Am 21.11.2019 um 22:56 schrieb Jacques Carette :

On 2019-Nov.-21 15:57 , Olaf Klinke wrote:

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Is there a way to have a function of type (a -> b) -> State a c -> State b c ? djinn says no. The reason ist that the a occurs both positively and negatively in State a c, that is, on the left and the right hand side of the -> in State.

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In the particular case of interest, I am interested in is actually b -> State a c -> (State (a,b) c), which is of course a special case of the above. djinn says yes. The suggested term is

\f -> \(a,b) -> let (a,c) = f a in ((a,b),c)

Thanks. I tried to play with the online djinn to ask about exactly this - and failed! How should I be asking?

I downloaded the djinn package and built it with cabal.

Also, something is weird about that term, since it doesn't match "b -> State a c -> (State (a,b) c)" at all. It does match "(b -> State a c) -> (State (a,b) c)" which is a completely different type (and not interesting...)

Sorry, I tried to simplify djinn's output and failed. The term should be \f -> \(a,b) -> let (c,a') = f a in (c,(a',b)) :: (a -> (c, a3)) -> (a, b) -> (c, (a3, b)) where you can let a3 = a to get State a c -> State (a,b) c That is, it maps a transformer of state a into a transformer of state (a,b) that leaves the b component unchanged. Olaf