Functional Programming in Bash

composition_function.sh

#!/bin/bash

# Function 1: Convert text to uppercase
to_uppercase() {
  echo "$1" | tr '[:lower:]' '[:upper:]'
}

# Function 2: Add a prefix to the text
add_prefix() { 
  echo "Prefix $1"
}

# Function 3: Display the final text
display_text() {
  echo "Final text: $1"
}

# Composition of functions
compose_functions() {
  local result="$1"
  shift
  for func in "$@"; do
    result="$($func "$result")"
  done
  echo "$result"
}

# Using function composition
text="example text"

result=$(compose_functions "$text" to_uppercase add_prefix display_text)

echo "$result"
# Output: Final text: Prefix EXAMPLE TEXT

higher_order_functions.sh

#!/bin/bash

# Higher-order function to apply a given function to each element of an array
map() {
  local func=$1
  local array=("${@:2}")
  local result=()

  for element in "${array[@]}"; do
    result+=("$("$func" "$element")")
  done

  echo "${result[@]}"
}

# Example usage
square() {
  local num=$1
  echo $((num * num))
}

array=(1 2 3 4 5)
result=($(map square "${array[@]}"))
echo "${result[@]}"
# Output: 1 4 9 16 25

immutability.sh

#!/bin/bash

my_function() {
  local var="Local"
  local -r read_only_var="Read-only"

  var="Modified"            # Modifying a local variable
  read_only_var="Modified"  # Attempting to modify an immutable variable
}

my_function
# Output: bash: read_only_var: readonly variable

lazy_evaluation_function.sh

#!/bin/bash

# Lazy function: Calculates and returns the list of even numbers up to a certain threshold
get_even_numbers_lazy() {
  local threshold=$1
  local numbers=()
  local current=0

  while (( current < threshold )); do
    numbers+=($current)
    current=$((current + 2))
  done

  echo "${numbers[@]}"
}

# Using the lazy function
numbers=$(get_even_numbers_lazy 10)

echo "Even numbers up to 10: ${numbers[@]}"
# Output: Even numbers up to 10: 0 2 4 6 8

pure_function.sh

#!/bin/bash

# Pure function to calculate the square of a number
square() {
  local num=$1
  echo $((num * num))
}

# Example usage
result=$(square 2)

echo "$result"
# Output: 4

recursion.sh

#!/bin/bash

# Recursive function to calculate the factorial of a number
factorial() {
  local num=$1

  if ((num <= 1)); then
    echo 1
  else
    local sub_factorial=$(factorial $((num - 1)))
    echo $((num * sub_factorial))
  fi
}

# Example usage
echo $(factorial 5)
# Output: 120