https://wiki.haskell.org/index.php?title=Difference_list&feed=atom&action=history
Difference list - Revision history
2024-03-28T19:52:36Z
Revision history for this page on the wiki
MediaWiki 1.35.5
https://wiki.haskell.org/index.php?title=Difference_list&diff=62571&oldid=prev
Enoksrd: /* Difference lists as functions */ Remove "of the second sort" (made no sense out of context of Wikipedia article it was copy-pasted from), and give code in addition to prose.
2018-08-01T16:40:02Z
<p><span dir="auto"><span class="autocomment">Difference lists as functions: </span> Remove "of the second sort" (made no sense out of context of Wikipedia article it was copy-pasted from), and give code in addition to prose.</span></p>
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<td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">Revision as of 16:40, 1 August 2018</td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>== Difference lists as functions ==</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>== Difference lists as functions ==</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td class="diff-marker">−</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>A difference list of <del class="diffchange diffchange-inline">the</del> <del class="diffchange diffchange-inline">second</del> <del class="diffchange diffchange-inline">sort</del> <del class="diffchange diffchange-inline">represents</del> <del class="diffchange diffchange-inline">lists</del> <del class="diffchange diffchange-inline">as</del> a function <hask>f</hask>, which when given <del class="diffchange diffchange-inline">a</del> list <hask><del class="diffchange diffchange-inline">x</del></hask>, returns the list that <hask>f</hask> represents, prepended to <hask><del class="diffchange diffchange-inline">x</del></hask>.</div></td>
<td class="diff-marker">+</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>A difference list<ins class="diffchange diffchange-inline"> representation</ins> of <ins class="diffchange diffchange-inline">a</ins> <ins class="diffchange diffchange-inline">list</ins> <ins class="diffchange diffchange-inline"><hask>xs</ins> <ins class="diffchange diffchange-inline">::</ins> <ins class="diffchange diffchange-inline">[T]</hask></ins> <ins class="diffchange diffchange-inline">is</ins> a function <hask>f<ins class="diffchange diffchange-inline"> :: [T] -> [T]</ins></hask>, which when given <ins class="diffchange diffchange-inline">another</ins> list <hask><ins class="diffchange diffchange-inline">ys :: [T]</ins></hask>, returns the list that <hask>f</hask> represents, prepended to <hask><ins class="diffchange diffchange-inline">ys</hask>. I.e. <hask>f ys = xs ++ ys</ins></hask>.</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Whether this kind of difference list is more efficient than another list representations depends on usage patterns.</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Whether this kind of difference list is more efficient than another list representations depends on usage patterns.</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>If an algorithm builds a list by concatenating smaller lists, which are themselves built by concatenating still smaller lists,</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>If an algorithm builds a list by concatenating smaller lists, which are themselves built by concatenating still smaller lists,</div></td>
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Enoksrd
https://wiki.haskell.org/index.php?title=Difference_list&diff=61675&oldid=prev
Jeffstyr: /* Examples */ Update hackage URLs
2017-03-28T13:46:04Z
<p><span dir="auto"><span class="autocomment">Examples: </span> Update hackage URLs</span></p>
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<td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">Revision as of 13:46, 28 March 2017</td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* [[ShowS]] type in the Prelude of Haskell</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* [[ShowS]] type in the Prelude of Haskell</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* The Endo monoid from <hask>Data.Monoid</hask> allows a simple difference list implementation. E.g. <hask>Endo ("bla"++)</hask> represents the list <hask>"bla"</hask>, <hask>mempty</hask> represents the empty list, <hask>mappend</hask> is list concatenation and <hask>appEndo dlist []</hask> converts the difference list <hask>dlist</hask> to a regular list.</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* The Endo monoid from <hask>Data.Monoid</hask> allows a simple difference list implementation. E.g. <hask>Endo ("bla"++)</hask> represents the list <hask>"bla"</hask>, <hask>mempty</hask> represents the empty list, <hask>mappend</hask> is list concatenation and <hask>appEndo dlist []</hask> converts the difference list <hask>dlist</hask> to a regular list.</div></td>
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<td class="diff-marker">−</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* Donald Bruce Stewart's [http://hackage.haskell.org<del class="diffchange diffchange-inline">/cgi-bin/hackage-scripts</del>/package/dlist difference list library]</div></td>
<td class="diff-marker">+</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* Donald Bruce Stewart's [http://hackage.haskell.org/package/dlist difference list library]</div></td>
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<td class="diff-marker">−</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* Bas van Dijk's [http://hackage.haskell.org<del class="diffchange diffchange-inline">/cgi-bin/hackage-scripts</del>/package/dstring difference strings library] (which is just a newtype wrapper around a DList Char)</div></td>
<td class="diff-marker">+</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* Bas van Dijk's [http://hackage.haskell.org/package/dstring difference strings library] (which is just a newtype wrapper around a DList Char)</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>== See also ==</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>== See also ==</div></td>
