lens-4.11.1: Lenses, Folds and Traversals
This package comes "Batteries Included" with many useful lenses for the types commonly used from the Haskell Platform, and with tools for automatically generating lenses and isomorphisms for user-supplied data types.
The combinators in
Control.Lens provide a highly generic toolbox for composing
families of getters, folds, isomorphisms, traversals, setters and lenses and their
An overview, with a large number of examples can be found in the README.
An introductory video on the style of code used in this library by Simon Peyton Jones is available from Skills Matter.
A video on how to use lenses and how they are constructed is available on youtube.
Slides for that second talk can be obtained from comonad.com.
More information on the care and feeding of lenses, including a brief tutorial and motivation for their types can be found on the lens wiki.
A small game of
pong and other more complex examples that manage their state using lenses can be found in the example folder.
Lenses, Folds and Traversals
With some signatures simplified, the core of the hierarchy of lens-like constructions looks like:
You can compose any two elements of the hierarchy above using
(.) from the
Prelude, and you can
use any element of the hierarchy as any type it linked to above it.
The result is their lowest upper bound in the hierarchy (or an error if that bound doesn't exist).
- You can use any
Foldor as a
- The composition of a
If you want to provide lenses and traversals for your own types in your own libraries, then you can do so without incurring a dependency on this (or any other) lens package at all.
e.g. for a data type:
data Foo a = Foo Int Int a
You can define lenses such as
-- bar :: Lens' (Foo a) Int bar :: Functor f => (Int -> f Int) -> Foo a -> f (Foo a) bar f (Foo a b c) = fmap (\a' -> Foo a' b c) (f a)
-- quux :: Lens (Foo a) (Foo b) a b quux :: Functor f => (a -> f b) -> Foo a -> f (Foo b) quux f (Foo a b c) = fmap (Foo a b) (f c)
without the need to use any type that isn't already defined in the
And you can define a traversal of multiple fields with
-- traverseBarAndBaz :: Traversal' (Foo a) Int traverseBarAndBaz :: Applicative f => (Int -> f Int) -> Foo a -> f (Foo a) traverseBarAndBaz f (Foo a b c) = Foo <$> f a <*> f b <*> pure c
What is provided in this library is a number of stock lenses and traversals for common haskell types, a wide array of combinators for working them, and more exotic functionality, (e.g. getters, setters, indexed folds, isomorphisms).