On 12/03/2013, at 10:00 AM, MigMit wrote: > > On Mar 12, 2013, at 12:44 AM, "Richard A. O'Keefe" <o...@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. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
