[Haskell-cafe] Applicative but not Monad

[Ross Paterson]

On Sun, Nov 01, 2009 at 04:20:18PM +0000, Conor McBride wrote:
> On 31 Oct 2009, at 10:39, Conor McBride wrote:
> >I have an example, perhaps not a datatype:
> >tomorrow-you-will-know
> 
> Elaborating, one day later,
> 
>   if you know something today, you can arrange to know it tomorrow
>   if will know a function tomorrow and its argument tomorrow, you
>     can apply them tomorrow
>   but if you will know tomorrow that you will know something the
>     day after, that does not tell you how to know the thing tomorrow

Yes, but if you will know tomorrow that you will know something
tomorrow, then you will know that thing tomorrow.

The applicative does coincide with a monad, just not the one you first
thought of (or/max rather than plus).