gistfile1.vim

" Maybe Monad
let Maybe = {}

function! Maybe.return(m)
  return Just(a:m)
endfunction

" m(self) >>= f
function! Maybe.bind(f)
  let Func = type(a:f) == type('') ? function(a:f) : a:f
  return self['bind_' . self.type](Func)
endfunction
function! Maybe.bind_Just(f)
  return a:f(self.m)
endfunction
function! Maybe.bind_Nothing(_)
  return Nothing()
endfunction

function! Maybe.show()
  return self['show_' . self.type](self)
endfunction
function! Maybe.show_Just(m)
  return 'Maybe "' . a:m.m.show() . '"'
endfunction
function! Maybe.show_Nothing(_)
  return 'Nothing'
endfunction

function! Maybe.fail(_)
  return Nothing()
endfunction

function! Just(m)
  return extend(copy(g:Maybe), {'type': 'Just', 'm': a:m})
endfunction

function! Nothing()
  return extend(copy(g:Maybe), {'type': 'Nothing'})
endfunction



" -- Sheep
function! Sheep(name, mother, father)
  let sheep = {'name': a:name, 'mother': a:mother, 'father': a:father}
  function! sheep.show()  " Helper function
    return self.name
  endfunction
  return sheep
endfunction
function! Name(sheep)
  return a:sheep.name
endfunction
function! Mother(sheep)
  return a:sheep.mother
endfunction
function! Father(sheep)
  return a:sheep.father
endfunction



function! FmaternalGrandfather(sheep)
  return g:Maybe.return(a:sheep).bind('Mother').bind('Father')
endfunction
function! FfathersMaternalGrandmother(sheep)
  return g:Maybe.return(a:sheep).bind('Father').bind('Mother').bind('Mother')
endfunction
function! FmothersPaternalGrandfather(sheep)
  return g:Maybe.return(a:sheep).bind('Mother').bind('Father').bind('Father')
endfunction


let breedSheep = {}
let breedSheep.adam   = Sheep("Adam", Nothing(), Nothing())
let breedSheep.eve    = Sheep("Eve", Nothing(), Nothing())
let breedSheep.uranus = Sheep("Uranus", Nothing(), Nothing())
let breedSheep.gaea   = Sheep("Gaea", Nothing(), Nothing())
let breedSheep.kronos = Sheep("Kronos", Just(breedSheep.gaea), Just(breedSheep.uranus))
let breedSheep.holly  = Sheep("Holly", Just(breedSheep.eve), Just(breedSheep.adam))
let breedSheep.roger  = Sheep("Roger", Just(breedSheep.eve), Just(breedSheep.kronos))
let breedSheep.molly  = Sheep("Molly", Just(breedSheep.holly), Just(breedSheep.roger))

let dolly = Sheep("Dolly", Just(breedSheep.molly), Nothing())


echo FmaternalGrandfather(dolly).show()
echo FfathersMaternalGrandmother(dolly).show()
echo FmothersPaternalGrandfather(dolly).show()