Re: Enum and bounded instances for (,) and Either

This raises a good point: enum as it currently exists is pretty bad and not well behaved ! On Sat, May 15, 2021 at 10:54 AM Zemyla wrote:

We can't write instances of Enum for (,) and Either until we have total versions of pred and succ.

On Wed, May 12, 2021, 20:11 Carter Schonwald wrote:

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*while we could

Perhaps we should provide these flavors via new types and thus folks could still benefit from these instances by way of deriving via ?

On Wed, May 12, 2021 at 6:38 PM Carter Schonwald < carter.schonwald@gmail.com> wrote:

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isnt there also the issue that while we good provide newtypes for various choices of product of enum instance => enum of products, there literally quite easily 3 valid instances we could concoct (and many more besides)

"row by row" "column by column" "diagonalization" etc? that is: each of these is just as valid?

On Wed, May 12, 2021 at 5:46 PM Edward Kmett wrote:

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Nothing indicates that minBound is >= 0.

So if you want to deal with a modulus properly you need to deal with a modulus of something like maxBound - minBound + 1 (after appropriately upcasting intermediate results to a large enough type to contain that range).

You also run into range issues pretty early on, and then scenarios like the instances for Float that are even wonkier.

If toEnum/fromEnum went to a larger type (Integer or Natural) and we didn't have the wonkier Float instances and the like this would be a much easier sell. As it is I find myself rather uncomfortable with the shakiness of the foundations this thing rests upon.

Sent from my iPhone

On May 12, 2021, at 10:53 AM, Sandy Maguire wrote:

 Hi all,

Found myself puzzled the other day when I wanted an (Enum a, Enum b) => Enum (a, b) instance, and was distraught that it didn't exist.

The following is a reasonable implementation:

instance (Bounded b, Enum a, Enum b) => Enum (a, b) where fromEnum (a, b) = (fromEnum (maxBound @b) + 1) * fromEnum a + fromEnum b toEnum n = let bound = fromEnum (maxBound @b) + 1 b = n `rem` bound a = n `div` bound in (toEnum a, toEnum b)

And, while we're at it, might as well add canonical instances for Either:

instance (Bounded a, Bounded b) => Bounded (Either a b) where minBound = Left minBound maxBound = Right maxBound

instance (Bounded a, Enum a, Enum b) => Enum (Either a b) where toEnum i = let bound = fromEnum (maxBound @a) + 1 in case i < bound of True -> Left $ toEnum i False -> Right $ toEnum $ i - bound fromEnum (Left a) = fromEnum a fromEnum (Right b) = fromEnum b + fromEnum (maxBound @a) + 1

Are there any reasons these instances are missing from base?

Cheers, Sandy

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