Polymorphic strict fields

[Duncan Coutts]

On Tue, 2007-05-01 at 09:50 -0700, Iavor Diatchki wrote:
> 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.

Ah ok. When you said "strictness annotations on polymorphic fields" I
assumed you meant just ordinary things like:

data A a = A !a

rather than local universal quantification.

Duncan