I should hope not. The identity element (return, coreturn, mempty, pure, Category.id) is almost never needed.
* http://hackage.haskell.org/package/semigroupoids * https://gist.github.com/3871764 On 25/10/12 04:49, Ben Franksen wrote: > First, let me make it clear that nowadays we are of course (I hope!) talking > about making not only Functor, but Applicative a super-class of Monad (so > Functor becomes a super-class by transitivity). > > Petr P wrote: >> The main objections were that it would break existing code and that it >> would lead to code duplication. The former is serious, [...] >> >> To address the first objection: > I don't buy this "it breaks lots of code" argument. Adding the missing > instances is a complete no-brainer; as you wrote: > >> instance Applicative ... where >> pure = return >> (<*>) = ap >> instance Functor ... where >> fmap = liftM > I do not think it is unreasonable to expect people to add such a simple and > practically automatic fix to some old programs in the interest of cleaning > up an old wart (and almost everyone agrees that this would be a good thing, > in principle). > > BTW, I guess most programs already contain the Functor instances (but maybe > not Applicative, as it is newer). > > I agree with Petr Pudlak that code duplication is not an issue, see above. > And yes, these "automatic" instances may have stronger super-class > constraints than strictly necessary. So what? The program didn't need the > Functor (or Applicative) instance anyway (or it already would have defined > it, in which case no change would be needed at all). > > "Default superclass instances" strike me as a complicated proposal for > solving trivial problems. The switch in Control.Exception (from data > Exception to class Exception) was much more disrupting, adapting programs > meant lots of changes everywhere exceptions are handled, not just adding > some trivial instances. Still people managed the transition. > > Cheers -- Tony Morris http://tmorris.net/ _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
