[Haskell-cafe] runSubStateT

[PkmX]

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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-- 
Chih-Mao Chen (PkmX)
System Software Laboratory
Department of Computer Science
National Chiao Tung University