Writing "Imperative Code" in Haskell: Lenses and the State Monad
Lenses: Basic Idea
A lens provides access into the middle of a data structure
A lens is a value of type Lens' s a
Lenses can be composed to "look deeper":
(.) :: Lens' r s -> Lens' s a -> Lens' r a
-- For example, if:
address :: Lens' Person Address
city :: Lens' Address City
-- Then:
address . city :: Lens' Person City
r
s
a
Lenses: Basic Functions
-- Get the focused element
view :: Lens' s a -> s -> a
-- Set the focused element
set :: Lens' s a -> a -> s -> s
-- Modify the focused element
over :: Lens' s a -> (a -> a) -> s -> s
a
s
-- "Get"
use :: MonadState s m => Lens' s a -> m a
-- "Set"
(.=) :: MonadState s m => Lens' s a -> a -> m ()
-- "Modify"
(%=) :: MonadState s m => Lens' s a -> (a -> a) -> m ()
