Lens.Micro.Mtl.Internal

When you're in a state monad, this function lets you operate on a part of your state. For instance, if your state was a record containing a position field, after zooming position would become your whole state (and when you modify it, the bigger structure would be modified as well).

(Your State / StateT or RWS / RWST can be anywhere in the stack, but you can't use zoom with arbitrary MonadState because it doesn't provide any methods to change the type of the state. See this issue for details.)

For the sake of the example, let's define some types first:

data Position = Position {
  _x, _y :: Int }

data Player = Player {
  _position :: Position,
  ... }

data Game = Game {
  _player :: Player,
  _obstacles :: [Position],
  ... }

concat <$> mapM makeLenses [''Position, ''Player, ''Game]

Now, here's an action that moves the player north-east:

moveNE :: State Game ()
moveNE = do
  player.position.x += 1
  player.position.y += 1

With zoom, you can use player.position to focus just on a part of the state:

moveNE :: State Game ()
moveNE = do
  zoom (player.position) $ do
    x += 1
    y += 1

You can just as well use it for retrieving things out of the state:

getCoords :: State Game (Int, Int)
getCoords = zoom (player.position) ((,) <$> use x <*> use y)

Or more explicitly:

getCoords = zoom (player.position) $ do
  x' <- use x
  y' <- use y
  return (x', y')

When you pass a traversal to zoom, it'll work as a loop. For instance, here we move all obstacles:

moveObstaclesNE :: State Game ()
moveObstaclesNE = do
  zoom (obstacles.each) $ do
    x += 1
    y += 1

If the action returns a result, all results would be combined with <> – the same way they're combined when ^. is passed a traversal. In this example, moveObstaclesNE returns a list of old coordinates of obstacles in addition to moving them:

moveObstaclesNE = do
  xys <- zoom (obstacles.each) $ do
    -- Get old coordinates.
    x' <- use x
    y' <- use y
    -- Update them.
    x .= x' + 1
    y .= y' + 1
    -- Return a single-element list with old coordinates.
    return [(x', y')]
  ...

Finally, you might need to write your own instances of Zoom if you use newtyped transformers in your monad stack. This can be done as follows:

import Lens.Micro.Mtl.Internal

type instance Zoomed (MyStateT s m) = Zoomed (StateT s m)

instance Monad m => Zoom (MyStateT s m) (MyStateT t m) s t where
    zoom l (MyStateT m) = MyStateT (zoom l m)

This is an equivalent of local which lets you apply a getter to your environment instead of merely applying a function (and it also lets you change the type of the environment).

local   :: (r -> r)   -> Reader r a -> Reader r a
magnify :: Getter r x -> Reader x a -> Reader r a

magnify works with Reader / ReaderT, RWS / RWST, and (->).

Here's an example of magnify being used to work with a part of a bigger config. First, the types:

data URL = URL {
  _protocol :: Maybe String,
  _path :: String }

data Config = Config {
  _base :: URL,
  ... }

makeLenses ''URL
makeLenses ''Config

Now, let's define a function which returns the base url:

getBase :: Reader Config String
getBase = do
  protocol <- fromMaybe "https" <$> view (base.protocol)
  path     <- view (base.path)
  return (protocol ++ path)

With magnify, we can factor out base:

getBase = magnify base $ do
  protocol <- fromMaybe "https" <$> view protocol
  path     <- view path
  return (protocol ++ path)

This concludes the example.

Finally, you should know writing instances of Magnify for your own types can be done as follows:

import Lens.Micro.Mtl.Internal

type instance Magnified (MyReaderT r m) = Magnified (ReaderT r m)

instance Monad m => Magnify (MyReaderT r m) (MyReaderT t m) r t where
    magnify l (MyReaderT m) = MyReaderT (magnify l m)