hxt-8.2.0: doc/hdoc/src/Control-Arrow-ArrowState.html
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<pre><a name="line-1"></a><span class='comment'>-- ------------------------------------------------------------</span>
<a name="line-2"></a>
<a name="line-3"></a><span class='comment'>{- |
<a name="line-4"></a> Module : Control.Arrow.ArrowState
<a name="line-5"></a> Copyright : Copyright (C) 2005 Uwe Schmidt
<a name="line-6"></a> License : MIT
<a name="line-7"></a>
<a name="line-8"></a> Maintainer : Uwe Schmidt (uwe\@fh-wedel.de)
<a name="line-9"></a> Stability : experimental
<a name="line-10"></a> Portability: multy parameter classes and functional depenedencies required
<a name="line-11"></a>
<a name="line-12"></a> Arrows for managing an explicit state
<a name="line-13"></a>
<a name="line-14"></a> State arrows work similar to state monads.
<a name="line-15"></a> A state value is threaded through the application of arrows.
<a name="line-16"></a>
<a name="line-17"></a>-}</span>
<a name="line-18"></a>
<a name="line-19"></a><span class='comment'>-- ------------------------------------------------------------</span>
<a name="line-20"></a>
<a name="line-21"></a><span class='keyword'>module</span> <span class='conid'>Control</span><span class='varop'>.</span><span class='conid'>Arrow</span><span class='varop'>.</span><span class='conid'>ArrowState</span>
<a name="line-22"></a> <span class='layout'>(</span> <span class='conid'>ArrowState</span><span class='layout'>(</span><span class='keyglyph'>..</span><span class='layout'>)</span>
<a name="line-23"></a> <span class='layout'>)</span>
<a name="line-24"></a><span class='keyword'>where</span>
<a name="line-25"></a>
<a name="line-26"></a><span class='keyword'>import</span> <span class='conid'>Control</span><span class='varop'>.</span><span class='conid'>Arrow</span>
<a name="line-27"></a>
<a name="line-28"></a><span class='comment'>-- | The interface for accessing and changing the state component.</span>
<a name="line-29"></a><span class='comment'>--</span>
<a name="line-30"></a><span class='comment'>-- Multi parameter classes and functional dependencies are required.</span>
<a name="line-31"></a>
<a name="line-32"></a><a name="ArrowState"></a><span class='keyword'>class</span> <span class='conid'>Arrow</span> <span class='varid'>a</span> <span class='keyglyph'>=></span> <span class='conid'>ArrowState</span> <span class='varid'>s</span> <span class='varid'>a</span> <span class='keyglyph'>|</span> <span class='varid'>a</span> <span class='keyglyph'>-></span> <span class='varid'>s</span> <span class='keyword'>where</span>
<a name="line-33"></a>
<a name="line-34"></a> <span class='comment'>-- | change the state of a state arrow by applying a function</span>
<a name="line-35"></a> <span class='comment'>-- for computing a new state from the old and the arrow input.</span>
<a name="line-36"></a> <span class='comment'>-- Result is the arrow input</span>
<a name="line-37"></a>
<a name="line-38"></a> <span class='varid'>changeState</span> <span class='keyglyph'>::</span> <span class='layout'>(</span><span class='varid'>s</span> <span class='keyglyph'>-></span> <span class='varid'>b</span> <span class='keyglyph'>-></span> <span class='varid'>s</span><span class='layout'>)</span> <span class='keyglyph'>-></span> <span class='varid'>a</span> <span class='varid'>b</span> <span class='varid'>b</span>
<a name="line-39"></a>
<a name="line-40"></a> <span class='comment'>-- | access the state with a function using the arrow input</span>
<a name="line-41"></a> <span class='comment'>-- as data for selecting state components.</span>
<a name="line-42"></a>
<a name="line-43"></a> <span class='varid'>accessState</span> <span class='keyglyph'>::</span> <span class='layout'>(</span><span class='varid'>s</span> <span class='keyglyph'>-></span> <span class='varid'>b</span> <span class='keyglyph'>-></span> <span class='varid'>c</span><span class='layout'>)</span> <span class='keyglyph'>-></span> <span class='varid'>a</span> <span class='varid'>b</span> <span class='varid'>c</span>
