[Haskell-cafe] Unable to make a HList-like to deal with Kleisli Arrows and State

[Gautier DI FOLCO]

2014-06-12 3:42 GMT+02:00 adam vogt <vogt.adam at gmail.com>:

> Hi Gautier,
>
> It seems like `type State s = StateT s Identity` causes the problem.
> You could supply Identity as an argument:
>
> data REnv (ts :: [*])
>           (i :: *)
>           (o :: *)
>           (s :: * -> (* -> *) -> * -> *)
>           (k :: (* -> *) -> * -> * -> *) where
>   Last  :: k (s t Identity) i o -> REnv (t ': '[]) i o s k
>   RCons :: k (s t Identity) i o' -> REnv ts o' o s k -> REnv (t ': ts) i o
> s k
>
>
> Or I guess you could keep the original REnv, and require a newtype
>
> newtype MyState s a = MyState (State s a)
>    deriving (MonadState s, Monad)
>
> Regards,
> Adam
>
>
> On Wed, Jun 11, 2014 at 3:31 PM, Gautier DI FOLCO
> <gautier.difolco at gmail.com> wrote:
> > Hi all,
> >
> > My goal is to compose Kleisli arrows with (State s) as parameteric
> Monadic
> > type.
> > So, I end up with the following code:
> >
> > data REnv :: [*] -> * -> * -> (* -> * -> *) -> ((* -> *) -> * -> * -> *)
> ->
> > * where
> >   Last  :: k (s t) i o -> REnv (t ': '[]) i o s k
> >   RCons :: k (s t) i o' -> REnv ts o' o s k -> REnv (t ': ts) i o s k
> >
> >
> > Which, of course, gives me the following error when I try to play with
> it:
> >
> > *H> :t Last genRanking
> >
> > <interactive>:1:6:
> >     Kind incompatibility when matching types:
> >       s :: * -> * -> *
> >       StateT Ranking :: (* -> *) -> * -> *
> >     Expected type: Kleisli (s t) Ticket Ranking
> >       Actual type: Kleisli (State Ranking) Ticket Ranking
> >     In the first argument of ‘Last’, namely ‘genRanking’
> >     In the expression: Last genRanking
> >
> > <interactive>:1:6:
> >     Kind incompatibility when matching types:
> >       t :: *
> >       Data.Functor.Identity.Identity :: * -> *
> >     Expected type: Kleisli (s t) Ticket Ranking
> >       Actual type: Kleisli (State Ranking) Ticket Ranking
> >     In the first argument of ‘Last’, namely ‘genRanking’
> >     In the expression: Last genRanking
> > *H> :t Last
> > Last :: k (s t) i o -> REnv '[t] i o s k
> > *H> :t genRanking
> > genRanking :: Kleisli (State Ranking) Ticket Ranking
> >
> >
> > I understand why it fails (State is * -> * -> *, while s is * -> *), but
> I
> > can't figure out how to write this, I tried (s t o), but it isn't correct
> > because k waits for a (* -> *).
> >
> >
> > If you have any ideas, I'm curious to read them.
> >
> > Thanks in advance for your help.
> >
> >
> >
> > _______________________________________________
> > Haskell-Cafe mailing list
> > Haskell-Cafe at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
>


Hi adam,

Thanks for your answer, I didn't realized that Identity could cause that.

Thanks,

Regards.
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