scratch/content/html/en/blog/Higher-order-function-in-zsh.md

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isHidden:       false
menupriority:   1
kind:           article
created_at:     2011-09-27T15:15:23+02:00
title: Higher order function in zsh
author_name: Yann Esposito
author_uri: yannesposito.com
# tags:
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<%= blogimage("main.jpg","Title image") %>

begindiv(intro)

<%= tldr %> some simple implementation of higher order function for zsh.

enddiv




<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>


<code class="zsh">
gif_to_png() { convert $1 ${1:r}.png && \rm -f $1 }
handle_resources() { map gif_to_png $1/resources/*.gif(.N) }
map handle_resources /path/to/projects/*(/N)
</code>




Before ⇒

<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>

After ⇒

<code class="zsh">
contain_no_s() { print $1 | grep -v s }

function verify_file_name {                               
    local project=$1:t
    contains_project_name() { print $1:t | grep $project }
    map "print -- X" $(filter contains_project_name $1/*(.N))
}

map show_project_matchin_file $( filter contain_no_s Projects/* )
</code>

Also, the first verstion is a bit easier to read. 
But the second one is clearly far superior in architecture.
Why?

<code class="zsh" file="functional.sh">
#!/usr/bin/env zsh

# Provide higer-order functions 

# usage:
#
# $ foo(){print "x: $1"}
# $ map foo a b c d
# x: a
# x: b
# x: c
# x: d
function map {
    local func_name=$1
    shift
    for elem in $@; print -- $(eval $func_name $elem)
}

# $ bar() { print $(($1 + $2)) }
# $ fold bar 0 1 2 3 4 5
# 15
# -- but also
# $ fold bar 0 $( seq 1 100 )
function fold {
    if (($#<2)) {
        print -- "ERROR fold use at least 2 arguments" >&2
        return 1
    }
    if (($#<3)) {
        print -- $2
        return 0
    } else {
        local acc
        local right
        local func_name=$1
        local init_value=$2
        local first_value=$3
        shift 3
        right=$( fold $func_name $init_value $@ )
        acc=$( eval "$func_name $first_value $right" )
        print -- $acc
        return 0
    }
}

# usage:
#
# $ baz() { print $1 | grep baz }
# $ filter baz titi bazaar biz
# bazaar
function filter {
    local predicate=$1
    local result
    typeset -a result
    shift
    for elem in $@; do
        if eval $predicate $elem >/dev/null; then
            result=( $result $elem )
        fi
    done
    print $result
}
</code>