lens: 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
indexed variants.
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).
For instance:
You can use any
Traversal
as aFold
or as aSetter
.The composition of a
Traversal
and aGetter
yields aFold
.
Minimizing Dependencies
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 Prelude
.
And you can define a traversal of multiple fields with Control.Applicative.Applicative
:
-- 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).
[Skip to Readme]
Modules
[Index]
- Control
- Exception
- Control.Lens
- Control.Lens.At
- Control.Lens.Combinators
- Control.Lens.Cons
- Control.Lens.Each
- Control.Lens.Empty
- Control.Lens.Equality
- Control.Lens.Extras
- Control.Lens.Fold
- Control.Lens.Getter
- Control.Lens.Indexed
- Control.Lens.Internal
- Control.Lens.Internal.Bazaar
- Control.Lens.Internal.ByteString
- Control.Lens.Internal.Coerce
- Control.Lens.Internal.Context
- Control.Lens.Internal.Deque
- Control.Lens.Internal.Exception
- Control.Lens.Internal.FieldTH
- Control.Lens.Internal.Fold
- Control.Lens.Internal.Getter
- Control.Lens.Internal.Indexed
- Control.Lens.Internal.Instances
- Control.Lens.Internal.Iso
- Control.Lens.Internal.Level
- Control.Lens.Internal.List
- Control.Lens.Internal.Magma
- Control.Lens.Internal.Prism
- Control.Lens.Internal.PrismTH
- Control.Lens.Internal.Reflection
- Control.Lens.Internal.Review
- Control.Lens.Internal.Setter
- Control.Lens.Internal.TH
- Control.Lens.Internal.Zoom
- Control.Lens.Iso
- Control.Lens.Lens
- Control.Lens.Level
- Control.Lens.Operators
- Control.Lens.Plated
- Control.Lens.Prism
- Control.Lens.Reified
- Control.Lens.Review
- Control.Lens.Setter
- Control.Lens.TH
- Control.Lens.Traversal
- Control.Lens.Tuple
- Control.Lens.Type
- Control.Lens.Wrapped
- Control.Lens.Zoom
- Monad
- Parallel
- Strategies
- Seq
- Data
- Array
- Bits
- ByteString
- Complex
- Data
- Dynamic
- HashSet
- IntSet
- List
- Map
- Sequence
- Set
- Text
- Tree
- Typeable
- Vector
- GHC
- Generics
- Generics
- Deriving
- Language
- Haskell
- Numeric
- System
- Exit
- FilePath
- IO
- Error
Flags
Manual Flags
Name | Description | Default |
---|---|---|
benchmark-uniplate | Disabled | |
inlining | Enabled | |
old-inline-pragmas | Disabled | |
dump-splices | Disabled | |
test-hlint | Enabled | |
test-doctests | Enabled | |
test-hunit | Enabled | |
test-properties | Enabled | |
safe | Disabled | |
trustworthy | Enabled | |
lib-werror | Disabled | |
j | Disabled |
Use -f <flag> to enable a flag, or -f -<flag> to disable that flag. More info
Downloads
- lens-4.9.tar.gz [browse] (Cabal source package)
- Package description (revised from the package)
Note: This package has metadata revisions in the cabal description newer than included in the tarball. To unpack the package including the revisions, use 'cabal get'.
