Lens Cheat Sheet


Access a field:

thing ^. field

Go deeper:

thing ^. field . field2

Transforming the result:

thing ^. field . to (*2)

Fold the result (if it's foldable):

thing ^. field . each

And do something to each value:

thing ^. field . each . to (*2)

Don't fold (return the elements):

thing ^.. field . each . to (*2)

Monadically (get a filename and return the handle):

thing ^! field . act (\x -> openFile x ReadMode)

Monadically, without folding (get a list of filenames, and return a list of handles):

thing ^!! field . each . act (\x -> openFile x ReadMode)


A field accessor's typeclass looks like this:

class Has_field a b | a -> b where
    field :: Lens' a b

An instance is a combination of getter and setter, using a simple combinator:

-- say we have Foo String String, and `field` refers to the first one
instance Has_field Foo String where
    field = lens getter setter
        where getter (Foo f1 _)     = f1
              setter (Foo f1 f2) v  = Foo v f2