monads.r

REBOL [
  Title: "Monads"
  Author: "Sgeo"
]

do-monad: func [
  monad "An object containing a /bind and a /return."
  assigns [block!]
  result [block!]
  /local ma f next-assigns arg
][
  either empty? assigns [
    monad/return do result
  ][
    arg: first assigns
    if not set-word? arg [1 / 0]
    arg: to word! arg
    ma: do/next next assigns 'next-assigns
    f: func reduce [arg] reduce [:do-monad monad next-assigns result]
    monad/bind :ma :f
  ]
]

identity-m: context [
  bind: func [ma f] [f ma]
  return: func [a] [:a]
]

list-m: context [
  bind: func [ma f [any-function!] /local result] [
    result: copy []
    foreach a ma [
      append result f a 
    ]
    result
  ]
  return: func [a] [reduce [a]]
]