Clock exercise shouldn't use Num because that promotes bad behavior

[isovector]

The clock exercise is really bad. The only meaningful semantics of a clock are of an absolute point in time, which aren't closed under addition. Thus Num is an exceedingly bad instance to give for them, and the exercise itself suggests ignoring most of the methods.

A much better solution is to have a addDelta :: Hour -> Minute -> Clock -> Clock

[petertseng]

While whoever wrote this exercise probably thought it was handy that you can use the + and - operators, I believe @isovector is correct here; it doesn't actually make sense to use them. The operation 08:20 + 09:01 (where both operands denote a time) is not sensible (the answer is NOT 17:21) because logically I can't add two times together. I can add nine hours and one minute to eight twenty, but I can't add one past nine to eight twenty.

I can add a duration to a time and get another time.
I can subtract a duration from a time and get another time.
I can subtract two times and get a duration. (but this is not covered in this exercise)
The typeclass Num does not express these desired semantics.
As we can see in http://hackage.haskell.org/package/base-4.11.1.0/docs/Prelude.html#t:Num , Num implies that you can do a + a and get an a, and as discussed above that is not sensible for times.
Therefore, as it currently stands, the clock exercise is not well-typed.

The proposed addDelta :: Hour -> Minute -> Clock -> Clock is a reasonable proposal that deals with the problems.

Assuming I haven't missed any files, these are the files that should be changed to resolve this issue:

• https://github.com/exercism/haskell/blob/master/exercises/clock/.meta/hints.md (remove mention of Num)
• https://github.com/exercism/haskell/blob/master/exercises/clock/README.md (regenerate using https://github.com/exercism/configlet or any compatible tool after editing hints.md, OR just make the exact same change as made to hints.md)
• https://github.com/exercism/haskell/blob/master/exercises/clock/examples/success-standard/src/Clock.hs
• https://github.com/exercism/haskell/blob/master/exercises/clock/src/Clock.hs
• https://github.com/exercism/haskell/blob/master/exercises/clock/test/Tests.hs

[tiniuclx]

I think the proposed changes make sense, I'll see what I can do about this. I'll have a go at solving the exercise as-is, then I'll write up a pull request for this issue.