In opposition of Functor as super-class of Monad
ben.franksen at online.de
Wed Oct 24 20:49:24 CEST 2012
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,
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.
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