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Higher order function in zsh
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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 >
< / div >
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< p > Why is it important to have these functions?
Simply because, the more I programmed with zsh the more I tended to work using functional programming style.< / p >
< 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 >
< p > Before ⇒< / p >
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< pre > < code class = "zsh" > # for each directory in projects dir
for toProject in /path/to/projects/*(/N); do
# toProject is /path/to/projects/foo
# project become foo (:t for tail)
project=${toProject:t}
for toResource in $toProject/resources/*.gif(.N); do
convert $toResource ${toResource:r}.png & & \
\rm -f $toResource
done
done
< / 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 >
< 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 >
< / 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 $1/resources/*.gif(.N) }
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map handle_resources /path/to/projects/*(/N)
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< / code > < / pre >
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< p > No more bloc!
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It might be a little bit harder to read if you’ re not used to functional programming notation.
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But it is more concise and robusts.< / p >
< p > Another example with some tests.< / p >
< p > Find all files in project not containing an < code > s< / code > which their name contains their project name:< / p >
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< p > Before ⇒< / p >
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< pre > < code class = "zsh" > for toProject in Projects/*; do
project=$toProject:t
if print -- project | grep -v s > /dev/null
then
print $project
for toResource in $toProject/*(.N); do
if print -- ${toResource:t} | grep $project > /dev/null; then
print -- "X $toResource"
fi
done
fi
done
< / 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
contains_project_name() { print $1:t | grep $project }
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/* )
< / code > < / pre >
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< p > Also, the first verstion is a bit easier to read.
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 >
< p > Actually I lack the lambda operator.
If someone has an idea on how to create anonymous functions, just tell me, thanks.< / p >
< 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
# usage:
#
# $ foo(){print "x: $1"}
# $ map foo a b c d
# x: a
# x: b
# x: c
# 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)) }
# $ fold bar 0 1 2 3 4 5
# 15
# -- but also
# $ fold bar 0 $( seq 1 100 )
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function fold {
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if (($#< 2)) {
print -- "ERROR fold use at least 2 arguments" > & 2
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return 1
}
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if (($#< 3)) {
print -- $2
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return 0
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} else {
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local acc
local right
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local func_name=$1
local init_value=$2
local first_value=$3
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shift 3
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right=$( fold $func_name $init_value $@ )
acc=$( eval "$func_name $first_value $right" )
print -- $acc
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return 0
}
}
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# usage:
#
# $ baz() { print $1 | grep baz }
# $ filter baz titi bazaar biz
# bazaar
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function filter {
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local predicate=$1
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local result
typeset -a result
shift
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for elem in $@; do
if eval $predicate $elem > /dev/null; then
result=( $result $elem )
fi
done
print $result
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}
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