Maintainer's Corner
For package maintainers and hackage trustees
Candidates
- No Candidates
Versions [RSS] | 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.0.1, 1.0.2, 1.0.3, 1.1, 1.1.1, 1.2, 1.3, 1.3.1, 1.4, 1.4.1, 1.5, 1.6, 1.7, 1.7.1, 1.8, 1.9, 1.9.1, 2.0, 2.1, 2.2, 2.3, 2.4, 2.4.0.2, 2.5, 2.6, 2.6.1, 2.7, 2.7.0.1, 2.8, 2.9, 3.0, 3.0.1, 3.0.2, 3.0.3, 3.0.4, 3.0.5, 3.0.6, 3.1, 3.2, 3.3, 3.4, 3.5, 3.5.1, 3.6, 3.6.0.1, 3.6.0.2, 3.6.0.3, 3.6.0.4, 3.7, 3.7.0.1, 3.7.0.2, 3.7.1, 3.7.1.1, 3.7.1.2, 3.7.2, 3.7.3, 3.7.4, 3.7.5, 3.7.6, 3.8, 3.8.0.1, 3.8.0.2, 3.8.1, 3.8.2, 3.8.3, 3.8.4, 3.8.5, 3.8.6, 3.8.7, 3.8.7.1, 3.8.7.2, 3.8.7.3, 3.9, 3.9.0.1, 3.9.0.2, 3.9.0.3, 3.9.1, 3.9.2, 3.10, 3.10.0.1, 3.10.1, 3.10.2, 3.10.3, 4.0, 4.0.1, 4.0.2, 4.0.3, 4.0.4, 4.0.5, 4.0.6, 4.0.7, 4.1, 4.1.1, 4.1.2, 4.1.2.1, 4.2, 4.3, 4.3.1, 4.3.2, 4.3.3, 4.4, 4.4.0.1, 4.4.0.2, 4.5, 4.6, 4.6.0.1, 4.7, 4.7.0.1, 4.8, 4.9, 4.9.1, 4.10, 4.11, 4.11.1, 4.12, 4.12.1, 4.12.2, 4.12.3, 4.13, 4.13.1, 4.13.2, 4.13.2.1, 4.14, 4.15, 4.15.1, 4.15.2, 4.15.3, 4.15.4, 4.16, 4.16.1, 4.17, 4.17.1, 4.18, 4.18.1, 4.19, 4.19.1, 4.19.2, 5, 5.0.1, 5.1, 5.1.1, 5.2, 5.2.1, 5.2.2, 5.2.3, 5.3, 5.3.1, 5.3.2 |
---|---|
Change log | CHANGELOG.markdown |
Dependencies | array (>=0.3.0.2 && <0.6), base (>=4.5 && <5), bifunctors (>=4 && <5), bytestring (>=0.9.1.10 && <0.11), comonad (>=4 && <5), containers (>=0.4.0 && <0.5.8), contravariant (>=0.3 && <2), distributive (>=0.3 && <1), exceptions (>=0.1.1 && <1), filepath (>=1.2.0.0 && <1.5), free (>=4 && <5), ghc-prim, hashable (>=1.1.2.3 && <1.3), kan-extensions (>=4.2.1 && <5), mtl (>=2.0.1 && <2.3), parallel (>=3.1.0.1 && <3.3), profunctors (>=4 && <5), reflection (>=1.1.6 && <2), semigroupoids (>=4 && <5), semigroups (>=0.8.4 && <1), tagged (>=0.4.4 && <1), template-haskell (>=2.4 && <2.11), text (>=0.11 && <1.3), transformers (>=0.2 && <0.5), transformers-compat (>=0.4 && <1), unordered-containers (>=0.2 && <0.3), vector (>=0.9 && <0.11), void (>=0.5 && <1) [details] |
License | BSD-3-Clause |
Copyright | Copyright (C) 2012-2015 Edward A. Kmett |
Author | Edward A. Kmett |
Maintainer | Edward A. Kmett <ekmett@gmail.com> |
Revised | Revision 2 made by ryanglscott at 2021-01-31T13:03:01Z |
Category | Data, Lenses, Generics |
Home page | http://github.com/ekmett/lens/ |
Bug tracker | http://github.com/ekmett/lens/issues |
Source repo | head: git clone git://github.com/ekmett/lens.git |
Uploaded | by EdwardKmett at 2015-04-02T23:35:35Z |
Distributions | Arch:5.2.3, Debian:4.18.1, Fedora:5.2.2, FreeBSD:4.12.3, LTSHaskell:5.2.3, NixOS:5.2.3, Stackage:5.3.2, openSUSE:5.2.3 |
Reverse Dependencies | 1433 direct, 6436 indirect [details] |
Downloads | 421000 total (710 in the last 30 days) |
Rating | 3.0 (votes: 50) [estimated by Bayesian average] |
Your Rating | |
Status | Docs available [build log] Last success reported on 2015-04-03 [all 1 reports] |