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

[Jacques Carette]

Ah, that looks perfect, thank you.  Jacques

On 2019-11-20 1:15 p.m., Oleg Grenrus wrote:
> 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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