[Haskell-cafe] Overloading

[Miguel Mitrofanov]

12.03.2013, 02:53, "Richard A. O'Keefe" <ok at cs.otago.ac.nz>:
> On 12/03/2013, at 10:00 AM, MigMit wrote:
>
>>  On Mar 12, 2013, at 12:44 AM, "Richard A. O'Keefe" <ok at cs.otago.ac.nz> wrote:
>>>  Prelude> :type (+)
>>>  (+) :: Num a => a -> a -> a
>>>
>>>  The predefined (+) in Haskell requires its arguments and its result
>>>  to be precisely the same type.
>>>
>>>  I think you had better justify the claim that Date+Period -> Date and
>>>  Date+Period -> Period are possible at the same time by showing us
>>>  actual code.
>>  Ehm...
>>
>>  import Prelude hiding (Num)
>>  class SumDP a where (+) :: Date -> Period -> a
>>  instance SumDP Date where date + period = <your_implementation_here>
>>  instance SumDP Period where date + period = <and_here>
>
> Notice the difference?
> I said that THE PREDEFINED (+) in Haskell requires its arguments
> and its result to be precisely the same type.
>
> This example is not the predefined (+); it's another variable
> entirely that happens to have the same short name and cannot
> also add integers.

So? You've said:

> I think you had better justify the claim that Date+Period -> Date and
> Date+Period -> Period are possible at the same time by showing us
> actual code.

You didn't say THIS (+) should be the "predefined" one. And, since you were replying to what Carlos said, and he didn't say it either, my code is still a valid example.

Of course, you can refine your request so that it would mention the "predefined" (+), but that would be off-topic here.