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Higher order function in zsh
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</h1>
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<p><img alt="Title image" src="/Scratch/img/blog/Higher-order-function-in-zsh/main.jpg" /></p>
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<div class="intro">
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<p><span class="sc"><abbr title="Too long; didn't read">tl;dr</abbr>: </span> some simple implementation of higher order function for zsh.</p>
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</div>
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<p>Why is it important to have these functions?
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Simply because, the more I programmed with zsh the more I tended to work using functional programming style.</p>
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<p>The minimal to have better code are the functions <code>map</code>, <code>filter</code> and <code>fold</code>.</p>
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<p>Let’s compare.
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First a program which convert all gif to png in many different directories of different projects.</p>
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<p>Before ⇒</p>
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<pre class="twilight">
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<span class="Comment"><span class="Comment">#</span> for each directory in projects dir</span>
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<span class="Keyword">for</span> toProject <span class="Keyword">in</span> /path/to/projects/*(/N)<span class="Keyword">;</span> <span class="Keyword">do</span>
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<span class="Comment"><span class="Comment">#</span> toProject is /path/to/projects/foo</span>
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<span class="Comment"><span class="Comment">#</span> project become foo (:t for tail)</span>
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project=<span class="Variable"><span class="Variable">${</span>toProject:t<span class="Variable">}</span></span>
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<span class="Keyword">for</span> toResource <span class="Keyword">in</span> <span class="Variable"><span class="Variable">$</span>toProject</span>/resources/*.gif(.N)<span class="Keyword">;</span> <span class="Keyword">do</span>
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convert <span class="Variable"><span class="Variable">$</span>toResource</span> <span class="Variable"><span class="Variable">${</span>toResource:r<span class="Variable">}</span></span>.png <span class="Keyword">&&</span> \
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<span class="Constant">\r</span>m -f <span class="Variable"><span class="Variable">$</span>toResource</span>
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<span class="Keyword">done</span>
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<span class="Keyword">done</span>
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</pre>
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<ul>
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<li>The <code>(/N)</code> means to select only directory and not to crash if there isn’t any.</li>
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<li>The <code>(.N)</code> means to select only files and not to crash if there isn’t any.</li>
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<li>The <code>:t</code> means tail; if <code>toto=/path/to/file.ext</code> then <code>${toto:t}=file.ext</code>.</li>
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</ul>
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<p>After ⇒</p>
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<pre class="twilight">
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<span class="Entity">gif_to_png</span>() { convert <span class="Variable"><span class="Variable">$</span>1</span> <span class="Variable"><span class="Variable">${</span>1:r<span class="Variable">}</span></span>.png <span class="Keyword">&&</span> <span class="Constant">\r</span>m -f <span class="Variable"><span class="Variable">$</span>1</span> }
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<span class="Entity">handle_resources</span>() { map gif_to_png <span class="Variable"><span class="Variable">$</span>1</span>/resources/*.gif(.N) }
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map handle_resources /path/to/projects/*(/N)
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</pre>
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<p>No more bloc!
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It might be a little bit harder to read if you’re not used to functional programming notation.
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But it is more concise and robusts.</p>
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<p>Another example with some tests.</p>
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<p>Find all files in project not containing an <code>s</code> which their name contains their project name:</p>
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<p>Before ⇒</p>
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<pre class="twilight">
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<span class="Keyword">for</span> toProject <span class="Keyword">in</span> Projects/*<span class="Keyword">;</span> <span class="Keyword">do</span>
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project=<span class="Variable"><span class="Variable">$</span>toProject</span>:t
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<span class="Keyword">if</span> print -- project <span class="Keyword">|</span> grep -v s <span class="Keyword">></span>/dev/null
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<span class="Keyword">then</span>
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print <span class="Variable"><span class="Variable">$</span>project</span>
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<span class="Keyword">for</span> toResource <span class="Keyword">in</span> <span class="Variable"><span class="Variable">$</span>toProject</span>/*(.N)<span class="Keyword">;</span> <span class="Keyword">do</span>
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<span class="Keyword">if</span> print -- <span class="Variable"><span class="Variable">${</span>toResource:t<span class="Variable">}</span></span> <span class="Keyword">|</span> grep <span class="Variable"><span class="Variable">$</span>project</span> <span class="Keyword">></span>/dev/null<span class="Keyword">;</span> <span class="Keyword">then</span>
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print -- <span class="String"><span class="String">"</span>X <span class="StringVariable"><span class="StringVariable">$</span>toResource</span><span class="String">"</span></span>
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<span class="Keyword">fi</span>
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<span class="Keyword">done</span>
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<span class="Keyword">fi</span>
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<span class="Keyword">done</span>
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</pre>
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<p>After ⇒</p>
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<pre class="twilight">
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<span class="Entity">contain_no_s</span>() { print <span class="Variable"><span class="Variable">$</span>1</span> <span class="Keyword">|</span> grep -v s }
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function verify_file_name {
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local project=<span class="Variable"><span class="Variable">$</span>1</span>:t
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<span class="Entity">contains_project_name</span>() { print <span class="Variable"><span class="Variable">$</span>1</span>:t <span class="Keyword">|</span> grep <span class="Variable"><span class="Variable">$</span>project</span> }
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map <span class="String"><span class="String">"</span>print -- X<span class="String">"</span></span> <span class="String"><span class="String">$(</span>filter contains_project_name <span class="StringVariable"><span class="StringVariable">$</span>1</span>/*<span class="String"><span class="String">(</span>.N<span class="String">)</span></span><span class="String">)</span></span>
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}
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map verify_file_name <span class="String"><span class="String">$(</span> filter contain_no_s Projects/* <span class="String">)</span></span>
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</pre>
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<p>Also, the first verstion is a bit easier to read.
