Proposol: make Data.Bits algebraic

[Carter Schonwald]

Yeah.  Agreed

It sounds like, in principle, Philip should circle back with how to add
some additional type classes and spell those out

BUT: currently we don’t have a good mechanism actually implemented in
Haskell / ghc for upgrading a type class hierarchy seamlessly with new type
classes at the middle or top of a type class hierarchy.  This raises the
bar for experiments and api changes because of the migration / breakage


If only we actually had one of those super class mechanisms implemented and
merged into ghc , a lot of cool stuff could happen 🕵️‍♂️🤔

On Mon, Mar 11, 2019 at 11:58 PM Zemyla <zemyla at gmail.com> wrote:

> Bits can't be algebraic, because the testBit function exists. Anything
> where finiteBitsMaybe returns Just (a number) can be tested for
> equality, and even if it returns Nothing, it can still be tested for
> inequality, or equality up to a certain index.
>
> On Mon, Mar 11, 2019 at 4:49 PM Johnson-Freyd, Philip Alden via
> Libraries <libraries at haskell.org> wrote:
> >
> > Hello,
> >
> > The current version of Data.Bits is non-algebraic, in so far as the Bits
> class has
> > 1. a superclass constraint on `Eq`
> > 2. operations `testBit` and `isSigned` which return `Bool`.
> >
> >
> >
> > This is quite annoying, as myriad potential uses of the Bits class cover
> types for which these concrete observations are not possible. It leads to
> comments like the following in SBV:
> >
> > -- | This instance is only defined so that we can define an instance for
> >
> > -- 'Data.Bits.Bits'. '==' and '/=' simply throw an error. Use
> >
> > -- 'Data.SBV.EqSymbolic' instead.
> >
> > We had this same issue for `Num` for years, until it eventually got
> fixed. And it would be nice to fix the situation for `Bits` as well.
> >
> > There are other aspects of the Bits API which are pretty clunky, but
> this is the one that is often a show stopper.
> >
> >
> > Philip
> >
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