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

12 Jun 2014

      2014-06-12 3:42 GMT+02:00 adam vogt :
...
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
 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.
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Hi adam,

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

Thanks,

Regards.