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Jeffstyr
https://wiki.haskell.org/index.php?title=Difference_list&diff=45370&oldid=prev
Killy9999 at 11:24, 22 April 2012
2012-04-22T11:24:22Z
<p></p>
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<td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">Revision as of 11:24, 22 April 2012</td>
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<td colspan="2" class="diff-lineno">Line 6:</td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>then use of difference lists can improve performance by effectively "flattening" the list building computations.</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>then use of difference lists can improve performance by effectively "flattening" the list building computations.</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td class="diff-marker">−</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>This can best be exemplified by <hask>show</hask> and <hask>shows</hask> of Prelude,</div></td>
<td class="diff-marker">+</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>This can best be exemplified by <hask>show</hask> and <hask>shows</hask> of Prelude,<ins class="diffchange diffchange-inline"> where the first one implements the naive approach and the second one uses difference lists.</ins></div></td>
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<td class="diff-marker">−</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>where the first one implements the naive approach and the second one uses difference lists.</div></td>
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<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Consider showing a binary tree.</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Consider showing a binary tree.</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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Killy9999
https://wiki.haskell.org/index.php?title=Difference_list&diff=37556&oldid=prev
Ollanta at 10:54, 25 November 2010
2010-11-25T10:54:52Z
<p></p>
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<td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">Revision as of 10:54, 25 November 2010</td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* <hask> shows L . (shows T . shows R) </hask></div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* <hask> shows L . (shows T . shows R) </hask></div></td>
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<td class="diff-marker">−</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* <hask> (shows LL . (<del class="diffchange diffchange-inline">showsLT</del> . shows LR)) . (shows T . shows R) </hask></div></td>
<td class="diff-marker">+</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* <hask> (shows LL . (<ins class="diffchange diffchange-inline">shows LT</ins> . shows LR)) . (shows T . shows R) </hask></div></td>
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<td class="diff-marker">−</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* <hask> ((shows LLL . (shows LLT . shows LLR)) . (<del class="diffchange diffchange-inline">showsLT</del> . shows LR)) . (shows T . shows R) </hask></div></td>
<td class="diff-marker">+</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* <hask> ((shows LLL . (shows LLT . shows LLR)) . (<ins class="diffchange diffchange-inline">shows LT</ins> . shows LR)) . (shows T . shows R) </hask></div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>You still need to resolve three (.) until you get to the first character of the result string,</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>You still need to resolve three (.) until you get to the first character of the result string,</div></td>
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Ollanta
https://wiki.haskell.org/index.php?title=Difference_list&diff=37555&oldid=prev
Ollanta at 10:53, 25 November 2010
2010-11-25T10:53:45Z
<p></p>
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<td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">Revision as of 10:53, 25 November 2010</td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div><hask>L-T-R</hask> gives</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div><hask>L-T-R</hask> gives</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* <hask> (show L) ++ (show T ++ (show R)) </hask></div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* <hask> (show L) ++ (show T ++ (show R)) </hask></div></td>
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<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* <hask> ((show LL) ++ (<del class="diffchange diffchange-inline">showLT</del> ++ (show LR))) ++ (show T ++ (show R)) </hask></div></td>
<td class="diff-marker">+</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* <hask> ((show LL) ++ (<ins class="diffchange diffchange-inline">show LT</ins> ++ (show LR))) ++ (show T ++ (show R)) </hask></div></td>
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<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* <hask> (((show LLL) ++ (show LLT ++ (show LLR))) ++ (show LT ++ (show LR))) ++ (show T ++ (show R)) </hask></div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* <hask> (((show LLL) ++ (show LLT ++ (show LLR))) ++ (show LT ++ (show LR))) ++ (show T ++ (show R)) </hask></div></td>
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Ollanta
https://wiki.haskell.org/index.php?title=Difference_list&diff=28069&oldid=prev
Lemming: Endo monoid
2009-04-28T23:18:51Z
<p>Endo monoid</p>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* [[ShowS]] type in the Prelude of Haskell</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* [[ShowS]] type in the Prelude of Haskell</div></td>
