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</script></head><body><div id="package-header"><ul class="links" id="page-menu"><li><a href="src/Control-Arrow-Transformer-Writer.html">Source</a></li><li><a href="index.html">Contents</a></li><li><a href="doc-index.html">Index</a></li></ul><p class="caption">arrows-0.4.4.1: Arrow classes and transformers</p></div><div id="content"><div id="module-header"><table class="info"><tr><th>Portability</th><td>non-portable (multi-parameter type classes)</td></tr><tr><th>Stability</th><td>experimental</td></tr><tr><th>Maintainer</th><td>ross@soi.city.ac.uk</td></tr><tr><th>Safe Haskell</th><td>Safe-Infered</td></tr></table><p class="caption">Control.Arrow.Transformer.Writer</p></div><div id="description"><p class="caption">Description</p><div class="doc"><p>Arrow transformer that adds accumulation of output.
</p></div></div><div id="synopsis"><p id="control.syn" class="caption expander" onclick="toggleSection('syn')">Synopsis</p><ul id="section.syn" class="hide" onclick="toggleSection('syn')"><li class="src short"><span class="keyword">newtype</span> <a href="#t:WriterArrow">WriterArrow</a> w a b c = <a href="#v:WriterArrow">WriterArrow</a> (a b (c, w))</li><li class="src short"><a href="#v:runWriter">runWriter</a> :: (<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:Arrow">Arrow</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a e b -> a e (b, w)</li><li class="src short"><span class="keyword">class</span> (<a href="Control-Arrow-Operations.html#t:ArrowWriter">ArrowWriter</a> w a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:Arrow">Arrow</a> a') => <a href="#t:ArrowAddWriter">ArrowAddWriter</a> w a a' | a -> a' <span class="keyword">where</span><ul class="subs"><li><a href="#v:liftWriter">liftWriter</a> :: a' e b -> a e b</li><li><a href="#v:elimWriter">elimWriter</a> :: a e b -> a' e (b, w)</li></ul></li></ul></div><div id="interface"><h1>Documentation</h1><div class="top"><p class="src"><span class="keyword">newtype</span> <a name="t:WriterArrow" class="def">WriterArrow</a> w a b c <a href="src/Control-Arrow-Transformer-Writer.html#WriterArrow" class="link">Source</a></p><div class="doc"><p>An arrow type that augments an existing arrow with accumulating
output. The <code><a href="Control-Arrow-Operations.html#t:ArrowWriter">ArrowWriter</a></code> class contains the relevant operations.
</p></div><div class="subs constructors"><p class="caption">Constructors</p><table><tr><td class="src"><a name="v:WriterArrow" class="def">WriterArrow</a> (a b (c, w))</td><td class="doc empty"> </td></tr></table></div><div class="subs instances"><p id="control.i:WriterArrow" class="caption collapser" onclick="toggleSection('i:WriterArrow')">Instances</p><div id="section.i:WriterArrow" class="show"><table><tr><td class="src">(<a href="Control-Arrow-Operations.html#t:ArrowError">ArrowError</a> ex a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="Control-Arrow-Operations.html#t:ArrowError">ArrowError</a> ex (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:Arrow">Arrow</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="Control-Arrow-Operations.html#t:ArrowWriter">ArrowWriter</a> w (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="Control-Arrow-Operations.html#t:ArrowState">ArrowState</a> s a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="Control-Arrow-Operations.html#t:ArrowState">ArrowState</a> s (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="Control-Arrow-Operations.html#t:ArrowReader">ArrowReader</a> r a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="Control-Arrow-Operations.html#t:ArrowReader">ArrowReader</a> r (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:Arrow">Arrow</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="Control-Arrow-Transformer-Writer.html#t:ArrowAddWriter">ArrowAddWriter</a> w (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a) a</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="Control-Arrow-Transformer-Error.html#t:ArrowAddError">ArrowAddError</a> ex a a', <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="Control-Arrow-Transformer-Error.html#t:ArrowAddError">ArrowAddError</a> ex (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a) (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a')</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="Control-Arrow-Transformer-Reader.html#t:ArrowAddReader">ArrowAddReader</a> r a a', <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="Control-Arrow-Transformer-Reader.html#t:ArrowAddReader">ArrowAddReader</a> r (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a) (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a')</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="Control-Arrow-Transformer-State.html#t:ArrowAddState">ArrowAddState</a> s a a', <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="Control-Arrow-Transformer-State.html#t:ArrowAddState">ArrowAddState</a> s (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a) (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a')</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:Arrow">Arrow</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="Control-Arrow-Transformer.html#t:ArrowTransformer">ArrowTransformer</a> (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w) a</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:Arrow">Arrow</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:Arrow">Arrow</a> (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:ArrowZero">ArrowZero</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:ArrowZero">ArrowZero</a> (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:ArrowPlus">ArrowPlus</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:ArrowPlus">ArrowPlus</a> (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:ArrowChoice">ArrowChoice</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:ArrowChoice">ArrowChoice</a> (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:ArrowApply">ArrowApply</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:ArrowApply">ArrowApply</a> (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:ArrowLoop">ArrowLoop</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:ArrowLoop">ArrowLoop</a> (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:Arrow">Arrow</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Category.html#t:Category">Category</a> (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="Control-Arrow-Operations.html#t:ArrowCircuit">ArrowCircuit</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="Control-Arrow-Operations.html#t:ArrowCircuit">ArrowCircuit</a> (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:Arrow">Arrow</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Monad.html#t:Functor">Functor</a> (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a b)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:Arrow">Arrow</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Applicative.html#t:Applicative">Applicative</a> (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a b)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:ArrowPlus">ArrowPlus</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Applicative.html#t:Alternative">Alternative</a> (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a b)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:ArrowPlus">ArrowPlus</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a b c)</td><td class="doc empty"> </td></tr></table></div></div></div><div class="top"><p class="src"><a name="v:runWriter" class="def">runWriter</a> :: (<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:Arrow">Arrow</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a e b -> a e (b, w)<a href="src/Control-Arrow-Transformer-Writer.html#runWriter" class="link">Source</a></p><div class="doc"><p>Encapsulation of a writer computation, providing the accumulated output.
</p><p>Typical usage in arrow notation:
</p><pre> proc p -> do
...
(result, output) <- (|runWriter cmd|)
</pre></div></div><div class="top"><p class="src"><span class="keyword">class</span> (<a href="Control-Arrow-Operations.html#t:ArrowWriter">ArrowWriter</a> w a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:Arrow">Arrow</a> a') => <a name="t:ArrowAddWriter" class="def">ArrowAddWriter</a> w a a' | a -> a' <span class="keyword">where</span><a href="src/Control-Arrow-Internals.html#ArrowAddWriter" class="link">Source</a></p><div class="doc"><p>Adding a <code><a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a></code> to an
arrow type, but not necessarily as the outer arrow transformer.
</p><p>Typically a composite arrow type is built by applying a series
of arrow transformer to a base arrow (usually either a function
arrow or a <code><a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:Kleisli">Kleisli</a></code> arrow. One can add a transformer to the
top of this stack using the <code><a href="Control-Arrow-Transformer.html#v:lift">lift</a></code>
method of the <code><a href="Control-Arrow-Transformer.html#t:ArrowTransformer">ArrowTransformer</a></code> class,
or remove a state transformer from the top of the stack using the
<code><a href="Control-Arrow-Transformer-Writer.html#v:runWriter">runWriter</a></code> encapsulation operator.
The methods of this class add and remove state transformers anywhere
in the stack. In the instance
</p><pre> instance Arrow a => ArrowAddWriter w (ArrowWriter w a) a
</pre><p>they are equivalent to <code><a href="Control-Arrow-Transformer.html#v:lift">lift</a></code> and
<code><a href="Control-Arrow-Transformer-Writer.html#v:runWriter">runWriter</a></code> respectively.
Instances are lifted through other transformers with
</p><pre> instance ArrowAddWriter w a a' =>
ArrowAddWriter w (FooArrow a) (FooArrow a')
</pre></div><div class="subs methods"><p class="caption">Methods</p><p class="src"><a name="v:liftWriter" class="def">liftWriter</a> :: a' e b -> a e b<a href="src/Control-Arrow-Internals.html#liftWriter" class="link">Source</a></p><div class="doc"><p>Lift a computation from an arrow to one with added output.
