I'm surprised noone has mentioned 'zoom' yet. It does not make your 
original example work directly, but almost. It may be very useful to 
keep in mind as a way to do whatever you want.

http://hackage.haskell.org/package/lens-4.19.1/docs/Control-Lens-Zoom.html#v:zoom

The idea of zoom is that you use it like so:

zoom my_lens my_monadic_action

where my_monadic_action is a monadic action that works on the 
sub-element (call it 's') of the state, and the entire thing 'zoom 
my_lens my_monadic_action' is then a monadic action that works on the 
whole element (call it 't'). my_lens is a lens that focuses on 's' 
within 't'.

So instead of using the lens all the time in the monadic action, 
you're basically saying: "Put this everywhere that you need to", and 
then you can do things with the sub-element 's'.

Juan.

PS: I hope I did not severely misunderstand what you wanted, but it's 
still a possibility.

--
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Scotland, with registration number SC005336.


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