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<title>src/Control/Lens/Prism.hs</title>
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<pre><a name="line-1"></a><span class='hs-comment'>{-# LANGUAGE CPP #-}</span>
<a name="line-2"></a><span class='hs-comment'>{-# LANGUAGE Rank2Types #-}</span>
<a name="line-3"></a>
<a name="line-4"></a><span class='hs-cpp'>#ifdef TRUSTWORTHY</span>
<a name="line-5"></a><span class='hs-comment'>{-# LANGUAGE Trustworthy #-}</span>
<a name="line-6"></a><span class='hs-cpp'>#endif</span>
<a name="line-7"></a>
<a name="line-8"></a><span class='hs-cpp'>#ifndef MIN_VERSION_base</span>
<a name="line-9"></a><span class='hs-cpp'>#define MIN_VERSION_base(x,y,z) 1</span>
<a name="line-10"></a><span class='hs-cpp'>#endif</span>
<a name="line-11"></a>
<a name="line-12"></a><span class='hs-comment'>-------------------------------------------------------------------------------</span>
<a name="line-13"></a><span class='hs-comment'>-- |</span>
<a name="line-14"></a><span class='hs-comment'>-- Module : Control.Lens.Prism</span>
<a name="line-15"></a><span class='hs-comment'>-- Copyright : (C) 2012-16 Edward Kmett</span>
<a name="line-16"></a><span class='hs-comment'>-- License : BSD-style (see the file LICENSE)</span>
<a name="line-17"></a><span class='hs-comment'>-- Maintainer : Edward Kmett <ekmett@gmail.com></span>
<a name="line-18"></a><span class='hs-comment'>-- Stability : provisional</span>
<a name="line-19"></a><span class='hs-comment'>-- Portability : non-portable</span>
<a name="line-20"></a><span class='hs-comment'>--</span>
<a name="line-21"></a><span class='hs-comment'>-------------------------------------------------------------------------------</span>
<a name="line-22"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Lens</span><span class='hs-varop'>.</span><span class='hs-conid'>Prism</span>
<a name="line-23"></a> <span class='hs-layout'>(</span>
<a name="line-24"></a> <span class='hs-comment'>-- * Prisms</span>
<a name="line-25"></a> <span class='hs-conid'>Prism</span><span class='hs-layout'>,</span> <span class='hs-conid'>Prism'</span>
<a name="line-26"></a> <span class='hs-layout'>,</span> <span class='hs-conid'>APrism</span><span class='hs-layout'>,</span> <span class='hs-conid'>APrism'</span>
<a name="line-27"></a> <span class='hs-comment'>-- * Constructing Prisms</span>
<a name="line-28"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>prism</span>
<a name="line-29"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>prism'</span>
<a name="line-30"></a> <span class='hs-comment'>-- * Consuming Prisms</span>
<a name="line-31"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>withPrism</span>
<a name="line-32"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>clonePrism</span>
<a name="line-33"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>outside</span>
<a name="line-34"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>aside</span>
<a name="line-35"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>without</span>
<a name="line-36"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>below</span>
<a name="line-37"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>isn't</span>
<a name="line-38"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>matching</span>
<a name="line-39"></a> <span class='hs-comment'>-- * Common Prisms</span>
<a name="line-40"></a> <span class='hs-layout'>,</span> <span class='hs-sel'>_Left</span>
<a name="line-41"></a> <span class='hs-layout'>,</span> <span class='hs-sel'>_Right</span>
<a name="line-42"></a> <span class='hs-layout'>,</span> <span class='hs-sel'>_Just</span>
<a name="line-43"></a> <span class='hs-layout'>,</span> <span class='hs-sel'>_Nothing</span>
<a name="line-44"></a> <span class='hs-layout'>,</span> <span class='hs-sel'>_Void</span>
<a name="line-45"></a> <span class='hs-layout'>,</span> <span class='hs-sel'>_Show</span>
<a name="line-46"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>only</span>
<a name="line-47"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>nearly</span>
<a name="line-48"></a> <span class='hs-comment'>-- * Prismatic profunctors</span>
<a name="line-49"></a> <span class='hs-layout'>,</span> <span class='hs-conid'>Choice</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span>
<a name="line-50"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-51"></a>
<a name="line-52"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Applicative</span>
<a name="line-53"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Lens</span><span class='hs-varop'>.</span><span class='hs-conid'>Internal</span><span class='hs-varop'>.</span><span class='hs-conid'>Prism</span>
<a name="line-54"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Lens</span><span class='hs-varop'>.</span><span class='hs-conid'>Lens</span>
<a name="line-55"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Lens</span><span class='hs-varop'>.</span><span class='hs-conid'>Review</span>
<a name="line-56"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Lens</span><span class='hs-varop'>.</span><span class='hs-conid'>Type</span>
<a name="line-57"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span>
<a name="line-58"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Functor</span><span class='hs-varop'>.</span><span class='hs-conid'>Identity</span>
<a name="line-59"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Profunctor</span>
<a name="line-60"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Profunctor</span><span class='hs-varop'>.</span><span class='hs-conid'>Rep</span>
<a name="line-61"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Profunctor</span><span class='hs-varop'>.</span><span class='hs-conid'>Sieve</span>
<a name="line-62"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Traversable</span>
<a name="line-63"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Void</span>
<a name="line-64"></a><span class='hs-cpp'>#if MIN_VERSION_base(4,7,0)</span>
