Refactor (~) to reduce the superclass stack
Note [The equality types story] in
TysPrim) we have
-- Hetereogeneous equality class a ~# b => a ~~ b instance a ~# b => a ~~ b -- Homogeneous equality class a ~~ b => (a :: k) ~ (b :: k) instance a ~~ b => a ~ b
(~#) is a superclass of
(~~) is a superclass of
(~). This means that in the common case of using
(~) we need two superclass selections to get to the
(~#) we want.
Nothing really wrong with that, but it bloats programs, and is confusing to read when debugging. I propose to change this to
-- Homogeneous equality class a ~# b => (a :: k) ~ (b :: k) instance a ~# b => a ~ b
That is, implement
(~) in precisely the same way as
(~) a tiny bit more baked-in to the compiler, but in exchange it behaves in the same way as
(~~), instead of behaving in a slightly different way.
There should be no observable effect for users.