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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For what it's worth, I do sometimes define a version of MonadReader's "local" for MonadState; it runs a State computation on a transformed State. I've heard this used often enough to wonder why it's not a standard part of MonadState. On Wed, Nov 20, 2019 at 3:41 PM Viktor Dukhovni wrote:

...

On Nov 20, 2019, at 1:08 PM, Jacques Carette wrote:

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.

Use case: a Stateful computation where a local sub-computation needs more (local) state that will be, in-time, de-allocated.

Perhaps I misunderstood your use-case, but it seems that with a local sub-computation that needs extra state you can just:

... st <- get (c, (st', _)) <- runStateT local (st, extra) put st' ...

where local :: StateT (a, b) m c.

-- Viktor.

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-- brandon s allbery kf8nh allbery.b@gmail.com