe] Re: Existencial quantification and polymorphic datatypes

[Jon Fairbairn]

Mauricio <briqueabraque at yahoo.com> writes:

> I'm trying, without success, to understand the difference
> between existencial quantification and polymorphic
> datatypes.

Polymorphic types are universally quantified;

so id:: forall t. t -> t

means that id works for every type t.  If haskell had a
symbol for existential quantification, 

  hid:: exists t. t -> t

would mean that hid only works on some type t, but it
doesn't say what it is (so you could only ever apply hid to
undefined.

> Can you give me a hint

Because being on the left of an arrow works something like
negation, a type like (exists t. t -> t) -> bool can be
written as  forall t . (t -> t) -> bool



-- 
Jón Fairbairn                                 Jon.Fairbairn at cl.cam.ac.uk
http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html  (updated 2008-04-26)