[Haskell-cafe] [Haskell] Functor => Applicative => Monad

Conor McBride conor at strictlypositive.org
Tue Dec 14 10:39:54 CET 2010

[switching to cafe]

On 14 Dec 2010, at 08:59, Sittampalam, Ganesh wrote:

> John Smith wrote:
>> I would like to formally propose that Monad become a subclass of
>> Applicative, with a call for consensus by 1 February.
> I would prefer that we have some proposal like class aliases  
> implemented
> before we start fundamental restructuring of basic type classes. This
> would help to limit the disruption these changes cause, which will be
> substantial.

I'm inclined to agree. What's even more galling than not having
Functor and Applicative instances for Monads is having to write

At the very least, can we open this particular vessel of vermicular
splendidity and verify that the inmates are still grinning with

If I recall, the class alias proposal was rather more ambitious
than necessary to solve this problem (though that is no crime),
but still a little wrinkled in the corners. Looking at the
relevant webpages, I see the proposal has lots of attractive
motivational examples, but less by way of definition. But perhaps,
somewhere, it has been defined enough for a prototype?

It's surely worth looking at some way to define default instances
for superclasses, provided they can be overridden or switched off.

Where did this train of though leave the rails, again?



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