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diff --git a/dev-haskell/lens/metadata.xml b/dev-haskell/lens/metadata.xml new file mode 100644 index 000000000000..e817d8842f5f --- /dev/null +++ b/dev-haskell/lens/metadata.xml @@ -0,0 +1,100 @@ +<?xml version="1.0" encoding="UTF-8"?> +<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd"> +<pkgmetadata> + <herd>haskell</herd> + <longdescription> + 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="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> + <flag name="test-doctests">Build and run the doctests test-suite.</flag> + <flag name="test-hlint">You can disable the hlint test suite with -f-test-hlint</flag> + <flag name="test-hunit">You can disable the hunit test suite with -f-test-hunit</flag> + <flag name="test-properties">Build the properties test if we're building tests</flag> + <flag name="trustworthy">Assert that we are trustworthy when we can</flag> + <flag name="lib-werror">Turn on ghc-options: -Werror</flag> + </use> + <upstream> + <remote-id type="github">ekmett/lens</remote-id> + </upstream> +</pkgmetadata> |