<a name="line-44"></a>
<a name="line-45"></a> <span class='comment'>-- | read the complete state, ignore arrow input</span>
<a name="line-46"></a> <span class='comment'>--</span>
<a name="line-47"></a> <span class='comment'>-- definition: @ getState = accessState (\\ s x -> s) @ </span>
<a name="line-48"></a>
<a name="line-49"></a> <span class='varid'>getState</span> <span class='keyglyph'>::</span> <span class='varid'>a</span> <span class='varid'>b</span> <span class='varid'>s</span>
<a name="line-50"></a> <span class='varid'>getState</span> <span class='keyglyph'>=</span> <span class='varid'>accessState</span> <span class='layout'>(</span><span class='keyglyph'>\</span> <span class='varid'>s</span> <span class='sel'>_x</span> <span class='keyglyph'>-></span> <span class='varid'>s</span><span class='layout'>)</span>
<a name="line-51"></a>
<a name="line-52"></a> <span class='comment'>-- | overwrite the old state</span>
<a name="line-53"></a> <span class='comment'>--</span>
<a name="line-54"></a> <span class='comment'>-- definition: @ setState = changeState (\\ s x -> x) @</span>
<a name="line-55"></a> <span class='varid'>setState</span> <span class='keyglyph'>::</span> <span class='varid'>a</span> <span class='varid'>s</span> <span class='varid'>s</span>
<a name="line-56"></a> <span class='varid'>setState</span> <span class='keyglyph'>=</span> <span class='varid'>changeState</span> <span class='layout'>(</span><span class='keyglyph'>\</span> <span class='sel'>_s</span> <span class='varid'>x</span> <span class='keyglyph'>-></span> <span class='varid'>x</span><span class='layout'>)</span> <span class='comment'>-- changeState (const id)</span>
<a name="line-57"></a>
<a name="line-58"></a> <span class='comment'>-- | change state (and ignore input) and return new state</span>
<a name="line-59"></a> <span class='comment'>--</span>
<a name="line-60"></a> <span class='comment'>-- convenience function,</span>
<a name="line-61"></a> <span class='comment'>-- usefull for generating e.g. unique identifiers:</span>
<a name="line-62"></a> <span class='comment'>--</span>
<a name="line-63"></a> <span class='comment'>-- example with SLA state list arrows</span>
<a name="line-64"></a> <span class='comment'>--</span>
<a name="line-65"></a> <span class='comment'>-- > newId :: SLA Int b String</span>
<a name="line-66"></a> <span class='comment'>-- > newId = nextState (+1)</span>
<a name="line-67"></a> <span class='comment'>-- > >>></span>
<a name="line-68"></a> <span class='comment'>-- > arr (('#':) . show)</span>
<a name="line-69"></a> <span class='comment'>-- > </span>
<a name="line-70"></a> <span class='comment'>-- > runSLA 0 (newId <+> newId <+> newId) undefined</span>
<a name="line-71"></a> <span class='comment'>-- > = ["#1", "#2", "#3"]</span>
<a name="line-72"></a>
<a name="line-73"></a> <span class='varid'>nextState</span> <span class='keyglyph'>::</span> <span class='layout'>(</span><span class='varid'>s</span> <span class='keyglyph'>-></span> <span class='varid'>s</span><span class='layout'>)</span> <span class='keyglyph'>-></span> <span class='varid'>a</span> <span class='varid'>b</span> <span class='varid'>s</span>
<a name="line-74"></a> <span class='varid'>nextState</span> <span class='varid'>sf</span> <span class='keyglyph'>=</span> <span class='varid'>changeState</span> <span class='layout'>(</span><span class='keyglyph'>\</span><span class='varid'>s</span> <span class='keyglyph'>-></span> <span class='varid'>const</span> <span class='layout'>(</span><span class='varid'>sf</span> <span class='varid'>s</span><span class='layout'>)</span><span class='layout'>)</span>
<a name="line-75"></a> <span class='varop'>>>></span>
<a name="line-76"></a> <span class='varid'>getState</span>
<a name="line-77"></a>
<a name="line-78"></a><span class='comment'>-- ------------------------------------------------------------</span>
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