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

Note that it is probably just missing some `error` calls in the right place. On Wed, May 12, 2021, 13:27 chessai wrote:

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 wrote:

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On Wed, 12 May 2021, Sandy Maguire wrote:

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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