<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html> <head> <!-- Generated by HsColour, http://code.haskell.org/~malcolm/hscolour/ --> <title>Control/Arrow/Operations.hs</title> <link type='text/css' rel='stylesheet' href='hscolour.css' /> </head> <body> <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'>-- Module : Control.Arrow.Operations</span> <a name="line-4"></a><span class='hs-comment'>-- Copyright : (c) Ross Paterson 2003</span> <a name="line-5"></a><span class='hs-comment'>-- License : BSD-style (see the LICENSE file in the distribution)</span> <a name="line-6"></a><span class='hs-comment'>--</span> <a name="line-7"></a><span class='hs-comment'>-- Maintainer : ross@soi.city.ac.uk</span> <a name="line-8"></a><span class='hs-comment'>-- Stability : experimental</span> <a name="line-9"></a><span class='hs-comment'>-- Portability : non-portable (multi-parameter type classes)</span> <a name="line-10"></a><span class='hs-comment'>--</span> <a name="line-11"></a><span class='hs-comment'>-- Subclasses of 'Arrow' providing additional operations.</span> <a name="line-12"></a><span class='hs-comment'>--</span> <a name="line-13"></a><span class='hs-comment'>-- The signatures are designed to be compatible with the proposed</span> <a name="line-14"></a><span class='hs-comment'>-- notation for arrows, cf. <<a href="http://www.haskell.org/arrows/">http://www.haskell.org/arrows/</a>>.</span> <a name="line-15"></a> <a name="line-16"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Arrow</span><span class='hs-varop'>.</span><span class='hs-conid'>Operations</span> <span class='hs-layout'>(</span> <a name="line-17"></a> <span class='hs-comment'>-- * Conventions</span> <a name="line-18"></a> <span class='hs-comment'>-- $conventions</span> <a name="line-19"></a> <a name="line-20"></a> <span class='hs-comment'>-- * State transformers</span> <a name="line-21"></a> <span class='hs-conid'>ArrowState</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span> <a name="line-22"></a> <span class='hs-comment'>-- * State readers</span> <a name="line-23"></a> <span class='hs-conid'>ArrowReader</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span> <a name="line-24"></a> <span class='hs-comment'>-- * Monoid writers</span> <a name="line-25"></a> <span class='hs-conid'>ArrowWriter</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span> <a name="line-26"></a> <span class='hs-comment'>-- * Errors</span> <a name="line-27"></a> <span class='hs-conid'>ArrowError</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span> <a name="line-28"></a> <span class='hs-varid'>tryInUnlessDefault</span><span class='hs-layout'>,</span> <a name="line-29"></a> <span class='hs-comment'>-- * Synchronous circuits</span> <a name="line-30"></a> <span class='hs-conid'>ArrowCircuit</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span> <a name="line-31"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</span> <a name="line-32"></a> <a name="line-33"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Arrow</span> <a name="line-34"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Monoid</span> <a name="line-35"></a> <a name="line-36"></a><span class='hs-comment'>-- $conventions</span> <a name="line-37"></a><span class='hs-comment'>-- The arrow classes defined in this module have names like @Arrow@/Foo/,</span> <a name="line-38"></a><span class='hs-comment'>-- and contain operations specific to such arrows. Some of these include</span> <a name="line-39"></a><span class='hs-comment'>-- a method @new@/Foo/, which maps computations to computations of the</span> <a name="line-40"></a><span class='hs-comment'>-- same arrow type, but exposing some of the internals of the arrow.</span> <a name="line-41"></a><span class='hs-comment'>--</span> <a name="line-42"></a><span class='hs-comment'>-- Arrow transformers have names like /Bar/@Arrow@, and are</span> <a name="line-43"></a><span class='hs-comment'>-- instances of appropriate arrow classes. For each arrow</span> <a name="line-44"></a><span class='hs-comment'>-- transformer, there is typically an encapsulation operator</span> <a name="line-45"></a><span class='hs-comment'>-- @run@/Bar/ that removes that transformer from the outside of an</span> <a name="line-46"></a><span class='hs-comment'>-- arrow type. The 'Control.Arrow.Transformer.lift' method of the</span> <a name="line-47"></a><span class='hs-comment'>-- 'Control.Arrow.Transformer.ArrowTransformer' class adds an arrow</span> <a name="line-48"></a><span class='hs-comment'>-- transformer to the outside of an arrow type.