<a name="line-65"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Coerce</span>
<a name="line-66"></a><span class='hs-cpp'>#elif defined(SAFE)</span>
<a name="line-67"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Profunctor</span><span class='hs-varop'>.</span><span class='hs-conid'>Unsafe</span>
<a name="line-68"></a><span class='hs-cpp'>#else</span>
<a name="line-69"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Unsafe</span><span class='hs-varop'>.</span><span class='hs-conid'>Coerce</span>
<a name="line-70"></a><span class='hs-cpp'>#endif</span>
<a name="line-71"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Prelude</span>
<a name="line-72"></a>
<a name="line-73"></a><span class='hs-cpp'>#ifdef HLINT</span>
<a name="line-74"></a><span class='hs-comment'>{-# ANN module "HLint: ignore Use camelCase" #-}</span>
<a name="line-75"></a><span class='hs-cpp'>#endif</span>
<a name="line-76"></a>
<a name="line-77"></a><span class='hs-comment'>-- $setup</span>
<a name="line-78"></a><span class='hs-comment'>-- >>> :set -XNoOverloadedStrings</span>
<a name="line-79"></a><span class='hs-comment'>-- >>> import Control.Lens</span>
<a name="line-80"></a><span class='hs-comment'>-- >>> import Numeric.Natural</span>
<a name="line-81"></a><span class='hs-comment'>-- >>> import Debug.SimpleReflect.Expr</span>
<a name="line-82"></a><span class='hs-comment'>-- >>> import Debug.SimpleReflect.Vars as Vars hiding (f,g)</span>
<a name="line-83"></a><span class='hs-comment'>-- >>> let isLeft (Left _) = True; isLeft _ = False</span>
<a name="line-84"></a><span class='hs-comment'>-- >>> let isRight (Right _) = True; isRight _ = False</span>
<a name="line-85"></a><span class='hs-comment'>-- >>> let f :: Expr -> Expr; f = Debug.SimpleReflect.Vars.f</span>
<a name="line-86"></a><span class='hs-comment'>-- >>> let g :: Expr -> Expr; g = Debug.SimpleReflect.Vars.g</span>
<a name="line-87"></a>
<a name="line-88"></a><span class='hs-comment'>------------------------------------------------------------------------------</span>
<a name="line-89"></a><span class='hs-comment'>-- Prism Internals</span>
<a name="line-90"></a><span class='hs-comment'>------------------------------------------------------------------------------</span>
<a name="line-91"></a>
<a name="line-92"></a><a name="APrism"></a><span class='hs-comment'>-- | If you see this in a signature for a function, the function is expecting a 'Prism'.</span>
<a name="line-93"></a><a name="APrism"></a><span class='hs-keyword'>type</span> <span class='hs-conid'>APrism</span> <span class='hs-varid'>s</span> <span class='hs-varid'>t</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Market</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span> <span class='hs-varid'>a</span> <span class='hs-layout'>(</span><span class='hs-conid'>Identity</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Market</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span> <span class='hs-varid'>s</span> <span class='hs-layout'>(</span><span class='hs-conid'>Identity</span> <span class='hs-varid'>t</span><span class='hs-layout'>)</span>
<a name="line-94"></a>
<a name="line-95"></a><a name="APrism'"></a><span class='hs-comment'>-- | @</span>
<a name="line-96"></a><a name="APrism'"></a><span class='hs-comment'>-- type APrism' = 'Simple' 'APrism'</span>
<a name="line-97"></a><a name="APrism'"></a><span class='hs-comment'>-- @</span>
<a name="line-98"></a><a name="APrism'"></a><span class='hs-keyword'>type</span> <span class='hs-conid'>APrism'</span> <span class='hs-varid'>s</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>APrism</span> <span class='hs-varid'>s</span> <span class='hs-varid'>s</span> <span class='hs-varid'>a</span> <span class='hs-varid'>a</span>
<a name="line-99"></a>
<a name="line-100"></a><a name="withPrism"></a><span class='hs-comment'>-- | Convert 'APrism' to the pair of functions that characterize it.</span>
<a name="line-101"></a><span class='hs-definition'>withPrism</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>APrism</span> <span class='hs-varid'>s</span> <span class='hs-varid'>t</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-layout'>(</span><span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>t</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>s</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Either</span> <span class='hs-varid'>t</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>r</span>
<a name="line-102"></a><span class='hs-cpp'>#if MIN_VERSION_base(4,7,0)</span>
<a name="line-103"></a><span class='hs-definition'>withPrism</span> <span class='hs-varid'>k</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>case</span> <span class='hs-varid'>coerce</span> <span class='hs-layout'>(</span><span class='hs-varid'>k</span> <span class='hs-layout'>(</span><span class='hs-conid'>Market</span> <span class='hs-conid'>Identity</span> <span class='hs-conid'>Right</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-keyword'>of</span>
<a name="line-104"></a> <span class='hs-conid'>Market</span> <span class='hs-varid'>bt</span> <span class='hs-varid'>seta</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>f</span> <span class='hs-varid'>bt</span> <span class='hs-varid'>seta</span>
<a name="line-105"></a><span class='hs-cpp'>#elif defined(SAFE)</span>
<a name="line-106"></a><span class='hs-definition'>withPrism</span> <span class='hs-varid'>k</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>case</span> <span class='hs-varid'>k</span> <span class='hs-layout'>(</span><span class='hs-conid'>Market</span> <span class='hs-conid'>Identity</span> <span class='hs-conid'>Right</span><span class='hs-layout'>)</span> <span class='hs-keyword'>of</span>
<a name="line-107"></a> <span class='hs-conid'>Market</span> <span class='hs-varid'>bt</span> <span class='hs-varid'>seta</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-varid'>runIdentity</span> <span class='hs-cpp'>#.</span> <span class='hs-varid'>bt</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-varid'>either</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varop'>.</span> <span class='hs-varid'>runIdentity</span><span class='hs-layout'>)</span> <span class='hs-conid'>Right</span> <span class='hs-varop'>.</span> <span class='hs-varid'>seta</span><span class='hs-layout'>)</span>
