[Haskell-cafe] (no subject)

[Corentin Dupont]

That, right, just to clarify, my instances are like that:

instance Applicative (Todo a) where
   pure                = Done
   Todo as <*> Todo bs = Todo $ as ++ bs
   Todo as <*> _       = Todo as
   Done f  <*> r       = fmap f r

instance Monad (Todo a) where
   return        = Done
   Todo as >>= _ = Todo as
   Done a >>= f  = f a


It's basically accumulating on the Todos. Either [a] b does not fit because
it does not accumulate on the a's.


On Tue, Aug 16, 2016 at 11:12 PM, David Kraeutmann <kane at kane.cx> wrote:

> I went from the assumption that he just wants something like
>
> instance Monad (Todo a)
>
> and that implies that b is the type inside of the monad.
>
>
> On 08/16/2016 08:23 PM, David Kraeutmann wrote:
> > This looks functionally identical to type Todo a b = Either [a] b, and
> Either has a Monad instance.
> > On 8/16/2016 8:21 PM, Corentin Dupont wrote:
> >> Hi guys,
> >> is there some library with a Monad (+Applicative, Functor...) instance
> of
> >> the following type:
> >>
> >> data Todo a b = Todo [a] | Done b
> >>
> >> Thanks!
> >> Corentin
> >>
> >>
> >>
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