Having these imports:
> import Control.Lens
Control.Lens> import qualified Data.Map as Map
and a map value defined as follows:
Control.Lens Map> let m = Map.fromList [('a', 1), ('c', 3), ('b', 2)]
I can get it's elements one by one like so:
Control.Lens Map> view (at 'b') m
Just 2
What I want to know is, having a set of keys such as this:
Control.Lens Map> import qualified Data.Set as Set
Control.Lens Map Set> let keys = Set.fromList ['d', 'c', 'b']
how to construct such a getter (I guess), using which I'll be able to get a set (or a list) of matching elements:
Control.Lens Map Set> view (**???**) m
[3, 2]
Notice that the result contains only 2 elements, because there's no match for a key 'd'.
The following will work if you only want a getter over multiple fields.
First, you need to make Accessor from lens an instance of Monoid (that instance is in in HEAD, but not released yet already defined in lens >= 4, so you only need to define the instance if you're working with an old version of the library).
import Data.Monoid
import Control.Lens
instance Monoid r => Monoid (Accessor r a) where
mempty = Accessor mempty
mappend (Accessor a) (Accessor b) = Accessor $ a <> b
You can then use that instance to combine multiple lenses/traversals into a single traversal:
>>> import qualified Data.Set as S
>>> import qualified Data.Map as M
>>> import Data.Foldable (foldMap)
>>> import Control.Lens
>>> let m = M.fromList [('a',1), ('b',2), ('c',3)]
>>> let k = S.fromList ['b','c','e']
>>> m ^.. foldMap at k
[Just 2,Just 3,Nothing]
>>> m ^.. foldMap ix k
[2,3]
foldMap uses the Monoid instance for Accessor and the Monoid instance for functions.