<a name="line-108"></a><span class='hs-cpp'>#else</span>
<a name="line-109"></a><span class='hs-definition'>withPrism</span> <span class='hs-varid'>k</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>case</span> <span class='hs-varid'>unsafeCoerce</span> <span class='hs-layout'>(</span><span class='hs-varid'>k</span> <span class='hs-layout'>(</span><span class='hs-conid'>Market</span> <span class='hs-conid'>Identity</span> <span class='hs-conid'>Right</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-keyword'>of</span>
<a name="line-110"></a> <span class='hs-conid'>Market</span> <span class='hs-varid'>bt</span> <span class='hs-varid'>seta</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>f</span> <span class='hs-varid'>bt</span> <span class='hs-varid'>seta</span>
<a name="line-111"></a><span class='hs-cpp'>#endif</span>
<a name="line-112"></a><span class='hs-comment'>{-# INLINE withPrism #-}</span>
<a name="line-113"></a>
<a name="line-114"></a><a name="clonePrism"></a><span class='hs-comment'>-- | Clone a 'Prism' so that you can reuse the same monomorphically typed 'Prism' for different purposes.</span>
<a name="line-115"></a><span class='hs-comment'>--</span>
<a name="line-116"></a><span class='hs-comment'>-- See 'Control.Lens.Lens.cloneLens' and 'Control.Lens.Traversal.cloneTraversal' for examples of why you might want to do this.</span>
<a name="line-117"></a><span class='hs-definition'>clonePrism</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>APrism</span> <span class='hs-varid'>s</span> <span class='hs-varid'>t</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Prism</span> <span class='hs-varid'>s</span> <span class='hs-varid'>t</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span>
<a name="line-118"></a><span class='hs-definition'>clonePrism</span> <span class='hs-varid'>k</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>withPrism</span> <span class='hs-varid'>k</span> <span class='hs-varid'>prism</span>
<a name="line-119"></a><span class='hs-comment'>{-# INLINE clonePrism #-}</span>
<a name="line-120"></a>
<a name="line-121"></a><span class='hs-comment'>------------------------------------------------------------------------------</span>
<a name="line-122"></a><span class='hs-comment'>-- Prism Combinators</span>
<a name="line-123"></a><span class='hs-comment'>------------------------------------------------------------------------------</span>
<a name="line-124"></a>
<a name="line-125"></a><a name="prism"></a><span class='hs-comment'>-- | Build a 'Control.Lens.Prism.Prism'.</span>
<a name="line-126"></a><span class='hs-comment'>--</span>
<a name="line-127"></a><span class='hs-comment'>-- @'Either' t a@ is used instead of @'Maybe' a@ to permit the types of @s@ and @t@ to differ.</span>
<a name="line-128"></a><span class='hs-comment'>--</span>
<a name="line-129"></a><span class='hs-definition'>prism</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>t</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>s</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Either</span> <span class='hs-varid'>t</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Prism</span> <span class='hs-varid'>s</span> <span class='hs-varid'>t</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span>
<a name="line-130"></a><span class='hs-definition'>prism</span> <span class='hs-varid'>bt</span> <span class='hs-varid'>seta</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>dimap</span> <span class='hs-varid'>seta</span> <span class='hs-layout'>(</span><span class='hs-varid'>either</span> <span class='hs-varid'>pure</span> <span class='hs-layout'>(</span><span class='hs-varid'>fmap</span> <span class='hs-varid'>bt</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-varop'>.</span> <span class='hs-varid'>right'</span>
<a name="line-131"></a><span class='hs-comment'>{-# INLINE prism #-}</span>
<a name="line-132"></a>
<a name="line-133"></a><a name="prism'"></a><span class='hs-comment'>-- | This is usually used to build a 'Prism'', when you have to use an operation like</span>
<a name="line-134"></a><span class='hs-comment'>-- 'Data.Typeable.cast' which already returns a 'Maybe'.</span>
<a name="line-135"></a><span class='hs-definition'>prism'</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>s</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>s</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Prism</span> <span class='hs-varid'>s</span> <span class='hs-varid'>s</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span>
<a name="line-136"></a><span class='hs-definition'>prism'</span> <span class='hs-varid'>bs</span> <span class='hs-varid'>sma</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>prism</span> <span class='hs-varid'>bs</span> <span class='hs-layout'>(</span><span class='hs-keyglyph'>\</span><span class='hs-varid'>s</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>maybe</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varid'>s</span><span class='hs-layout'>)</span> <span class='hs-conid'>Right</span> <span class='hs-layout'>(</span><span class='hs-varid'>sma</span> <span class='hs-varid'>s</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span>
<a name="line-137"></a><span class='hs-comment'>{-# INLINE prism' #-}</span>
<a name="line-138"></a>
<a name="line-139"></a><span class='hs-comment'>-- | Use a 'Prism' as a kind of first-class pattern.</span>
<a name="line-140"></a><span class='hs-comment'>--</span>
<a name="line-141"></a><span class='hs-comment'>-- @'outside' :: 'Prism' s t a b -> 'Lens' (t -> r) (s -> r) (b -> r) (a -> r)@</span>
<a name="line-142"></a>
<a name="line-143"></a><a name="outside"></a><span class='hs-comment'>-- TODO: can we make this work with merely Strong?</span>
