[Haskell-cafe] Transforming from on State-transformer to another

[Oleg Grenrus]

You need something like Lens to "zoom" into the state. As state monad 
can not only `get` but also `put` the state, the plain a -> b function 
is not enough: you need Lens a b.

See 
https://hackage.haskell.org/package/lens-4.18.1/docs/Control-Lens-Zoom.html#v:zoom 
or 
https://hackage.haskell.org/package/optics-extra-0.2/docs/Optics-Zoom.html#v:zoom

- Oleg

On 20.11.2019 20.08, Jacques Carette wrote:
> Is there a way to have a function of type (a -> b) -> State a c -> 
> State b c ?
>
> 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.
>
> This matches 'first' on Data.Bifunctor, but State is not a Bifunctor. 
> Nor a Profunctor, AFAIK.
>
> Use case: a Stateful computation where a local sub-computation needs 
> more (local) state that will be, in-time, de-allocated.
>
> I guess the usual solution is probably to use 2 stacked State, but I'm 
> curious if there's another way.
>
> Jacques
>
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