Optics.Zoom

This class allows us to zoom in, changing the State supplied by many different monad transformers, potentially quite deep in a monad transformer stack.

Its functions can be used to run a monadic action in a larger State than it was defined in, using a Lens', an AffineTraversal' or a Traversal'.

This is commonly used to lift actions in a simpler State Monad into a State Monad with a larger State type.

When used with a Traversal' over multiple values, the actions for each target are executed sequentially and the results are aggregated.

This can be used to edit pretty much any Monad transformer stack with a State in it!

>>> flip L.evalState ('a','b') $ zoom _1 $ use equality'a'

>>> flip S.execState ('a','b') $ zoom _1 $ equality .= 'c'('c','b')

>>> flip L.execState [(1,2),(3,4)] $ zoomMany traversed $ _2 %= (*10)[(1,20),(3,40)]

>>> flip S.runState [('a',"b"),('c',"d")] $ zoomMany traversed $ _2 <%= (\x -> x <> x)("bbdd",[('a',"bb"),('c',"dd")])

>>> flip S.evalState ("a","b") $ zoomMany each (use equality)"ab"

This class allows us to magnify part of the environment, changing the environment supplied by many different Monad transformers. Unlike zoom this can change the environment of a deeply nested Monad transformer.

Its functions can be used to run a monadic action in a larger environment than it was defined in, using a Getter or an AffineFold.

They act like local, but can in many cases change the type of the environment as well.

They're commonly used to lift actions in a simpler Reader Monad into a Monad with a larger environment type.

They can be used to edit pretty much any Monad transformer stack with an environment in it:

>>> (1,2) & magnify _2 (+1)3

>>> flip runReader (1,2) $ magnify _1 ask1

>>> flip runReader (1,2,[10..20]) $ magnifyMaybe (_3 % _tail) askJust [11,12,13,14,15,16,17,18,19,20]

Extends Magnify with an ability to magnify using a Fold over multiple targets so that actions for each one are executed sequentially and the results are aggregated.

There is however no sensible instance of MagnifyMany for StateT.