<a name="line-144"></a><span class='hs-definition'>outside</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Representable</span> <span class='hs-varid'>p</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>APrism</span> <span class='hs-varid'>s</span> <span class='hs-varid'>t</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Lens</span> <span class='hs-layout'>(</span><span class='hs-varid'>p</span> <span class='hs-varid'>t</span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-varid'>p</span> <span class='hs-varid'>s</span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-varid'>p</span> <span class='hs-varid'>b</span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-varid'>p</span> <span class='hs-varid'>a</span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span>
<a name="line-145"></a><span class='hs-definition'>outside</span> <span class='hs-varid'>k</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>withPrism</span> <span class='hs-varid'>k</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>bt</span> <span class='hs-varid'>seta</span> <span class='hs-varid'>f</span> <span class='hs-varid'>ft</span> <span class='hs-keyglyph'>-></span>
<a name="line-146"></a> <span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-varid'>lmap</span> <span class='hs-varid'>bt</span> <span class='hs-varid'>ft</span><span class='hs-layout'>)</span> <span class='hs-varop'><&></span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>fa</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>tabulate</span> <span class='hs-varop'>$</span> <span class='hs-varid'>either</span> <span class='hs-layout'>(</span><span class='hs-varid'>sieve</span> <span class='hs-varid'>ft</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-varid'>sieve</span> <span class='hs-varid'>fa</span><span class='hs-layout'>)</span> <span class='hs-varop'>.</span> <span class='hs-varid'>seta</span>
<a name="line-147"></a><span class='hs-comment'>{-# INLINE outside #-}</span>
<a name="line-148"></a>
<a name="line-149"></a><a name="without"></a><span class='hs-comment'>-- | Given a pair of prisms, project sums.</span>
<a name="line-150"></a><span class='hs-comment'>--</span>
<a name="line-151"></a><span class='hs-comment'>-- Viewing a 'Prism' as a co-'Lens', this combinator can be seen to be dual to 'Control.Lens.Lens.alongside'.</span>
<a name="line-152"></a><span class='hs-definition'>without</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>APrism</span> <span class='hs-varid'>s</span> <span class='hs-varid'>t</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span>
<a name="line-153"></a> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>APrism</span> <span class='hs-varid'>u</span> <span class='hs-varid'>v</span> <span class='hs-varid'>c</span> <span class='hs-varid'>d</span>
<a name="line-154"></a> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Prism</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>s</span> <span class='hs-varid'>u</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>t</span> <span class='hs-varid'>v</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>a</span> <span class='hs-varid'>c</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>b</span> <span class='hs-varid'>d</span><span class='hs-layout'>)</span>
<a name="line-155"></a><span class='hs-definition'>without</span> <span class='hs-varid'>k</span> <span class='hs-keyglyph'>=</span>
<a name="line-156"></a> <span class='hs-varid'>withPrism</span> <span class='hs-varid'>k</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>bt</span> <span class='hs-varid'>seta</span> <span class='hs-varid'>k'</span> <span class='hs-keyglyph'>-></span>
<a name="line-157"></a> <span class='hs-varid'>withPrism</span> <span class='hs-varid'>k'</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>dv</span> <span class='hs-varid'>uevc</span> <span class='hs-keyglyph'>-></span>
<a name="line-158"></a> <span class='hs-varid'>prism</span> <span class='hs-layout'>(</span><span class='hs-varid'>bimap</span> <span class='hs-varid'>bt</span> <span class='hs-varid'>dv</span><span class='hs-layout'>)</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>su</span> <span class='hs-keyglyph'>-></span>
<a name="line-159"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>su</span> <span class='hs-keyword'>of</span>
<a name="line-160"></a> <span class='hs-conid'>Left</span> <span class='hs-varid'>s</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>bimap</span> <span class='hs-conid'>Left</span> <span class='hs-conid'>Left</span> <span class='hs-layout'>(</span><span class='hs-varid'>seta</span> <span class='hs-varid'>s</span><span class='hs-layout'>)</span>
<a name="line-161"></a> <span class='hs-conid'>Right</span> <span class='hs-varid'>u</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>bimap</span> <span class='hs-conid'>Right</span> <span class='hs-conid'>Right</span> <span class='hs-layout'>(</span><span class='hs-varid'>uevc</span> <span class='hs-varid'>u</span><span class='hs-layout'>)</span>
<a name="line-162"></a><span class='hs-comment'>{-# INLINE without #-}</span>
<a name="line-163"></a>
<a name="line-164"></a><a name="aside"></a><span class='hs-comment'>-- | Use a 'Prism' to work over part of a structure.</span>
<a name="line-165"></a><span class='hs-comment'>--</span>
<a name="line-166"></a><span class='hs-definition'>aside</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>APrism</span> <span class='hs-varid'>s</span> <span class='hs-varid'>t</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Prism</span> <span class='hs-layout'>(</span><span class='hs-varid'>e</span><span class='hs-layout'>,</span> <span class='hs-varid'>s</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-varid'>e</span><span class='hs-layout'>,</span> <span class='hs-varid'>t</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-varid'>e</span><span class='hs-layout'>,</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-varid'>e</span><span class='hs-layout'>,</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span>
