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.