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#... or https://hackage.haskell.org/package/optics-extra-0.2/docs/Optics-Zoom.html#v...
- 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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