<a name="line-167"></a><span class='hs-definition'>aside</span> <span class='hs-varid'>k</span> <span class='hs-keyglyph'>=</span>
<a name="line-168"></a> <span class='hs-varid'>withPrism</span> <span class='hs-varid'>k</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>bt</span> <span class='hs-varid'>seta</span> <span class='hs-keyglyph'>-></span>
<a name="line-169"></a> <span class='hs-varid'>prism</span> <span class='hs-layout'>(</span><span class='hs-varid'>fmap</span> <span class='hs-varid'>bt</span><span class='hs-layout'>)</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-layout'>(</span><span class='hs-varid'>e</span><span class='hs-layout'>,</span><span class='hs-varid'>s</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span>
<a name="line-170"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>seta</span> <span class='hs-varid'>s</span> <span class='hs-keyword'>of</span>
<a name="line-171"></a> <span class='hs-conid'>Left</span> <span class='hs-varid'>t</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Left</span> <span class='hs-layout'>(</span><span class='hs-varid'>e</span><span class='hs-layout'>,</span><span class='hs-varid'>t</span><span class='hs-layout'>)</span>
<a name="line-172"></a> <span class='hs-conid'>Right</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Right</span> <span class='hs-layout'>(</span><span class='hs-varid'>e</span><span class='hs-layout'>,</span><span class='hs-varid'>a</span><span class='hs-layout'>)</span>
<a name="line-173"></a><span class='hs-comment'>{-# INLINE aside #-}</span>
<a name="line-174"></a>
<a name="line-175"></a><a name="below"></a><span class='hs-comment'>-- | 'lift' a 'Prism' through a 'Traversable' functor, giving a Prism that matches only if all the elements of the container match the 'Prism'.</span>
<a name="line-176"></a><span class='hs-comment'>--</span>
<a name="line-177"></a><span class='hs-comment'>-- >>> [Left 1, Right "foo", Left 4, Right "woot"]^..below _Right</span>
<a name="line-178"></a><span class='hs-comment'>-- []</span>
<a name="line-179"></a><span class='hs-comment'>--</span>
<a name="line-180"></a><span class='hs-comment'>-- >>> [Right "hail hydra!", Right "foo", Right "blah", Right "woot"]^..below _Right</span>
<a name="line-181"></a><span class='hs-comment'>-- [["hail hydra!","foo","blah","woot"]]</span>
<a name="line-182"></a><span class='hs-definition'>below</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Traversable</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>APrism'</span> <span class='hs-varid'>s</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Prism'</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-varid'>s</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span>
<a name="line-183"></a><span class='hs-definition'>below</span> <span class='hs-varid'>k</span> <span class='hs-keyglyph'>=</span>
<a name="line-184"></a> <span class='hs-varid'>withPrism</span> <span class='hs-varid'>k</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>bt</span> <span class='hs-varid'>seta</span> <span class='hs-keyglyph'>-></span>
<a name="line-185"></a> <span class='hs-varid'>prism</span> <span class='hs-layout'>(</span><span class='hs-varid'>fmap</span> <span class='hs-varid'>bt</span><span class='hs-layout'>)</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>s</span> <span class='hs-keyglyph'>-></span>
<a name="line-186"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>traverse</span> <span class='hs-varid'>seta</span> <span class='hs-varid'>s</span> <span class='hs-keyword'>of</span>
<a name="line-187"></a> <span class='hs-conid'>Left</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Left</span> <span class='hs-varid'>s</span>
<a name="line-188"></a> <span class='hs-conid'>Right</span> <span class='hs-varid'>t</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Right</span> <span class='hs-varid'>t</span>
<a name="line-189"></a><span class='hs-comment'>{-# INLINE below #-}</span>
<a name="line-190"></a>
<a name="line-191"></a><a name="isn't"></a><span class='hs-comment'>-- | Check to see if this 'Prism' doesn't match.</span>
<a name="line-192"></a><span class='hs-comment'>--</span>
<a name="line-193"></a><span class='hs-comment'>-- >>> isn't _Left (Right 12)</span>
<a name="line-194"></a><span class='hs-comment'>-- True</span>
<a name="line-195"></a><span class='hs-comment'>--</span>
<a name="line-196"></a><span class='hs-comment'>-- >>> isn't _Left (Left 12)</span>
<a name="line-197"></a><span class='hs-comment'>-- False</span>
<a name="line-198"></a><span class='hs-comment'>--</span>
<a name="line-199"></a><span class='hs-comment'>-- >>> isn't _Empty []</span>
<a name="line-200"></a><span class='hs-comment'>-- False</span>
<a name="line-201"></a><span class='hs-definition'>isn't</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>APrism</span> <span class='hs-varid'>s</span> <span class='hs-varid'>t</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>s</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Bool</span>
<a name="line-202"></a><span class='hs-definition'>isn't</span> <span class='hs-varid'>k</span> <span class='hs-varid'>s</span> <span class='hs-keyglyph'>=</span>
<a name="line-203"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>matching</span> <span class='hs-varid'>k</span> <span class='hs-varid'>s</span> <span class='hs-keyword'>of</span>
<a name="line-204"></a> <span class='hs-conid'>Left</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>True</span>
<a name="line-205"></a> <span class='hs-conid'>Right</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>False</span>
<a name="line-206"></a><span class='hs-comment'>{-# INLINE isn't #-}</span>
<a name="line-207"></a>
<a name="line-208"></a><a name="matching"></a><span class='hs-comment'>-- | Retrieve the value targeted by a 'Prism' or return the</span>
