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<h1>
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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><code class="zsh"># for each directory in projects dir
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for toProject in /path/to/projects/*(/N); do
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# toProject is /path/to/projects/foo
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# project become foo (:t for tail)
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project=${toProject:t}
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for toResource in $toProject/resources/*.gif(.N); do
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convert $toResource ${toResource:r}.png && \
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\rm -f $toResource
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done
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done
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</code></pre>
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<ul>
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<li>The <code>(/N)</code> means to select only directory and not to crash if there isn’t any.</li>
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<li>The <code>(.N)</code> means to select only files and not to crash if there isn’t any.</li>
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<li>The <code>:t</code> means tail; if <code>toto=/path/to/file.ext</code> then <code>${toto:t}=file.ext</code>.</li>
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</ul>
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<p>After ⇒</p>
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<pre><code class="zsh">gif_to_png() { convert $1 ${1:r}.png && \rm -f $1 }
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handle_resources() { map gif_to_png $1/resources/*.gif(.N) }
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map handle_resources /path/to/projects/*(/N)
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</code></pre>
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<p>No more bloc!
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It might be a little bit harder to read if you’re not used to functional programming notation.
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But it is more concise and robusts.</p>
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<p>Another example with some tests.</p>
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<p>Find all files in project not containing an <code>s</code> which their name contains their project name:</p>
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<p>Before ⇒</p>
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<pre><code class="zsh">for toProject in Projects/*; do
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project=$toProject:t
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if print -- project | grep -v s >/dev/null
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then
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print $project
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for toResource in $toProject/*(.N); do
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if print -- ${toResource:t} | grep $project >/dev/null; then
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print -- "X $toResource"
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fi
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done
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fi
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done
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</code></pre>
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<p>After ⇒</p>
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<pre><code class="zsh">contain_no_s() { print $1 | grep -v s }
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function verify_file_name {
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local project=$1:t
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contains_project_name() { print $1:t | grep $project }
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map "print -- X" $(filter contains_project_name $1/*(.N))
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}
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map verify_file_name $( filter contain_no_s Projects/* )
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</code></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="codefile"><a href="/Scratch/en/blog/Higher-order-function-in-zsh/code/functional.sh">➥ functional.sh</a></div>
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<pre><code class="zsh">#!/usr/bin/env zsh
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# Provide higer-order functions
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# usage:
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#
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# $ foo(){print "x: $1"}
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# $ map foo a b c d
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# x: a
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# x: b
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# x: c
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# x: d
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function map {
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local func_name=$1
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shift
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for elem in $@; print -- $(eval $func_name $elem)
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}
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# $ bar() { print $(($1 + $2)) }
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# $ fold bar 0 1 2 3 4 5
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# 15
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# -- but also
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# $ fold bar 0 $( seq 1 100 )
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function fold {
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if (($#<2)) {
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print -- "ERROR fold use at least 2 arguments" >&2
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return 1
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}
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if (($#<3)) {
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print -- $2
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return 0
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} else {
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local acc
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local right
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local func_name=$1
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local init_value=$2
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local first_value=$3
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shift 3
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right=$( fold $func_name $init_value $@ )
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acc=$( eval "$func_name $first_value $right" )
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print -- $acc
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return 0
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}
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}
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# usage:
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#
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# $ baz() { print $1 | grep baz }
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# $ filter baz titi bazaar biz
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# bazaar
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function filter {
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local predicate=$1
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local result
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typeset -a result
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shift
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for elem in $@; do
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if eval $predicate $elem >/dev/null; then
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result=( $result $elem )
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fi
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done
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print $result
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}
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</code></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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<div id="lastmod">
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Created: 09/28/2011
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Modified: 04/26/2012
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</div>
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<div>
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Entirely done with
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<a href="http://www.vim.org">Vim</a>
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and
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<a href="http://nanoc.stoneship.org">nanoc</a>
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