ghc-lens - Lenses, Folds and Traversals

Property Value
Distribution openSUSE Leap 15.1
Repository openSUSE Oss all
Package filename ghc-lens-4.17-lp151.1.3.x86_64.rpm
Package name ghc-lens
Package version 4.17
Package release lp151.1.3
Package architecture x86_64
Package type rpm
Category Development/Libraries/Haskell
License BSD-2-Clause
Maintainer -
Download size 723.46 KB
Installed size 5.17 MB
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
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
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:
<Hierarchy.png (Local Copy)>
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 a 'Fold' or as a 'Setter'.
* The composition of a 'Traversal' and a 'Getter' yields a 'Fold'.
/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
> -- 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).


Package Version Architecture Repository
ghc-lens - - -


Name Value - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -


Name Value
ghc-lens = 4.17-lp151.1.3
ghc-lens(x86-64) = 4.17-lp151.1.3 -


Type URL
Binary Package ghc-lens-4.17-lp151.1.3.x86_64.rpm
Source Package ghc-lens-4.17-lp151.1.3.src.rpm

Install Howto

Install ghc-lens rpm package:

# zypper install ghc-lens




2018-08-02 -
- Add lens at version 4.17.

See Also

Package Description
ghc-lens-devel-4.17-lp151.1.3.x86_64.rpm Haskell lens library development files
ghc-libiserv-8.6.3-lp151.3.13.x86_64.rpm Haskell libiserv library
ghc-libiserv-devel-8.6.3-lp151.3.13.x86_64.rpm Haskell libiserv library development files
ghc-libraries-8.6.4-lp151.3.13.x86_64.rpm GHC development libraries meta package
ghc-libxml-sax-0.7.5-lp151.3.2.x86_64.rpm Bindings for the libXML2 SAX interface
ghc-libxml-sax-devel-0.7.5-lp151.3.2.x86_64.rpm Haskell libxml-sax library development files
ghc-libyaml- Low-level, streaming YAML interface
ghc-libyaml-devel- Haskell libyaml library development files
ghc-lifted-async- Run lifted IO operations asynchronously and wait for their results
ghc-lifted-async-devel- Haskell lifted-async library development files
ghc-lifted-base- Lifted IO operations from the base library
ghc-lifted-base-devel- Haskell lifted-base library development files
ghc-logict- A backtracking logic-programming monad
ghc-logict-devel- Haskell logict library development files
ghc-loop-0.3.0-lp151.2.5.x86_64.rpm Fast loops (for when GHC can't optimize forM_)