<a name="line-209"></a><span class='hs-comment'>-- original value while allowing the type to change if it does</span>
<a name="line-210"></a><span class='hs-comment'>-- not match.</span>
<a name="line-211"></a><span class='hs-comment'>--</span>
<a name="line-212"></a><span class='hs-comment'>-- >>> matching _Just (Just 12)</span>
<a name="line-213"></a><span class='hs-comment'>-- Right 12</span>
<a name="line-214"></a><span class='hs-comment'>--</span>
<a name="line-215"></a><span class='hs-comment'>-- >>> matching _Just (Nothing :: Maybe Int) :: Either (Maybe Bool) Int</span>
<a name="line-216"></a><span class='hs-comment'>-- Left Nothing</span>
<a name="line-217"></a><span class='hs-definition'>matching</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>APrism</span> <span class='hs-varid'>s</span> <span class='hs-varid'>t</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>s</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Either</span> <span class='hs-varid'>t</span> <span class='hs-varid'>a</span>
<a name="line-218"></a><span class='hs-definition'>matching</span> <span class='hs-varid'>k</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>withPrism</span> <span class='hs-varid'>k</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-keyword'>_</span> <span class='hs-varid'>seta</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>seta</span>
<a name="line-219"></a><span class='hs-comment'>{-# INLINE matching #-}</span>
<a name="line-220"></a>
<a name="line-221"></a><span class='hs-comment'>------------------------------------------------------------------------------</span>
<a name="line-222"></a><span class='hs-comment'>-- Common Prisms</span>
<a name="line-223"></a><span class='hs-comment'>------------------------------------------------------------------------------</span>
<a name="line-224"></a>
<a name="line-225"></a><span class='hs-comment'>-- | This 'Prism' provides a 'Traversal' for tweaking the 'Left' half of an 'Either':</span>
<a name="line-226"></a><span class='hs-comment'>--</span>
<a name="line-227"></a><span class='hs-comment'>-- >>> over _Left (+1) (Left 2)</span>
<a name="line-228"></a><span class='hs-comment'>-- Left 3</span>
<a name="line-229"></a><span class='hs-comment'>--</span>
<a name="line-230"></a><span class='hs-comment'>-- >>> over _Left (+1) (Right 2)</span>
<a name="line-231"></a><span class='hs-comment'>-- Right 2</span>
<a name="line-232"></a><span class='hs-comment'>--</span>
<a name="line-233"></a><span class='hs-comment'>-- >>> Right 42 ^._Left :: String</span>
<a name="line-234"></a><span class='hs-comment'>-- ""</span>
<a name="line-235"></a><span class='hs-comment'>--</span>
<a name="line-236"></a><span class='hs-comment'>-- >>> Left "hello" ^._Left</span>
<a name="line-237"></a><span class='hs-comment'>-- "hello"</span>
<a name="line-238"></a><span class='hs-comment'>--</span>
<a name="line-239"></a><span class='hs-comment'>-- It also can be turned around to obtain the embedding into the 'Left' half of an 'Either':</span>
<a name="line-240"></a><span class='hs-comment'>--</span>
<a name="line-241"></a><span class='hs-comment'>-- >>> _Left # 5</span>
<a name="line-242"></a><span class='hs-comment'>-- Left 5</span>
<a name="line-243"></a><span class='hs-comment'>--</span>
<a name="line-244"></a><span class='hs-comment'>-- >>> 5^.re _Left</span>
<a name="line-245"></a><span class='hs-comment'>-- Left 5</span>
<a name="line-246"></a><span class='hs-sel'>_Left</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Prism</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>a</span> <span class='hs-varid'>c</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>b</span> <span class='hs-varid'>c</span><span class='hs-layout'>)</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span>
<a name="line-247"></a><span class='hs-sel'>_Left</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>prism</span> <span class='hs-conid'>Left</span> <span class='hs-varop'>$</span> <span class='hs-varid'>either</span> <span class='hs-conid'>Right</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varop'>.</span> <span class='hs-conid'>Right</span><span class='hs-layout'>)</span>
<a name="line-248"></a><span class='hs-comment'>{-# INLINE _Left #-}</span>
<a name="line-249"></a>
<a name="line-250"></a><span class='hs-comment'>-- | This 'Prism' provides a 'Traversal' for tweaking the 'Right' half of an 'Either':</span>
<a name="line-251"></a><span class='hs-comment'>--</span>
<a name="line-252"></a><span class='hs-comment'>-- >>> over _Right (+1) (Left 2)</span>
<a name="line-253"></a><span class='hs-comment'>-- Left 2</span>
<a name="line-254"></a><span class='hs-comment'>--</span>
<a name="line-255"></a><span class='hs-comment'>-- >>> over _Right (+1) (Right 2)</span>
<a name="line-256"></a><span class='hs-comment'>-- Right 3</span>
<a name="line-257"></a><span class='hs-comment'>--</span>
<a name="line-258"></a><span class='hs-comment'>-- >>> Right "hello" ^._Right</span>
<a name="line-259"></a><span class='hs-comment'>-- "hello"</span>
<a name="line-260"></a><span class='hs-comment'>--</span>
<a name="line-261"></a><span class='hs-comment'>-- >>> Left "hello" ^._Right :: [Double]</span>
<a name="line-262"></a><span class='hs-comment'>-- []</span>
<a name="line-263"></a><span class='hs-comment'>--</span>
<a name="line-264"></a><span class='hs-comment'>-- It also can be turned around to obtain the embedding into the 'Right' half of an 'Either':</span>
<a name="line-265"></a><span class='hs-comment'>--</span>
<a name="line-266"></a><span class='hs-comment'>-- >>> _Right # 5</span>
<a name="line-267"></a><span class='hs-comment'>-- Right 5</span>
<a name="line-268"></a><span class='hs-comment'>--</span>
<a name="line-269"></a><span class='hs-comment'>-- >>> 5^.re _Right</span>
<a name="line-270"></a><span class='hs-comment'>-- Right 5</span>
<a name="line-271"></a><span class='hs-sel'>_Right</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Prism</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>c</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>c</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span>
