[Haskell-cafe] Applicative but not Monad

Conor McBride conor at strictlypositive.org
Sun Nov 1 11:20:18 EST 2009


On 31 Oct 2009, at 10:39, Conor McBride wrote:

> Hi
> On 30 Oct 2009, at 16:14, Yusaku Hashimoto wrote:
>> Hello cafe,
>> Do you know any data-type which is Applicative but not Monad?
> [can resist anything but temptation]
> 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

Put otherwise, unit-delay is applicative but not monadic. I've been
using this to organise exactly what happens when in those wacky
miraculous-looking circular programs. It seems quite promising,
so far...



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