How can I use Control.Lens to update the ith element of a list?

Question

I have some datatypes along the line of

data Outer = Outer { _list :: [ Inner ] }
data Inner = Inner { _bool :: Bool }

using Control.Lens, I can access the _bool of the ith Inner (inside a 'State Outer' monad) like this

boolValue <- gets (^. list . to (!! i) . inner)

I would like to also be able to update this value with something like

list ^. (to (!! i)) ^. inner %= True

However (by my understanding), the 'to' function only creates a getter, not a true lens that can be used as either getter or setter.

So, how can I convert (!! i) into a lens that will allow me to update this field?

Answer 1

You can't* turn (!!) into any lens-like thing other than a Getter -- but there's a function to do this sort of thing: ix, for accessing things at indices. It's actually a Traversal, not a Lens -- which, here, just means that it can fail (if the index is out of bounds) -- but as long as the index is in the list, it'll work.

There's another problem, though -- (^.) is also an operator that's used exclusively for getting values. It's incompatible with e.g. (%=), which takes a lens-like thing as its first argument. And: (%=) is for mapping a function over the existing value; if you just want to set, you can use (.=). So you probably want something like:

list . ix i . inner .= True

* There actually is a function that can do this -- it's called upon -- but it uses wonderful evil black magic and you shouldn't use it, at least not for this (and probably not for any real code).