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<pre><a name="line-1"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-2"></a><span class='hs-comment'>-- |</span>
<a name="line-3"></a><span class='hs-comment'>-- Copyright : (C) 2014-2015 Edward Kmett</span>
<a name="line-4"></a><span class='hs-comment'>-- License : BSD-style (see the file LICENSE)</span>
<a name="line-5"></a><span class='hs-comment'>-- Maintainer : Edward Kmett <ekmett@gmail.com></span>
<a name="line-6"></a><span class='hs-comment'>-- Stability : experimental</span>
<a name="line-7"></a><span class='hs-comment'>-- Portability : non-portable</span>
<a name="line-8"></a><span class='hs-comment'>--</span>
<a name="line-9"></a><span class='hs-comment'>-- One of most commonly-asked questions about this package is whether</span>
<a name="line-10"></a><span class='hs-comment'>-- it provides lenses for working with 'Data.Map.Map'. It does, but their uses</span>
<a name="line-11"></a><span class='hs-comment'>-- are perhaps obscured by their genericity. This module exists to provide</span>
<a name="line-12"></a><span class='hs-comment'>-- documentation for them.</span>
<a name="line-13"></a><span class='hs-comment'>--</span>
<a name="line-14"></a><span class='hs-comment'>-- 'Data.Map.Map' is an instance of 'Control.Lens.At.At', so we have a lenses</span>
<a name="line-15"></a><span class='hs-comment'>-- on values at keys:</span>
<a name="line-16"></a><span class='hs-comment'>--</span>
<a name="line-17"></a><span class='hs-comment'>-- >>> Map.fromList [(1, "world")] ^.at 1</span>
<a name="line-18"></a><span class='hs-comment'>-- Just "world"</span>
<a name="line-19"></a><span class='hs-comment'>--</span>
<a name="line-20"></a><span class='hs-comment'>-- >>> at 1 .~ Just "world" $ Map.empty</span>
<a name="line-21"></a><span class='hs-comment'>-- fromList [(1,"world")]</span>
<a name="line-22"></a><span class='hs-comment'>--</span>
<a name="line-23"></a><span class='hs-comment'>-- >>> at 0 ?~ "hello" $ Map.empty</span>
<a name="line-24"></a><span class='hs-comment'>-- fromList [(0,"hello")]</span>
<a name="line-25"></a><span class='hs-comment'>--</span>
<a name="line-26"></a><span class='hs-comment'>-- We can traverse, fold over, and map over key-value pairs in a</span>
<a name="line-27"></a><span class='hs-comment'>-- 'Data.Map.Map', thanks to its 'Control.Lens.Indexed.TraversableWithIndex',</span>
<a name="line-28"></a><span class='hs-comment'>-- 'Control.Lens.Indexed.FoldableWithIndex', and</span>
<a name="line-29"></a><span class='hs-comment'>-- 'Control.Lens.Indexed.FunctorWithIndex' instances.</span>
<a name="line-30"></a><span class='hs-comment'>--</span>
<a name="line-31"></a><span class='hs-comment'>-- >>> imap const $ Map.fromList [(1, "Venus")]</span>
<a name="line-32"></a><span class='hs-comment'>-- fromList [(1,1)]</span>
<a name="line-33"></a><span class='hs-comment'>--</span>
<a name="line-34"></a><span class='hs-comment'>-- >>> ifoldMap (\i _ -> Sum i) $ Map.fromList [(2, "Earth"), (3, "Mars")]</span>
<a name="line-35"></a><span class='hs-comment'>-- Sum {getSum = 5}</span>
<a name="line-36"></a><span class='hs-comment'>--</span>
<a name="line-37"></a><span class='hs-comment'>-- >>> itraverse_ (curry print) $ Map.fromList [(4, "Jupiter")]</span>
<a name="line-38"></a><span class='hs-comment'>-- (4,"Jupiter")</span>
<a name="line-39"></a><span class='hs-comment'>--</span>
<a name="line-40"></a><span class='hs-comment'>-- >>> itoList $ Map.fromList [(5, "Saturn")]</span>
<a name="line-41"></a><span class='hs-comment'>-- [(5,"Saturn")]</span>
<a name="line-42"></a><span class='hs-comment'>--</span>
<a name="line-43"></a><span class='hs-comment'>-- A related class, 'Control.Lens.At.Ixed', allows us to use</span>
<a name="line-44"></a><span class='hs-comment'>-- 'Control.Lens.At.ix' to traverse a value at a particular key.</span>
<a name="line-45"></a><span class='hs-comment'>--</span>
<a name="line-46"></a><span class='hs-comment'>-- >>> ix 2 %~ ("New " ++) $ Map.fromList [(2, "Earth")]</span>
<a name="line-47"></a><span class='hs-comment'>-- fromList [(2,"New Earth")]</span>
<a name="line-48"></a><span class='hs-comment'>--</span>
<a name="line-49"></a><span class='hs-comment'>-- >>> preview (ix 8) $ Map.empty</span>
<a name="line-50"></a><span class='hs-comment'>-- Nothing</span>
<a name="line-51"></a><span class='hs-comment'>--</span>
<a name="line-52"></a><span class='hs-comment'>-- Additionally, 'Data.Map.Map' has 'Control.Lens.Traversal.TraverseMin' and</span>
<a name="line-53"></a><span class='hs-comment'>-- 'Control.Lens.Traversal.TraverseMax' instances, which let us traverse over</span>
<a name="line-54"></a><span class='hs-comment'>-- the value at the least and greatest keys, respectively.</span>
<a name="line-55"></a><span class='hs-comment'>--</span>
<a name="line-56"></a><span class='hs-comment'>-- >>> preview traverseMin $ Map.fromList [(5, "Saturn"), (6, "Uranus")]</span>
<a name="line-57"></a><span class='hs-comment'>-- Just "Saturn"</span>
<a name="line-58"></a><span class='hs-comment'>--</span>
<a name="line-59"></a><span class='hs-comment'>-- >>> preview traverseMax $ Map.fromList [(5, "Saturn"), (6, "Uranus")]</span>
<a name="line-60"></a><span class='hs-comment'>-- Just "Uranus"</span>
<a name="line-61"></a><span class='hs-comment'>--</span>
<a name="line-62"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-63"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Map</span><span class='hs-varop'>.</span><span class='hs-conid'>Lens</span> <span class='hs-conid'>()</span> <span class='hs-keyword'>where</span>
<a name="line-64"></a><span class='hs-comment'>-- $setup</span>
<a name="line-65"></a><span class='hs-comment'>-- >>> import Control.Lens</span>
<a name="line-66"></a><span class='hs-comment'>-- >>> import Data.Monoid</span>
<a name="line-67"></a><span class='hs-comment'>-- >>> import qualified Data.Map as Map</span>
<a name="line-68"></a><span class='hs-comment'>-- >>> :set -XNoOverloadedStrings</span>
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