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@: <https://github.com/ekmett/lens#lens-lenses-folds-and-traversals>
-
+
A video on how to use lenses and how they are constructed is available from youtube: <http://youtu.be/cefnmjtAolY?hd=1>
-
+
Slides can be obtained here: <http://comonad.com/haskell/Lenses-Folds-and-Traversals-NYC.pdf>
-
+
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: <https://github.com/ekmett/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: <https://github.com/ekmett/lens/blob/master/examples/>
-
+
/Lenses, Folds and Traversals/
-
+
The core of the hierarchy of lens-like constructions looks like:
-
-
+
+
<<http://i.imgur.com/4fHw3Fd.png>>
-
+
Local copy (<Hierarchy.png>)
-
+
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)
-
+
> -- 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 :: 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).
</longdescription>
<use>
- <flag name="benchmark-uniplate">Enable benchmarking against Neil Mitchell's
- uniplate library for comparative performance analysis. Defaults to being
- turned off to avoid the extra dependency.</flag>
- <flag name="inlining">Generate inline pragmas when using
- template-haskell. This defaults to enabled, but you can
- to shut it off to benchmark the relative performance impact,
- or as last ditch effort to address compile errors resulting
- from the myriad versions of template-haskell that all purport to be 2.8.</flag>
- <flag name="old-inline-pragmas">Some 7.6.1-rc1 users report their TH still
- uses old style inline pragmas. This lets them turn on inlining.</flag>
+ <flag name="benchmark-uniplate">
+ Enable benchmarking against Neil Mitchell's
+ uniplate library for comparative performance analysis. Defaults to being
+ turned off to avoid the extra dependency.
+ </flag>
+ <flag name="inlining">
+ Generate inline pragmas when using
+ template-haskell. This defaults to enabled, but you can
+ to shut it off to benchmark the relative performance impact,
+ or as last ditch effort to address compile errors resulting
+ from the myriad versions of template-haskell that all purport to be 2.8.
+ </flag>
+ <flag name="old-inline-pragmas">Some 7.6.1-rc1 users report their TH still uses old style inline pragmas. This lets them turn on inlining.</flag>
<flag name="safe">Disallow unsafeCoerce</flag>
<flag name="dump-splices">Build and run the doctests test-suite.</flag>
<flag name="j">Attempt a parallel build with GHC 7.8.</flag>
projects / gentoo.git / commitdiff