[Haskell-cafe] arr considered harmful

[Conal Elliott]

>
> If you require the circuit to be parametric in the value types, you can
> limit the types of function you can pass to arr to simple plumbing.
> See the netlist example at the end of my "Fun of Programming" slides (
> http://www.soi.city.ac.uk/~ross/papers/fop.html).
>

I'm running into this same issue: I have something (another circuits
formulation) that's almost an arrow but doesn't support arr. I'd like to
use arrow notation, but then I run afoul of my missing arr. I'd like to
understand Ross's suggestion and how to apply it. (I've read the "FoP"
slides.)

Ross: do you mean to say that you were able to implement arr and thus run
your circuit examples via the standard arrow desugarer?

Ryan: did you get a working solution to the problem you described for your
Circuit arrow?

Thanks.  -- Conal



On Mon, Oct 31, 2011 at 6:52 PM, Paterson, Ross <R.Paterson at city.ac.uk>wrote:

> Ryan Ingram writes:
> > Most of the conversion from arrow syntax into arrows uses 'arr' to move
> components around. However, arr is totally opaque to the arrow itself, and
> prevents describing some very useful objects as arrows.
>
> > For example, I would love to be able to use the arrow syntax to define
> objects of this type:
>
> > data Circuit a b where
> >     Const :: Bool -> Circuit () Bool
> >     Wire :: Circuit a a
> >     Delay :: Circuit a a
> >     And :: Circuit (Bool,Bool) Bool
> >     Or :: Circuit (Bool,Bool) Bool
> >     Not :: Circuit Bool Bool
> >     Then :: Circuit a b -> Circuit b c -> Circuit a c
> >     Pair :: Circuit a c -> Circuit b d -> Circuit (a,b) (c,d)
> >     First :: Circuit a b -> Circuit (a,c) (b,c)
> >     Swap :: Circuit (a,b) (b,a)
> >     AssocL :: Circuit ((a,b),c) (a,(b,c))
> >     AssocR :: Circuit (a,(b,c)) ((a,b),c)
> >     Loop :: Circuit (a,b) (a,c) -> Circuit b c
> > etc.
>
> > Then we can have code that examines this concrete data representation,
> converts it to VHDL, optimizes it, etc.
>
> > However, due to the presence of the opaque 'arr', there's no way to make
> this type an arrow without adding an 'escape hatch'
> >     Arr :: (a -> b) -> Circuit a b
> > which breaks the abstraction: circuit is supposed to represent an actual
> boolean circuit; (Arr not) is not a valid circuit because we've lost the
> information about the existence of a 'Not' gate.
>
> If you require the circuit to be parametric in the value types, you can
> limit the types of function you can pass to arr to simple plumbing.
> See the netlist example at the end of my "Fun of Programming" slides (
> http://www.soi.city.ac.uk/~ross/papers/fop.html).
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