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But the second one is clearly far superior in architecture.
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I don’t want to argue why here.
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Just believe me that the functional programming approach is superior.</p>
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<p>Actually I lack the lambda operator.
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If someone has an idea on how to create anonymous functions, just tell me, thanks.</p>
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<p>Here is the source code:</p>
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<div class="code"><div class="file"><a href="/Scratch/en/blog/Higher-order-function-in-zsh/code/functional.sh"> ➥ functional.sh </a></div><div class="withfile">
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<pre class="twilight">
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<span class="Comment"><span class="Comment">#</span>!/usr/bin/env zsh</span>
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<span class="Comment"><span class="Comment">#</span> Provide higer-order functions </span>
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<span class="Comment"><span class="Comment">#</span> usage:</span>
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<span class="Comment"><span class="Comment">#</span></span>
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<span class="Comment"><span class="Comment">#</span> $ foo(){print "x: $1"}</span>
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<span class="Comment"><span class="Comment">#</span> $ map foo a b c d</span>
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<span class="Comment"><span class="Comment">#</span> x: a</span>
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<span class="Comment"><span class="Comment">#</span> x: b</span>
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<span class="Comment"><span class="Comment">#</span> x: c</span>
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<span class="Comment"><span class="Comment">#</span> x: d</span>
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function map {
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local func_name=<span class="Variable"><span class="Variable">$</span>1</span>
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shift
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<span class="Keyword">for</span> elem <span class="Keyword">in</span> <span class="Variable"><span class="Variable">$</span>@</span><span class="Keyword">;</span> print -- <span class="String"><span class="String">$(</span>eval <span class="StringVariable"><span class="StringVariable">$</span>func_name</span> <span class="StringVariable"><span class="StringVariable">$</span>elem</span><span class="String">)</span></span>
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}
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<span class="Comment"><span class="Comment">#</span> $ bar() { print $(($1 + $2)) }</span>
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<span class="Comment"><span class="Comment">#</span> $ fold bar 0 1 2 3 4 5</span>
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<span class="Comment"><span class="Comment">#</span> 15</span>
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<span class="Comment"><span class="Comment">#</span> -- but also</span>
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<span class="Comment"><span class="Comment">#</span> $ fold bar 0 $( seq 1 100 )</span>
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function fold {
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<span class="Keyword">if</span> ((<span class="Variable"><span class="Variable">$</span>#</span><span class="Keyword"><</span>2)) {
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print -- <span class="String"><span class="String">"</span>ERROR fold use at least 2 arguments<span class="String">"</span></span> <span class="Keyword">>&2</span>
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return 1
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}
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<span class="Keyword">if</span> ((<span class="Variable"><span class="Variable">$</span>#</span><span class="Keyword"><</span>3)) {
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print -- <span class="Variable"><span class="Variable">$</span>2</span>
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return 0
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} <span class="Keyword">else</span> {
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local acc
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local right
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local func_name=<span class="Variable"><span class="Variable">$</span>1</span>
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local init_value=<span class="Variable"><span class="Variable">$</span>2</span>
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local first_value=<span class="Variable"><span class="Variable">$</span>3</span>
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shift 3
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right=<span class="String"><span class="String">$(</span> fold <span class="StringVariable"><span class="StringVariable">$</span>func_name</span> <span class="StringVariable"><span class="StringVariable">$</span>init_value</span> <span class="StringVariable"><span class="StringVariable">$</span>@</span> <span class="String">)</span></span>
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acc=<span class="String"><span class="String">$(</span> eval <span class="String"><span class="String">"</span><span class="StringVariable"><span class="StringVariable">$</span>func_name</span> <span class="StringVariable"><span class="StringVariable">$</span>first_value</span> <span class="StringVariable"><span class="StringVariable">$</span>right</span><span class="String">"</span></span> <span class="String">)</span></span>
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print -- <span class="Variable"><span class="Variable">$</span>acc</span>
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return 0
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}
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}
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<span class="Comment"><span class="Comment">#</span> usage:</span>
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<span class="Comment"><span class="Comment">#</span></span>
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<span class="Comment"><span class="Comment">#</span> $ baz() { print $1 | grep baz }</span>
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<span class="Comment"><span class="Comment">#</span> $ filter baz titi bazaar biz</span>
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<span class="Comment"><span class="Comment">#</span> bazaar</span>
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function filter {
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local predicate=<span class="Variable"><span class="Variable">$</span>1</span>
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local result
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typeset -a result
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shift
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<span class="Keyword">for</span> elem <span class="Keyword">in</span> <span class="Variable"><span class="Variable">$</span>@</span><span class="Keyword">;</span> <span class="Keyword">do</span>
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<span class="Keyword">if</span> eval <span class="Variable"><span class="Variable">$</span>predicate</span> <span class="Variable"><span class="Variable">$</span>elem</span> <span class="Keyword">></span>/dev/null<span class="Keyword">;</span> <span class="Keyword">then</span>
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result=( <span class="Variable"><span class="Variable">$</span>result</span> <span class="Variable"><span class="Variable">$</span>elem</span> )
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<span class="Keyword">fi</span>
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<span class="Keyword">done</span>
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print <span class="Variable"><span class="Variable">$</span>result</span>
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}
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</pre>
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<a rel="license" href="http://creativecommons.org/licenses/by-sa/3.0/">Copyright ©, Yann Esposito</a>
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Created: 09/28/2011
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Modified: 10/26/2011
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