Add `SaturatingEnum`?

[martijnbastiaan]

https://old.reddit.com/r/haskell/comments/jtjgej/where_lions_roam_haskell_hardware_on_the_veldt/gceunep/?context=3

[gergoerdi]

Does this need a typeclass? I have this in RetroClash and have had a ton of uses for them:

nextIdx :: (Eq a, Enum a, Bounded a) => a -> a
nextIdx = fromMaybe minBound . succIdx

prevIdx :: (Eq a, Enum a, Bounded a) => a -> a
prevIdx = fromMaybe maxBound . predIdx

moreIdx :: (Eq a, Enum a, Bounded a) => a -> a
moreIdx = fromMaybe maxBound . succIdx

lessIdx :: (Eq a, Enum a, Bounded a) => a -> a
lessIdx = fromMaybe minBound . predIdx

succIdx :: (Eq a, Enum a, Bounded a) => a -> Maybe a
succIdx x | x == maxBound = Nothing
          | otherwise = Just $ succ x

predIdx :: (Eq a, Enum a, Bounded a) => a -> Maybe a
predIdx x | x == minBound = Nothing
          | otherwise = Just $ pred x

[martijnbastiaan]

I was mostly thinking of satSucc and satPred which are currently defined as members of SaturatingNum: http://hackage.haskell.org/package/clash-prelude-1.2.4/docs/Clash-Class-Num.html#v:satSucc. We could pull those out into a separate type class SaturatingEnum. We'd remain backwards compatible and allow types that can't implement Num to have satSucc still. Thier default implementations could definitely be based on Eq/Enum/Bounded though!

[paddytheplaster]

Does this need a typeclass? I have this in RetroClash and have had a ton of uses for them:

nextIdx :: (Eq a, Enum a, Bounded a) => a -> a
nextIdx = fromMaybe minBound . succIdx

prevIdx :: (Eq a, Enum a, Bounded a) => a -> a
prevIdx = fromMaybe maxBound . predIdx

moreIdx :: (Eq a, Enum a, Bounded a) => a -> a
moreIdx = fromMaybe maxBound . succIdx

lessIdx :: (Eq a, Enum a, Bounded a) => a -> a
lessIdx = fromMaybe minBound . predIdx

succIdx :: (Eq a, Enum a, Bounded a) => a -> Maybe a
succIdx x | x == maxBound = Nothing
          | otherwise = Just $ succ x

predIdx :: (Eq a, Enum a, Bounded a) => a -> Maybe a
predIdx x | x == minBound = Nothing
          | otherwise = Just $ pred x

A class defines an API. You can use functions like yours as a default implementation and programmers may override the functions in the API. If you define functions at the top level, users can no longer override them.

Regards,

Paddy

[gergoerdi]

Yeah I'm well aware what typeclasses are for... The below definitions are also using typeclasses, but instead of new degrees of freedom, they build on `Enum`, `Bounded` and `Eq`. So the question here is whether there are useful `SaturatingEnum` instances that would differ from what you get based on the above three typeclasses.

…

On Thu, Nov 26, 2020, 18:16 paddytheplaster ***@***.***> wrote: Does this need a typeclass? I have this in RetroClash <https://github.com/gergoerdi/retroclash-lib/blob/master/src/RetroClash/Utils.hs> and have had a ton of uses for them: nextIdx :: (Eq a, Enum a, Bounded a) => a -> a nextIdx = fromMaybe minBound . succIdx prevIdx :: (Eq a, Enum a, Bounded a) => a -> a prevIdx = fromMaybe maxBound . predIdx moreIdx :: (Eq a, Enum a, Bounded a) => a -> a moreIdx = fromMaybe maxBound . succIdx lessIdx :: (Eq a, Enum a, Bounded a) => a -> a lessIdx = fromMaybe minBound . predIdx succIdx :: (Eq a, Enum a, Bounded a) => a -> Maybe a succIdx x | x == maxBound = Nothing | otherwise = Just $ succ x predIdx :: (Eq a, Enum a, Bounded a) => a -> Maybe a predIdx x | x == minBound = Nothing | otherwise = Just $ pred x A class defines an API. You can use functions like yours as a default implementation and programmers may override the functions in the API. If you define functions at the top level, users can no longer override them. Regards, Paddy — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#1592 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAA4KKOBI3RWILA7GNHTVPLSRYTIJANCNFSM4TWN2ROQ> .

[gergoerdi]

For example, when is `satSucc SatBound x` equal to `x`? I would argue it should be `maxBound`. If you have a `SatEnum` typeclass, then this can be by convention at most. But if the definition of `satSucc` is based on the `Bound` instance, then it will hold by definition.

…

On Thu, Nov 26, 2020 at 6:21 PM Dr. ÉRDI Gergő ***@***.***> wrote: Yeah I'm well aware what typeclasses are for... The below definitions are also using typeclasses, but instead of new degrees of freedom, they build on `Enum`, `Bounded` and `Eq`. So the question here is whether there are useful `SaturatingEnum` instances that would differ from what you get based on the above three typeclasses. On Thu, Nov 26, 2020, 18:16 paddytheplaster ***@***.***> wrote: > Does this need a typeclass? I have this in RetroClash > <https://github.com/gergoerdi/retroclash-lib/blob/master/src/RetroClash/Utils.hs> > and have had a ton of uses for them: > > nextIdx :: (Eq a, Enum a, Bounded a) => a -> a > nextIdx = fromMaybe minBound . succIdx > > prevIdx :: (Eq a, Enum a, Bounded a) => a -> a > prevIdx = fromMaybe maxBound . predIdx > > moreIdx :: (Eq a, Enum a, Bounded a) => a -> a > moreIdx = fromMaybe maxBound . succIdx > > lessIdx :: (Eq a, Enum a, Bounded a) => a -> a > lessIdx = fromMaybe minBound . predIdx > > succIdx :: (Eq a, Enum a, Bounded a) => a -> Maybe a > succIdx x | x == maxBound = Nothing > | otherwise = Just $ succ x > > predIdx :: (Eq a, Enum a, Bounded a) => a -> Maybe a > predIdx x | x == minBound = Nothing > | otherwise = Just $ pred x > > A class defines an API. You can use functions like yours as a default > implementation and programmers may override the functions in the API. If > you define functions at the top level, users can no longer override them. > > Regards, > > Paddy > > — > You are receiving this because you commented. > Reply to this email directly, view it on GitHub > <#1592 (comment)>, > or unsubscribe > <https://github.com/notifications/unsubscribe-auth/AAA4KKOBI3RWILA7GNHTVPLSRYTIJANCNFSM4TWN2ROQ> > . >