<a name="line-272"></a><span class='hs-sel'>_Right</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>prism</span> <span class='hs-conid'>Right</span> <span class='hs-varop'>$</span> <span class='hs-varid'>either</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varop'>.</span> <span class='hs-conid'>Left</span><span class='hs-layout'>)</span> <span class='hs-conid'>Right</span>
<a name="line-273"></a><span class='hs-comment'>{-# INLINE _Right #-}</span>
<a name="line-274"></a>
<a name="line-275"></a><span class='hs-comment'>-- | This 'Prism' provides a 'Traversal' for tweaking the target of the value of 'Just' in a 'Maybe'.</span>
<a name="line-276"></a><span class='hs-comment'>--</span>
<a name="line-277"></a><span class='hs-comment'>-- >>> over _Just (+1) (Just 2)</span>
<a name="line-278"></a><span class='hs-comment'>-- Just 3</span>
<a name="line-279"></a><span class='hs-comment'>--</span>
<a name="line-280"></a><span class='hs-comment'>-- Unlike 'Data.Traversable.traverse' this is a 'Prism', and so you can use it to inject as well:</span>
<a name="line-281"></a><span class='hs-comment'>--</span>
<a name="line-282"></a><span class='hs-comment'>-- >>> _Just # 5</span>
<a name="line-283"></a><span class='hs-comment'>-- Just 5</span>
<a name="line-284"></a><span class='hs-comment'>--</span>
<a name="line-285"></a><span class='hs-comment'>-- >>> 5^.re _Just</span>
<a name="line-286"></a><span class='hs-comment'>-- Just 5</span>
<a name="line-287"></a><span class='hs-comment'>--</span>
<a name="line-288"></a><span class='hs-comment'>-- Interestingly,</span>
<a name="line-289"></a><span class='hs-comment'>--</span>
<a name="line-290"></a><span class='hs-comment'>-- @</span>
<a name="line-291"></a><span class='hs-comment'>-- m '^?' '_Just' ≡ m</span>
<a name="line-292"></a><span class='hs-comment'>-- @</span>
<a name="line-293"></a><span class='hs-comment'>--</span>
<a name="line-294"></a><span class='hs-comment'>-- >>> Just x ^? _Just</span>
<a name="line-295"></a><span class='hs-comment'>-- Just x</span>
<a name="line-296"></a><span class='hs-comment'>--</span>
<a name="line-297"></a><span class='hs-comment'>-- >>> Nothing ^? _Just</span>
<a name="line-298"></a><span class='hs-comment'>-- Nothing</span>
<a name="line-299"></a><span class='hs-sel'>_Just</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Prism</span> <span class='hs-layout'>(</span><span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-conid'>Maybe</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span>
<a name="line-300"></a><span class='hs-sel'>_Just</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>prism</span> <span class='hs-conid'>Just</span> <span class='hs-varop'>$</span> <span class='hs-varid'>maybe</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-conid'>Nothing</span><span class='hs-layout'>)</span> <span class='hs-conid'>Right</span>
<a name="line-301"></a><span class='hs-comment'>{-# INLINE _Just #-}</span>
<a name="line-302"></a>
<a name="line-303"></a><span class='hs-comment'>-- | This 'Prism' provides the 'Traversal' of a 'Nothing' in a 'Maybe'.</span>
<a name="line-304"></a><span class='hs-comment'>--</span>
<a name="line-305"></a><span class='hs-comment'>-- >>> Nothing ^? _Nothing</span>
<a name="line-306"></a><span class='hs-comment'>-- Just ()</span>
<a name="line-307"></a><span class='hs-comment'>--</span>
<a name="line-308"></a><span class='hs-comment'>-- >>> Just () ^? _Nothing</span>
<a name="line-309"></a><span class='hs-comment'>-- Nothing</span>
<a name="line-310"></a><span class='hs-comment'>--</span>
<a name="line-311"></a><span class='hs-comment'>-- But you can turn it around and use it to construct 'Nothing' as well:</span>
<a name="line-312"></a><span class='hs-comment'>--</span>
<a name="line-313"></a><span class='hs-comment'>-- >>> _Nothing # ()</span>
<a name="line-314"></a><span class='hs-comment'>-- Nothing</span>
<a name="line-315"></a><span class='hs-sel'>_Nothing</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Prism'</span> <span class='hs-layout'>(</span><span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-conid'>()</span>
<a name="line-316"></a><span class='hs-sel'>_Nothing</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>prism'</span> <span class='hs-layout'>(</span><span class='hs-varid'>const</span> <span class='hs-conid'>Nothing</span><span class='hs-layout'>)</span> <span class='hs-varop'>$</span> <span class='hs-varid'>maybe</span> <span class='hs-layout'>(</span><span class='hs-conid'>Just</span> <span class='hs-conid'>()</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-varid'>const</span> <span class='hs-conid'>Nothing</span><span class='hs-layout'>)</span>
<a name="line-317"></a><span class='hs-comment'>{-# INLINE _Nothing #-}</span>
<a name="line-318"></a>
<a name="line-319"></a><span class='hs-comment'>-- | 'Void' is a logically uninhabited data type.</span>
<a name="line-320"></a><span class='hs-comment'>--</span>
<a name="line-321"></a><span class='hs-comment'>-- This is a 'Prism' that will always fail to match.</span>
<a name="line-322"></a><span class='hs-sel'>_Void</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Prism</span> <span class='hs-varid'>s</span> <span class='hs-varid'>s</span> <span class='hs-varid'>a</span> <span class='hs-conid'>Void</span>
<a name="line-323"></a><span class='hs-sel'>_Void</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>prism</span> <span class='hs-varid'>absurd</span> <span class='hs-conid'>Left</span>
<a name="line-324"></a><span class='hs-comment'>{-# INLINE _Void #-}</span>
<a name="line-325"></a>
<a name="line-326"></a><a name="only"></a><span class='hs-comment'>-- | This 'Prism' compares for exact equality with a given value.</span>
<a name="line-327"></a><span class='hs-comment'>--</span>
<a name="line-328"></a><span class='hs-comment'>-- >>> only 4 # ()</span>
<a name="line-329"></a><span class='hs-comment'>-- 4</span>
