Yes, I remember seeing a talk by Edward Kmett (at some Scala user group I think) where he said lenses allowed you to focus on a substate in the state monad. Paul On Tue Jan 27 2015 at 11:21:35 PM PkmX wrote:

Isn't this exactly what the `zoom` function does from `Control.Lens.Zoom`?

...

flip execState (1, 1) $ zoom _1 $ modify succ (2, 1)

On Wed, Jan 28, 2015 at 6:14 AM, Christopher Done wrote:

...

runSubStateT :: Monad m => (s -> s') -> (s' -> s) -> StateT s' m a -> StateT s m a runSubStateT to from m = StateT (\s -> liftM (\(a,s') -> (a,from s')) (runStateT m (to s)))

Anyone ever needed something like this? Does it already exist in some form in the standard libraries?

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-- Chih-Mao Chen (PkmX) System Software Laboratory Department of Computer Science National Chiao Tung University _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe