require! 'prelude-ls': {fold, sum}

# String → Number
decimal_of_roman = do
  # [Number, Number] → String → [Number, Number]
  _convert = ([acc, last_value], ch) ->
    current_value = { M:1000 D:500 C:100 L:50 X:10 V:5 I:1 }[ch] ? 0
    op = if last_value < current_value then (-) else (+)
    [op(acc, last_value), current_value]
  # fold the string and sum the resulting tuple (array)
  fold(_convert, [0, 0]) >> sum

{[rom, decimal_of_roman rom] for rom in <[ MCMXC MMVII MDCLXVII MMMCLIX MCMLXXVII MMX ]>}