</span> <a name="line-49"></a><span class='hs-comment'>--</span> <a name="line-50"></a><span class='hs-comment'>-- Typically a composite arrow type is built by applying a series of arrow</span> <a name="line-51"></a><span class='hs-comment'>-- transformers to a base arrow (usually either a function arrow or a</span> <a name="line-52"></a><span class='hs-comment'>-- 'Kleisli' arrow. The 'Control.Arrow.Transformer.lift' method and the</span> <a name="line-53"></a><span class='hs-comment'>-- @run@/Bar/ function operate only on the arrow transformer at the top</span> <a name="line-54"></a><span class='hs-comment'>-- of this stack. For more sophisticated manipulation of this stack of</span> <a name="line-55"></a><span class='hs-comment'>-- arrow transformers, many arrow transformers provide an @ArrowAdd@/Bar/</span> <a name="line-56"></a><span class='hs-comment'>-- class, with methods methods @lift@/Bar/ and @elim@/Bar/ to add and remove</span> <a name="line-57"></a><span class='hs-comment'>-- the transformer anywhere in the stack.</span> <a name="line-58"></a> <a name="line-59"></a><span class='hs-comment'>-- | An arrow type that provides a read-only state (an environment).</span> <a name="line-60"></a><span class='hs-comment'>-- If you also need to modify the state, use 'ArrowState'.</span> <a name="line-61"></a> <a name="line-62"></a><a name="ArrowReader"></a><span class='hs-keyword'>class</span> <span class='hs-conid'>Arrow</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>ArrowReader</span> <span class='hs-varid'>r</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-varid'>r</span> <span class='hs-keyword'>where</span> <a name="line-63"></a> <span class='hs-comment'>-- | Obtain the current value of the state.</span> <a name="line-64"></a> <span class='hs-varid'>readState</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span> <span class='hs-varid'>r</span> <a name="line-65"></a> <a name="line-66"></a> <span class='hs-comment'>-- | Run a subcomputation in the same arrow, but with a different</span> <a name="line-67"></a> <span class='hs-comment'>-- environment. The environment of the outer computation is</span> <a name="line-68"></a> <span class='hs-comment'>-- unaffected.</span> <a name="line-69"></a> <span class='hs-comment'>--</span> <a name="line-70"></a> <span class='hs-comment'>-- Typical usage in arrow notation:</span> <a name="line-71"></a> <span class='hs-comment'>--</span> <a name="line-72"></a> <span class='hs-comment'>-- > proc p -> ...</span> <a name="line-73"></a> <span class='hs-comment'>-- > (|newReader cmd|) env</span> <a name="line-74"></a> <a name="line-75"></a> <span class='hs-varid'>newReader</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-varid'>e</span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span> <span class='hs-layout'>(</span><span class='hs-varid'>e</span><span class='hs-layout'>,</span><span class='hs-varid'>r</span><span class='hs-layout'>)</span> <span class='hs-varid'>b</span> <a name="line-76"></a> <a name="line-77"></a><span class='hs-comment'>-- | An arrow type that provides a modifiable state,</span> <a name="line-78"></a><span class='hs-comment'>-- based of section 9 of /Generalising Monads to Arrows/, by John Hughes,</span> <a name="line-79"></a><span class='hs-comment'>-- /Science of Computer Programming/ 37:67-111, May 2000.</span> <a name="line-80"></a> <a name="line-81"></a><a name="ArrowState"></a><span class='hs-keyword'>class</span> <span class='hs-conid'>Arrow</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>ArrowState</span> <span class='hs-varid'>s</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-varid'>s</span> <span class='hs-keyword'>where</span> <a name="line-82"></a> <span class='hs-comment'>-- | Obtain the current value of the state.