Enum and bounded instances for (,) and Either
Sandy Maguire
sandy at sandymaguire.me
Wed May 12 17:52:56 UTC 2021
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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