Polymorphic components, so far

[Iavor Diatchki]

Hello,
(Apologies for the two emails, I accidentally hit the send button on
my client before I had finished the first e-mail...)

* Rank-2 vs Rank-n types.  I think that this is the most important
issue that we need to resolve which is why I am placing it first :-)
Our options (please feel free to suggest others)
  - option 1: Hugs style rank-2 types (what I described , very
briefly, on the ticket)
     * Based on "From Hindley Milner Types to First-Class Structures"
     * Predicative
     * Requires function with rank-2 types to be applied to all their
polymorphic arguments.
  - option 2: GHC 6.4 style rank-N types. As far as I understand,
these are the details:
     * Based on "Putting Type Annotations to Work".
     * Predicative
     * We do not compare schemes for equality but, rather, for
generality, using a kind of sub-typing.
     * Function type constructors are special (there are two of them)
because of co/contra variance issues.
  - option 3: GHC 6.6 style rank-N types.  This one I am less familiar
with but here is my understanding:
      * Based on "Boxy types: type inference for higher-rank types and
impredicativity"
      * Impredicative (type variables may be bound to schemes)
      * Sometimes we compare schemes for equality (this is
demonstrated by the example on ticket 57) and we also use the
sub-typing by generality on schemes
      * Again, function types are special

So far, Andres and Stephanie prefer a system based on rank-N types
(which flavor?), and I prefer the rank-2 design.  Atze would like a
more expressive system that accepts the example presented on the
ticket.

I think this is all.
-Iavor