[Haskell-cafe] Applicative of Applicative

[Alexander Solla]

I haven't read all the code, but it seems to me that the most likely
candidate is that your old <*> is breaking the applicative laws.  <*>
"can't" depend on the values it's applied to (but based on what you said,
it looks like your old <*> was making choices)

On Thu, Mar 26, 2015 at 11:24 AM, martin <martin.drautzburg at web.de> wrote:

> Am 03/24/2015 um 04:13 AM schrieb Chris Wong:
> > Hi Martin,
> >
> > On Tue, Mar 24, 2015 at 10:02 AM, martin <martin.drautzburg at web.de>
> wrote:
> >> Hello all,
> >>
> >> I've been playing with temporal values, i.e. values which change over
> time at discrete points in time. I thought it
> >> would be good to make it an instance of Applicative and I was pleased
> with the results. I may be re-inventing some of
> >> frp here, but hey.
>
> >
> > Have you considered making Temporal a Monad? All monads by definition
> > provide a `join :: m (m a) -> m a` which flattens their nested
> > structure.
>
> I just tried that, but I started with join and I wanted to get >>= for
> free.
>
> data Change a = Chg {
>             ct :: Time, -- "change time"
>             cv :: a     -- "change value"
>         }
>
> data Temporal a = Temporal {
>     td :: a,         -- "temporal default"
>     tc :: [Change a] -- "temporal changes"
> } deriving (Show)
>
> -- apply a function to the value of a change
> cvf :: (a->b) -> Change a -> Change b
> cvf f e = Chg (ct e) (f $ cv e)
>
>
> instance Functor Temporal where
>         fmap f (Temporal xd xs) = Temporal (f xd) (map (cvf f) xs)
>
>
> I beleive join is a bit too long to post here. But is seems to work with
> these examples
>
> exNat :: Temporal Int
> exNat  = Temporal 0 (map (\x -> Chg (2*x) (fromIntegral x)) [1..100000])
>
> ext2 :: Temporal Int
> ext2   = Temporal 10 [Chg 5 0]
>
> exNested2 = Temporal exNat [Chg 7 ext2]
>
> *Main> tJoin exNested2
> Temporal {td = 0, tc = [(2,1),(4,2),(6,3),(7,10)]}
>
> It starts with exNat but only up to the Time=10 where ext2 is scheduled
> and adds the default at Time=10. Since ext2 has
> no further changes after Time=10, this is it.
>
>
> Then I defined Modad as
>
> instance Monad Temporal where
>         return x    = Temporal x []
>         ma >>= f    = tJoin $ fmap f ma
>
>
> And Applicative
>
> instance Applicative Temporal where
>         pure x = Temporal x []
>         (<*>)  = ap
>
>
> But here is what I get
>
> *Main> (*) <$> exNat <*> ext2
> Temporal {td = 0, tc = [(2,10),(4,20),(6,30),(8,40),(10,50) ...
>
> This is NOT what I expected. Before I had a hand-crafted <*> function, and
> this gave me
>
> *Main> (*) <$> exNat <*> ext2
> Temporal {td = 0, tc = [(2,10),(4,20),(5,0),(6,0),(8,0) ...
>
> You see the values all drop to zero beyond Time=5, because ext2 drops to
> zero there and I am multiplying.
>
> Where do you think things went awry? Do you think its my tJoin function,
> or is there something wrong in the way I
> defined those typeclasses with respect to each other? Or did I voilate one
> of the laws? How could I find out?
>
>
>
>
>
>
>
>
>
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