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

[Viktor Dukhovni]

> On Nov 20, 2019, at 1:08 PM, Jacques Carette <carette at mcmaster.ca> 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.