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 _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries