Polymorphic strict fields

[Iavor Diatchki]

Hello,

On 5/1/07, Duncan Coutts <duncan.coutts at worc.ox.ac.uk> wrote:
> On Mon, 2007-04-30 at 19:47 -0700, Iavor Diatchki wrote:
>
> > All of this leads me to think that perhaps we should not allow
> > strictness annotations on polymorphic fields.  Would people find this
> > too restrictive?
>
> Yes.
>
> Our current implementation of stream fusion relies on this:
>
> data Stream a = forall s. Unlifted s =>
>                           Stream !(s -> Step a s)  -- ^ a stepper function
>                                  !s                -- ^ an initial state
>
> We use strictness on polymorphic (class constrained) fields to simulate
> unlifted types. We pretend that the stream state types are all unlifted
> and have various strict/unlifted type constructors:

This declaration uses existential and not universal quantification.
More concretely, there exists some type that classifies the state of
the stream but the users of the stream do not know what it is (by the
way I saw Don talk about this stream stuff and I think that it is
quite cool!).  A polymorphic field is one where the ``forall`` is
associated with the field (it comes after the constructor), it allows
you to store polymorphic values in a datatype.  Here is an example:

data T = T (forall a. a -> a)
make = T id
use (T f) = (f True, f 'a')

Hope this helps
-Iavor