I am +1 on these instances existing, but I just discovered via
quickcheck-classes that the given code does not uphold succ-pred or
pred-succ identity:
```
Enum: Succ Pred Identity *** Failed! Falsified (after 2 tests):
Description: succ (pred x) = x
a = (0,1)
succ (pred x) = (-1,1)
Enum: Pred Succ Identity *** Failed! Falsified (after 1 test):
Description: pred (succ x) = x
a = (0,0)
pred (succ x) = (1,0)
*** Failed! Falsified (after 2 tests)..
Enum: Succ Pred Identity *** Failed! Falsified (after 2 tests):
Description: succ (pred x) = x
a = Left 0
succ (pred x) = Right (-9223372036854775808)
Enum: Pred Succ Identity *** Failed! Falsified (after 1 test):
Description: pred (succ x) = x
a = Left 0
pred (succ x) = Right (-9223372036854775808)
```
Perhaps this is indicative of something wrong with the proposed instance?
Thanks
On Wed, May 12, 2021 at 11:01 AM Henning Thielemann
On Wed, 12 May 2021, Sandy Maguire wrote:
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:
This is certainly an instance that fulfills some laws, but do we really want it or is it more likely an accident if someone writes, say, succ (a,b)?
We already allow "max (a,b) (c,d)", which can be pretty counterintuitive. I think the Ord instance for pairs is currently mostly used for Set and Map, but a separate class for this purpose would have been better. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries