I should have been clearer sorry. I should hope not that Functor <- Applicative <- Monad. Perhaps I do not understand the purpose of this thread, but "fixing" the hierarchy in this way is a mistake of similar magnitude to the original position -- one that I would cringe at seeing repeated. That is why I thought such a discussion was on-topic. On 25/10/12 10:12, Ben Franksen wrote:
Tony Morris wrote:
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 Off-topic. Feel free to start a new thread named "The bombastic one-and-true class hierarchy I always wanted to have". These proposals have their merits, and I greatly respect the category theoretic knowledge that went into them -- but this is another discussion. This thread refers to a rather modest correction in the standard libraries, not a complete re-design. The idea is to fix something that is widely accepted as an unfortunate ommision (in fact, Oleg's comment is one of the very few that question the idea of adding super class constraints to Monad in principle).
BTW, it is unclear what your "I hope not" refers to, since in both of the hierarchies you linked to Applicative *is* a super class of Monad.
Cheers
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
On 25/10/12 04:49, Ben Franksen wrote: 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/