[Haskell-cafe] [weird stuff] The Dodgy Diagonal

[Donald Bruce Stewart]

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