</span> <a name="line-83"></a> <span class='hs-varid'>fetch</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-varid'>e</span> <span class='hs-varid'>s</span> <a name="line-84"></a> <span class='hs-comment'>-- | Assign a new value to the state.</span> <a name="line-85"></a> <span class='hs-varid'>store</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-varid'>s</span> <span class='hs-conid'>()</span> <a name="line-86"></a> <a name="line-87"></a><span class='hs-comment'>-- | An arrow type that collects additional output (of some 'Monoid' type).</span> <a name="line-88"></a> <a name="line-89"></a><a name="ArrowWriter"></a><span class='hs-keyword'>class</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</span><span class='hs-layout'>,</span> <span class='hs-conid'>Arrow</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>ArrowWriter</span> <span class='hs-varid'>w</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-varid'>w</span> <span class='hs-keyword'>where</span> <a name="line-90"></a> <span class='hs-comment'>-- | Add a piece of additional output.</span> <a name="line-91"></a> <span class='hs-varid'>write</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-varid'>w</span> <span class='hs-conid'>()</span> <a name="line-92"></a> <a name="line-93"></a> <span class='hs-comment'>-- | Run a subcomputation in the same arrow, making its additional</span> <a name="line-94"></a> <span class='hs-comment'>-- output accessible.</span> <a name="line-95"></a> <span class='hs-comment'>--</span> <a name="line-96"></a> <span class='hs-comment'>-- Typical usage in arrow notation:</span> <a name="line-97"></a> <span class='hs-comment'>--</span> <a name="line-98"></a> <span class='hs-comment'>-- > proc p -> do</span> <a name="line-99"></a> <span class='hs-comment'>-- > ...</span> <a name="line-100"></a> <span class='hs-comment'>-- > (value, output) <- (|newWriter cmd|)</span> <a name="line-101"></a> <a name="line-102"></a> <span class='hs-varid'>newWriter</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-varid'>e</span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span> <span class='hs-varid'>e</span> <span class='hs-layout'>(</span><span class='hs-varid'>b</span><span class='hs-layout'>,</span><span class='hs-varid'>w</span><span class='hs-layout'>)</span> <a name="line-103"></a> <a name="line-104"></a><span class='hs-comment'>-- | An arrow type that includes errors (or exceptions).</span> <a name="line-105"></a><span class='hs-comment'>--</span> <a name="line-106"></a><span class='hs-comment'>-- Minimal definition: 'raise' and 'tryInUnless'.</span> <a name="line-107"></a><span class='hs-comment'>--</span> <a name="line-108"></a><span class='hs-comment'>-- /TODO:/ the operations here are inconsistent with other arrow transformers.</span> <a name="line-109"></a> <a name="line-110"></a><a name="ArrowError"></a><span class='hs-keyword'>class</span> <span class='hs-conid'>Arrow</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>ArrowError</span> <span class='hs-varid'>ex</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-varid'>ex</span> <span class='hs-keyword'>where</span> <a name="line-111"></a> <span class='hs-comment'>-- | Raise an error.</span> <a name="line-112"></a> <span class='hs-varid'>raise</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-varid'>ex</span> <span class='hs-varid'>b</span> <a name="line-113"></a> <a name="line-114"></a> <span class='hs-comment'>-- | Traditional exception construct.</span> <a name="line-115"></a> <span class='hs-comment'>--</span> <a name="line-116"></a> <span class='hs-comment'>-- Typical usage in arrow notation:</span> <a name="line-117"></a> <span class='hs-comment'>--</span> <a name="line-118"></a> <span class='hs-comment'>-- > proc p -> ...