Functional programming...in Bash

wtfp.sh

#!/usr/bin/env bash

map() {
  local fn="$1"

  local input
  while read -r input; do
    "${fn}" "${input}"
  done
}

reduce() {
  local fn="$1"
  local init="$2"

  local input
  local output="${init}"

  while read -r input; do
    output="$("${fn}" "${output}" "${input}")"
  done

  echo "${output}"
}

filter() {
  local fn="$1"

  local input
  while read -r input; do
    "${fn}" "${input}" && echo "${input}"
  done
}

add() {
  echo $(( $1 + $2 ))
}

multiply() {
  echo $(( $1 * $2 ))
}

concat() {
  echo "${1}${2}"
}

increment() {
  echo $(( $1 + 1 ))
}

square() {
  echo $(( $1 * $1 ))
}

even() {
  return $(( $1 % 2 ))
}

sum() {
  reduce add 0
}

map increment | map square | filter even | sum