type class Boolean

[Oliver Charles]

I've wanted it when writing DSLs. For example, when doing SQL query generation, if you have some Expr a type to represent database expressions of type 'a', it'd be nice to be able to have && "just work" on them, much like how we can add a Num (Expr a) instance. If other functions were written in terms of Bool, these are trivially useful on Expr now, but without that they have to be re-implemented.

On Mon, 21 Dec 2020, at 6:54 PM, Tom Ellis wrote:
> On Mon, Dec 21, 2020 at 07:19:49PM +0100, Ben Franksen wrote:
> > So, a simple type class Boolean with instances for Bool and
> > functions returning Booleans should cover the majority of use cases;
> > more instances could be added of course. Something like
> [...]
> > instance Boolean Bool where
> >   (&&) = (Prelude.&&)
> >   (||) = (Prelude.||)
> >   not = Prelude.not
> >   top = True
> >   bottom = False
> > 
> > instance Boolean b => Boolean (a->b) where
> >   (f && g) x = f x && g x
> >   (f || g) x = f x || g x
> >   (not f) x = not (f x)
> >   top = const top
> >   bottom = const bottom
> 
> I think it's worth seeing more instances. As it is I don't understand
> in what situations one would use these polymorphically and therefore
> why `liftA2 (&&)`, `fmap not`, `pure True` and friends wouldn't
> suffice.
> 
> Tom
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