Basic functional programming in defining map, reduce, and select.

basic_fp.rb

# Do note I'm being very purposely verbose on some of these for the sake of clarity

map = -> list, fn {
  if list.empty?
    []
  else
    head, *tail = list
    
    [fn.call(head)].concat map.call(tail, fn)
  end
}

# [3] pry(main)> map.call([1,2,3], -> x { x * 2 })
# => [2, 4, 6]

reduce = -> list, fn, accumulator = nil {
  if list.empty?
    accumulator
  else
    head, *tail = list
    
    reduce.call(tail, fn, fn.call(accumulator, head))
  end
}

# [5] pry(main)> reduce.call([1,2,3], -> acc, i { acc + i }, 0)
# => 6

select = -> list, fn {
  if list.empty?
    []
  else
    head, *tail = list
    
    (fn.call(head) ? [head] : []).concat(select.call(tail, fn))
  end
}

# [7] pry(main)> select.call([1,2,3], -> i { i.even? })
# => [2]