<a name="line-330"></a><span class='hs-comment'>--</span>
<a name="line-331"></a><span class='hs-comment'>-- >>> 5 ^? only 4</span>
<a name="line-332"></a><span class='hs-comment'>-- Nothing</span>
<a name="line-333"></a><span class='hs-definition'>only</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Eq</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=></span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Prism'</span> <span class='hs-varid'>a</span> <span class='hs-conid'>()</span>
<a name="line-334"></a><span class='hs-definition'>only</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>prism'</span> <span class='hs-layout'>(</span><span class='hs-keyglyph'>\</span><span class='hs-conid'>()</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-varop'>$</span> <span class='hs-varid'>guard</span> <span class='hs-varop'>.</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-varop'>==</span><span class='hs-layout'>)</span>
<a name="line-335"></a><span class='hs-comment'>{-# INLINE only #-}</span>
<a name="line-336"></a>
<a name="line-337"></a>
<a name="line-338"></a><span class='hs-comment'>-- | This 'Prism' compares for approximate equality with a given value and a predicate for testing,</span>
<a name="line-339"></a><span class='hs-comment'>-- an example where the value is the empty list and the predicate checks that a list is empty (same</span>
<a name="line-340"></a><span class='hs-comment'>-- as 'Control.Lens.Empty._Empty' with the 'Control.Lens.Empty.AsEmpty' list instance):</span>
<a name="line-341"></a><span class='hs-comment'>--</span>
<a name="line-342"></a><span class='hs-comment'>-- >>> nearly [] null # ()</span>
<a name="line-343"></a><span class='hs-comment'>-- []</span>
<a name="line-344"></a><span class='hs-comment'>-- >>> [1,2,3,4] ^? nearly [] null</span>
<a name="line-345"></a><span class='hs-comment'>-- Nothing</span>
<a name="line-346"></a><span class='hs-comment'>--</span>
<a name="line-347"></a><span class='hs-comment'>-- @'nearly' [] 'Prelude.null' :: 'Prism'' [a] ()@</span>
<a name="line-348"></a><span class='hs-comment'>--</span>
<a name="line-349"></a><span class='hs-comment'>-- To comply with the 'Prism' laws the arguments you supply to @nearly a p@ are somewhat constrained.</span>
<a name="line-350"></a><span class='hs-comment'>--</span>
<a name="line-351"></a><span class='hs-comment'>-- We assume @p x@ holds iff @x ≡ a@. Under that assumption then this is a valid 'Prism'.</span>
<a name="line-352"></a><span class='hs-comment'>--</span>
<a name="line-353"></a><span class='hs-comment'>-- This is useful when working with a type where you can test equality for only a subset of its</span>
<a name="line-354"></a><span class='hs-comment'>-- values, and the prism selects such a value.</span>
<a name="line-355"></a>
<a name="line-356"></a><a name="nearly"></a><span class='hs-definition'>nearly</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Bool</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Prism'</span> <span class='hs-varid'>a</span> <span class='hs-conid'>()</span>
<a name="line-357"></a><span class='hs-definition'>nearly</span> <span class='hs-varid'>a</span> <span class='hs-varid'>p</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>prism'</span> <span class='hs-layout'>(</span><span class='hs-keyglyph'>\</span><span class='hs-conid'>()</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-varop'>$</span> <span class='hs-varid'>guard</span> <span class='hs-varop'>.</span> <span class='hs-varid'>p</span>
<a name="line-358"></a><span class='hs-comment'>{-# INLINE nearly #-}</span>
<a name="line-359"></a>
<a name="line-360"></a><span class='hs-comment'>-- | This is an improper prism for text formatting based on 'Read' and 'Show'.</span>
<a name="line-361"></a><span class='hs-comment'>--</span>
<a name="line-362"></a><span class='hs-comment'>-- This 'Prism' is \"improper\" in the sense that it normalizes the text formatting, but round tripping</span>
<a name="line-363"></a><span class='hs-comment'>-- is idempotent given sane 'Read'/'Show' instances.</span>
<a name="line-364"></a><span class='hs-comment'>--</span>
<a name="line-365"></a><span class='hs-comment'>-- >>> _Show # 2</span>
<a name="line-366"></a><span class='hs-comment'>-- "2"</span>
<a name="line-367"></a><span class='hs-comment'>--</span>
<a name="line-368"></a><span class='hs-comment'>-- >>> "EQ" ^? _Show :: Maybe Ordering</span>
<a name="line-369"></a><span class='hs-comment'>-- Just EQ</span>
<a name="line-370"></a><span class='hs-comment'>--</span>
<a name="line-371"></a><span class='hs-comment'>-- @</span>
<a name="line-372"></a><span class='hs-comment'>-- '_Show' ≡ 'prism'' 'show' 'readMaybe'</span>
<a name="line-373"></a><span class='hs-comment'>-- @</span>
<a name="line-374"></a><span class='hs-sel'>_Show</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Read</span> <span class='hs-varid'>a</span><span class='hs-layout'>,</span> <span class='hs-conid'>Show</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Prism'</span> <span class='hs-conid'>String</span> <span class='hs-varid'>a</span>
<a name="line-375"></a><span class='hs-sel'>_Show</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>prism</span> <span class='hs-varid'>show</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>s</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyword'>case</span> <span class='hs-varid'>reads</span> <span class='hs-varid'>s</span> <span class='hs-keyword'>of</span>
<a name="line-376"></a> <span class='hs-keyglyph'>[</span><span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span><span class='hs-str'>""</span><span class='hs-layout'>)</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Right</span> <span class='hs-varid'>a</span>
<a name="line-377"></a> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Left</span> <span class='hs-varid'>s</span>
<a name="line-378"></a><span class='hs-comment'>{-# INLINE _Show #-}</span>
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