The lens package

[Tags: bsd3, library]

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.

More information on the care and feeding of lenses, including a tutorial and motivation for their types can be found on the lens wiki.

https://github.com/ekmett/lens/wiki

A small game that manages its state using lenses can be found in the example folder.

https://github.com/ekmett/lens/blob/master/examples/Pong.hs

Lenses, Folds and Traversals

The core of this hierarchy 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 links to above it.

The result is their lowest upper bound in the hierarchy (or an error f that bound doesn't exist).

For instance:

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 :: Simple 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)
 -- baz :: 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 :: Simple 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).


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Properties

Versions0.1, 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
Change logNone available
Dependenciesarray (>=0.3.0.2 && <0.5), base (>=4.3 && <5), bytestring (>=0.9.1.10 && <0.10), containers (>=0.4.2 && <0.6), ghc-prim, mtl (>=2.0.1 && <2.2), parallel (>=3.1.0.1 && <3.3), template-haskell (>=2.4 && <2.8), text (>=0.11.1.5 && <0.12), transformers (==0.3.*) [details]
LicenseBSD3
CopyrightCopyright (C) 2012 Edward A. Kmett
AuthorEdward A. Kmett
MaintainerEdward A. Kmett <ekmett@gmail.com>
Stabilityprovisional
CategoryData, Lenses
Home pagehttp://github.com/ekmett/lens/
Bug trackerhttp://github.com/ekmett/lens/issues
Source repositoryhead: git clone git://github.com/ekmett/lens.git
UploadedThu Aug 9 20:41:31 UTC 2012 by EdwardKmett
DistributionsDebian:4.7.0.1, FreeBSD:4.3.3, LTSHaskell:4.7.0.1, NixOS:4.12.1, Stackage:4.12.3
Downloads119696 total (3228 in last 30 days)
Votes
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StatusDocs uploaded by user
Build status unknown [no reports yet]

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For package maintainers and hackage trustees

Readme for lens-1.8

lens

Build Status

This package provides families of lenses, isomorphisms, folds, traversals, getters and setters.

These lenses are compatible with those from lens-family, lens-family-core and lens-family-th, but they provide a great deal of additional flexibility in their composition.

An overview of the derivation of setters, folds, traversals, getters and lenses can be found on the lens wiki under Tutorial.

Lens Hierarchy

Example

ghci> :m + Control.Lens Data.Text.Lens
ghci> anyOf (traverse.text) (=='y') ["hello"^.packed, "goodbye"^.packed]
True

Contact Information

Contributions and bug reports are welcome!

Please feel free to contact me through github or on the #haskell IRC channel on irc.freenode.net.

-Edward Kmett