Proposal: Add Compositor class as superclass of Arrow

[Philippa Cowderoy]

On Sun, 21 Oct 2007, apfelmus wrote:

> Twan van Laarhoven wrote:
> > My proposal would be the following. The important things are that:
> >  1. It incorporates Conal's deep arrow,
> >  2. as well as everything that is needed for functional references/lenses
> > and bijective/invertible functions.
> 
> I'd opt for more research for that proposal to answer the following
> essential questions:
> - Do the classes correspond to already-known categories, i.e. are the class
> names optimal?

I suspect the correspondance with the existing Arrow classes is probably 
more important from the average Haskeller's point of view anyway.

> These questions likely have nice answers for many of the classes, but
> CategoryZero, CategoryPlus, CategoryChoice and in particular CategoryFun
> may be hard nuts.
> 

I think CategoryChoice is fairly natural if we've already got 
CategoryPair. Variants on a parsing lib that goes straight to a concrete 
syntax tree would be an obvious instance - such a lib should be trivially 
invertible to give a prettyprinter as well.

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