[Haskell-cafe] [weird stuff] The Dodgy Diagonal
Donald Bruce Stewart
dons at cse.unsw.edu.au
Sat Jul 14 21:11:52 EDT 2007
ctm:
> Hi Stefan
>
> Thanks for a very enlightening reply.
>
> >In GHC 6.7.20070712 and Yhc, this is perfectly safe.
>
>
> >In GRIN based systems like Jhc, this is *not* safe, since after
> >evaluation comparisons are done using the full tag.
>
>
> It's now occurred to me that at a cost of some noise, I could have
> done things
> a little differently
>
> On 14 Jul 2007, at 20:37, Stefan O'Rear wrote:
>
> >On Sat, Jul 14, 2007 at 12:06:30PM +0100, Conor McBride wrote:
> >>A peculiar query for folks who know more about the internals of
> >>Haskell
> >>compilers than I do. I attach the full code with all the bits and
> >>pieces,
> >>but let me pull out the essentials in order to state the problem.
> >>
> >>>newtype Id x = Id x -- element
> >>>newtype K1 a x = K1 a -- constant
>
> > newtype Up1 f p q x = U1 (f (p x) (q x))
> > type Sum1 = Up1 Either
> > type Prod1 = Up1 (,)
>
> >>
> >>>newtype Fst x y = Fst x
> >>>newtype Snd x y = Snd y
> >>>newtype K2 a x y = K2 a
>
> > newtype Up2 f p q x y = U2 (f (p x y) (q x y))
> > type Sum2 = Up2 Either
> > type Prod2 = Up2 (,)
>
>
> >>>class (Bifunctor b, Functor f) => Diag b f | b -> f where
> >>> diag :: b x x -> f x
> >>> gaid :: f x -> b x x
>
> and then all of my coercions would (recursively) have been between
> newtype-isotopes.
>
> Would that have more universal traction? I realise, of course that it's
> all voodoo, and I shouldn't trust a thing.
I do wonder if there's some type families/associated type/witness games
that could be played here. Any thoughts on that, Conor?
http://haskell.org/haskellwiki/GHC/Indexed_types
-- Don
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