问题
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?
回答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).
回答2:
Use element
, which is a Traversal
to the specified list element:
list . element i . inner %= True :: Outer -> Outer
If you want to get the list element you must do so using a Maybe
since the list element might not be there:
myList :: Outer
myList ^? list . element i . inner :: Maybe Bool
来源:https://stackoverflow.com/questions/17006679/how-can-i-use-control-lens-to-update-the-ith-element-of-a-list