</p><p>Typical usage in arrow notation:
</p><pre> proc p -> ...
(|liftWriter cmd|)
</pre></div><p class="src"><a name="v:elimWriter" class="def">elimWriter</a> :: a e b -> a' e (b, w)<a href="src/Control-Arrow-Internals.html#elimWriter" class="link">Source</a></p><div class="doc"><p>Elimination of an output writer from a computation,
providing the accumulated output.
</p><p>Typical usage in arrow notation:
</p><pre> proc p -> do
...
(result, output) <- (|elimWriter cmd|)
</pre></div></div><div class="subs instances"><p id="control.i:ArrowAddWriter" class="caption collapser" onclick="toggleSection('i:ArrowAddWriter')">Instances</p><div id="section.i:ArrowAddWriter" class="show"><table><tr><td class="src"><a href="Control-Arrow-Transformer-Writer.html#t:ArrowAddWriter">ArrowAddWriter</a> w a a' => <a href="Control-Arrow-Transformer-Writer.html#t:ArrowAddWriter">ArrowAddWriter</a> w (<a href="Control-Arrow-Transformer-Automaton.html#t:Automaton">Automaton</a> a) (<a href="Control-Arrow-Transformer-Automaton.html#t:Automaton">Automaton</a> a')</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:Arrow">Arrow</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="Control-Arrow-Transformer-Writer.html#t:ArrowAddWriter">ArrowAddWriter</a> w (<a href="Control-Arrow-Transformer-Writer.html#t:WriterArrow">WriterArrow</a> w a) a</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="Control-Arrow-Transformer-Writer.html#t:ArrowAddWriter">ArrowAddWriter</a> w a a', <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:ArrowChoice">ArrowChoice</a> a, <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Arrow.html#t:ArrowChoice">ArrowChoice</a> a') => <a href="Control-Arrow-Transformer-Writer.html#t:ArrowAddWriter">ArrowAddWriter</a> w (<a href="Control-Arrow-Transformer-Error.html#t:ErrorArrow">ErrorArrow</a> ex a) (<a href="Control-Arrow-Transformer-Error.html#t:ErrorArrow">ErrorArrow</a> ex a')</td><td class="doc empty"> </td></tr><tr><td class="src"><a href="Control-Arrow-Transformer-Writer.html#t:ArrowAddWriter">ArrowAddWriter</a> w a a' => <a href="Control-Arrow-Transformer-Writer.html#t:ArrowAddWriter">ArrowAddWriter</a> w (<a href="Control-Arrow-Transformer-State.html#t:StateArrow">StateArrow</a> s a) (<a href="Control-Arrow-Transformer-State.html#t:StateArrow">StateArrow</a> s a')</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="Control-Arrow-Transformer-Writer.html#t:ArrowAddWriter">ArrowAddWriter</a> w a a', <a href="/usr/share/doc/ghc/html/libraries/base-4.5.0.0/Control-Applicative.html#t:Applicative">Applicative</a> f) => <a href="Control-Arrow-Transformer-Writer.html#t:ArrowAddWriter">ArrowAddWriter</a> w (<a href="Control-Arrow-Transformer-Static.html#t:StaticArrow">StaticArrow</a> f a) (<a href="Control-Arrow-Transformer-Static.html#t:StaticArrow">StaticArrow</a> f a')</td><td class="doc empty"> </td></tr><tr><td class="src"><a href="Control-Arrow-Transformer-Writer.html#t:ArrowAddWriter">ArrowAddWriter</a> s a a' => <a href="Control-Arrow-Transformer-Writer.html#t:ArrowAddWriter">ArrowAddWriter</a> s (<a href="Control-Arrow-Transformer-Reader.html#t:ReaderArrow">ReaderArrow</a> r a) (<a href="Control-Arrow-Transformer-Reader.html#t:ReaderArrow">ReaderArrow</a> r a')</td><td class="doc empty"> </td></tr></table></div></div></div></div></div><div id="footer"><p>Produced by <a href="http://www.haskell.org/haddock/">Haddock</a> version 2.10.0</p></div></body></html>
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