</span> <a name="line-119"></a> <span class='hs-comment'>-- > body `handle` \ex -> handler</span> <a name="line-120"></a> <a name="line-121"></a> <span class='hs-varid'>handle</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-varid'>e</span> <span class='hs-varid'>b</span> <span class='hs-comment'>-- ^ computation that may raise errors</span> <a name="line-122"></a> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span> <span class='hs-layout'>(</span><span class='hs-varid'>e</span><span class='hs-layout'>,</span><span class='hs-varid'>ex</span><span class='hs-layout'>)</span> <span class='hs-varid'>b</span> <span class='hs-comment'>-- ^ computation to handle errors</span> <a name="line-123"></a> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span> <span class='hs-varid'>e</span> <span class='hs-varid'>b</span> <a name="line-124"></a> <span class='hs-varid'>handle</span> <span class='hs-varid'>f</span> <span class='hs-varid'>h</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>tryInUnless</span> <span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-varid'>arr</span> <span class='hs-varid'>snd</span><span class='hs-layout'>)</span> <span class='hs-varid'>h</span> <a name="line-125"></a> <a name="line-126"></a> <span class='hs-comment'>-- | Exception construct in the style of /Exceptional Syntax/,</span> <a name="line-127"></a> <span class='hs-comment'>-- by Nick Benton and Andrew Kennedy, /JFP/ 11(4):395-410, July 2001.</span> <a name="line-128"></a> <span class='hs-comment'>--</span> <a name="line-129"></a> <span class='hs-comment'>-- Typical usage in arrow notation:</span> <a name="line-130"></a> <span class='hs-comment'>--</span> <a name="line-131"></a> <span class='hs-comment'>-- > proc p -> ...</span> <a name="line-132"></a> <span class='hs-comment'>-- > (|tryInUnless</span> <a name="line-133"></a> <span class='hs-comment'>-- > body</span> <a name="line-134"></a> <span class='hs-comment'>-- > (\res -> success)</span> <a name="line-135"></a> <span class='hs-comment'>-- > (\ex -> handler)</span> <a name="line-136"></a> <span class='hs-comment'>-- > |)</span> <a name="line-137"></a> <span class='hs-varid'>tryInUnless</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-varid'>e</span> <span class='hs-varid'>b</span> <span class='hs-comment'>-- ^ computation that may raise errors</span> <a name="line-138"></a> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</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> <span class='hs-varid'>c</span> <span class='hs-comment'>-- ^ computation to receive successful results</span> <a name="line-139"></a> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span> <span class='hs-layout'>(</span><span class='hs-varid'>e</span><span class='hs-layout'>,</span><span class='hs-varid'>ex</span><span class='hs-layout'>)</span> <span class='hs-varid'>c</span> <span class='hs-comment'>-- ^ computation to handle errors</span> <a name="line-140"></a> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span> <span class='hs-varid'>e</span> <span class='hs-varid'>c</span> <a name="line-141"></a> <a name="line-142"></a> <span class='hs-comment'>-- | Handler that returns the error as a value.</span> <a name="line-143"></a> <span class='hs-varid'>newError</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-varid'>e</span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span> <span class='hs-varid'>e</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>ex</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <a name="line-144"></a> <span class='hs-varid'>newError</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>handle</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-varop'>>>></span> <span class='hs-varid'>arr</span> <span class='hs-conid'>Right</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-varid'>arr</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varop'>.</span> <span class='hs-varid'>snd</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <a name="line-145"></a> <a name="line-146"></a><span class='hs-comment'>-- | A suitable value for 'tryInUnless' when the arrow type belongs to</span> <a name="line-147"></a><span class='hs-comment'>-- 'ArrowChoice'. To use it, you must define either 'handle' or 'newError'.