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<td class="diff-marker">+</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* The Endo monoid from <hask>Data.Monoid</hask> allows a simple difference list implementation. E.g. <hask>Endo ("bla"++)</hask> represents the list <hask>"bla"</hask>, <hask>mempty</hask> represents the empty list, <hask>mappend</hask> is list concatenation and <hask>appEndo dlist []</hask> converts the difference list <hask>dlist</hask> to a regular list.</div></td>
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<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* Donald Bruce Stewart's [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dlist difference list library]</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* Donald Bruce Stewart's [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dlist difference list library]</div></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* Bas van Dijk's [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dstring difference strings library] (which is just a newtype wrapper around a DList Char)</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* Bas van Dijk's [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dstring difference strings library] (which is just a newtype wrapper around a DList Char)</div></td>
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<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>== See also ==</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>== See also ==</div></td>
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</table>
Lemming
https://wiki.haskell.org/index.php?title=Difference_list&diff=26868&oldid=prev
Basvandijk: Added reference to dstring
2009-03-05T14:26:57Z
<p>Added reference to dstring</p>
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<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* [[ShowS]] type in the Prelude of Haskell</div></td>
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<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* Donald Bruce Stewart's [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dlist difference list library]</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* Donald Bruce Stewart's [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dlist difference list library]</div></td>
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<td class="diff-marker">+</td>
<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* Bas van Dijk's [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dstring difference strings library] (which is just a newtype wrapper around a DList Char)</div></td>
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Basvandijk
https://wiki.haskell.org/index.php?title=Difference_list&diff=18071&oldid=prev
Lemming: Wikipedia
2008-01-04T10:05:46Z
<p>Wikipedia</p>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* Haskell-Cafe: [http://www.haskell.org/pipermail/haskell-cafe/2008-January/037405.html Difference lists and ShowS]</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* Haskell-Cafe: [http://www.haskell.org/pipermail/haskell-cafe/2008-January/037405.html Difference lists and ShowS]</div></td>
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<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* Wikipedia on [http://en.wikipedia.org/wiki/Difference_list Difference lists]</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>[[Category:Idioms]]</div></td>
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Lemming
https://wiki.haskell.org/index.php?title=Difference_list&diff=18060&oldid=prev
Lemming: extract from Haskell-Cafe session
2008-01-03T15:39:16Z
<p>extract from Haskell-Cafe session</p>
<p><b>New page</b></p><div>== Difference lists as functions ==<br />
<br />
A difference list of the second sort represents lists as a function <hask>f</hask>, which when given a list <hask>x</hask>, returns the list that <hask>f</hask> represents, prepended to <hask>x</hask>.<br />
Whether this kind of difference list is more efficient than another list representations depends on usage patterns.<br />
If an algorithm builds a list by concatenating smaller lists, which are themselves built by concatenating still smaller lists,<br />
then use of difference lists can improve performance by effectively "flattening" the list building computations.<br />
<br />
This can best be exemplified by <hask>show</hask> and <hask>shows</hask> of Prelude,<br />
where the first one implements the naive approach and the second one uses difference lists.<br />
Consider showing a binary tree.<br />
<br />
<hask>L-T-R</hask> gives<br />
* <hask> (show L) ++ (show T ++ (show R)) </hask><br />
* <hask> ((show LL) ++ (showLT ++ (show LR))) ++ (show T ++ (show R)) </hask><br />
* <hask> (((show LLL) ++ (show LLT ++ (show LLR))) ++ (show LT ++ (show LR))) ++ (show T ++ (show R)) </hask><br />
<br />
If the tree is large, you end up with a pretty large left association for the left subtree.<br />
True, there's lot of right association, too, but bad enough.<br />
<br />
With difference lists you write<br />
<br />
* <hask> shows L . (shows T . shows R) </hask><br />
* <hask> (shows LL . (showsLT . shows LR)) . (shows T . shows R) </hask><br />
* <hask> ((shows LLL . (shows LLT . shows LLR)) . (showsLT . shows LR)) . (shows T . shows R) </hask><br />
<br />
You still need to resolve three (.) until you get to the first character of the result string,<br />
but for the subsequent characters you do not need to resolve those dots.<br />
In the end, resolution of all (.) may need some time but then concatenation is performed entirely right-associative.<br />
<br />
== Examples ==<br />
<br />
* [[ShowS]] type in the Prelude of Haskell<br />
* Donald Bruce Stewart's [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dlist difference list library]<br />
<br />
== See also ==<br />
<br />
* Haskell-Cafe: [http://www.haskell.org/pipermail/haskell-cafe/2008-January/037405.html Difference lists and ShowS]<br />
<br />
[[Category:Idioms]]</div>
Lemming