PROPOSAL: Make Applicative a superclass of Monad

[Henning Thielemann]

On Tue, 24 Jun 2008, Malcolm Wallace wrote:

> Ashley Yakeley <ashley at semantic.org> wrote:
>
>> Make Applicative (in Control.Applicative) a superclass of Monad (in
>> Control.Monad).
>
> I'm strongly against.
>
> Some datatypes have several different possible implementations of
> Applicative, for instance, they may be either lazy or strict.  The
> PolyParse library relies crucially on the fact that the Monad and
> Applicative instances for the Parser type have different strictness
> behaviours.
>
> I worry that if Applicative were to become a superclass of Monad, this
> way of partitioning my API into lazy and strict portions may no longer
> be possible, or least, a good deal less convenient.

Although I also see no compatible way to make Applicative a superclass of 
Monad, I think it should be so. Intuitively it is a superclass and it 
would surprise me if a library has inconsistent instances of Applicative 
and Monad. So I count your example as argument _for_ making Applicative a 
superclass of Monad in order to reduce the risk of surprises for the 
library user.