</span> <a name="line-148"></a> <a name="line-149"></a><a name="tryInUnlessDefault"></a><span class='hs-definition'>tryInUnlessDefault</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>ArrowError</span> <span class='hs-varid'>ex</span> <span class='hs-varid'>a</span><span class='hs-layout'>,</span> <span class='hs-conid'>ArrowChoice</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <a name="line-150"></a> <span class='hs-varid'>a</span> <span class='hs-varid'>e</span> <span class='hs-varid'>b</span> <span class='hs-comment'>-- ^ computation that may raise errors</span> <a name="line-151"></a> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</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> <span class='hs-varid'>c</span> <span class='hs-comment'>-- ^ computation to receive successful results</span> <a name="line-152"></a> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span> <span class='hs-layout'>(</span><span class='hs-varid'>e</span><span class='hs-layout'>,</span><span class='hs-varid'>ex</span><span class='hs-layout'>)</span> <span class='hs-varid'>c</span> <span class='hs-comment'>-- ^ computation to handle errors</span> <a name="line-153"></a> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span> <span class='hs-varid'>e</span> <span class='hs-varid'>c</span> <a name="line-154"></a><span class='hs-definition'>tryInUnlessDefault</span> <span class='hs-varid'>f</span> <span class='hs-varid'>s</span> <span class='hs-varid'>h</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>arr</span> <span class='hs-varid'>id</span> <span class='hs-varop'>&&&</span> <span class='hs-varid'>newError</span> <span class='hs-varid'>f</span> <span class='hs-varop'>>>></span> <span class='hs-varid'>arr</span> <span class='hs-varid'>dist</span> <span class='hs-varop'>>>></span> <span class='hs-varid'>h</span> <span class='hs-varop'>|||</span> <span class='hs-varid'>s</span> <a name="line-155"></a> <span class='hs-keyword'>where</span> <span class='hs-varid'>dist</span> <span class='hs-layout'>(</span><span class='hs-varid'>e</span><span class='hs-layout'>,</span> <span class='hs-conid'>Left</span> <span class='hs-varid'>ex</span><span class='hs-layout'>)</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'>ex</span><span class='hs-layout'>)</span> <a name="line-156"></a> <span class='hs-varid'>dist</span> <span class='hs-layout'>(</span><span class='hs-varid'>e</span><span class='hs-layout'>,</span> <span class='hs-conid'>Right</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</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'>b</span><span class='hs-layout'>)</span> <a name="line-157"></a> <a name="line-158"></a><span class='hs-comment'>-- tryInUnless (and thus handle) could be replaced by newError if:</span> <a name="line-159"></a><span class='hs-comment'>-- 1. When ArrowChoice is available, tryInUnless and newError are equivalent.</span> <a name="line-160"></a><span class='hs-comment'>-- 2. When tryInUnless is available, so is ArrowChoice.</span> <a name="line-161"></a><span class='hs-comment'>-- (Counterexample: general CoKleisli)</span> <a name="line-162"></a> <a name="line-163"></a><span class='hs-comment'>-- | An arrow type that can be used to interpret synchronous circuits.</span> <a name="line-164"></a> <a name="line-165"></a><a name="ArrowCircuit"></a><span class='hs-keyword'>class</span> <span class='hs-conid'>ArrowLoop</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>ArrowCircuit</span> <span class='hs-varid'>a</span> <span class='hs-keyword'>where</span> <a name="line-166"></a> <span class='hs-comment'>-- | A delay component.</span> <a name="line-167"></a> <span class='hs-varid'>delay</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>b</span> <span class='hs-comment'>-- ^ the value to return initially.</span> <a name="line-168"></a> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span> <span class='hs-varid'>b</span> <span class='hs-comment'>-- ^ an arrow that propagates its input</span> <a name="line-169"></a> <span class='hs-comment'>-- with a one-tick delay.</span> </pre></body> </html>
distrib
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Fedora
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18
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x86_64
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media
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updates
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by-pkgid
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765adc18ec52512b3512a9992c59746e
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files
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80
