[Haskell-cafe] runSubStateT

[Paul Brauner]

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 <pkmx.tw at gmail.com> 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 <chrisdone at gmail.com>
> 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?
> >
> > Ciao
> > _______________________________________________
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> > http://www.haskell.org/mailman/listinfo/haskell-cafe
>
> --
> Chih-Mao Chen (PkmX)
> System Software Laboratory
> Department of Computer Science
> National Chiao Tung University
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>
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