-- Convert numbers to Roman

-- Sometimes, complicated if and case constructs can be encoded as data,
-- thus leaving the algorithm itself a simple 'map' operation
-- In the following example, the variable 'digitPattern' encodes the
-- 'subtraction rule' and 'five rule' of the roman number system
-- which otherwise would have to be encoded with tedious 'if' and 'case'

-- See jsfiddle.net / rplantiko/ehL5qwpq/ for the original JS version

toRoman n = 
  let 
    digitPattern = [ [],    [0],   [0,0],   [0,0,0], [0,1],
                     [1], [1,0], [1,0,0], [1,0,0,0], [0,2]]
    symbols = "IVXLCDMↁↂ"
    r( n, s ) | n < 10   = map (\x->symbols!!(x+2*s)) (digitPattern!!n)  
              | n >= 10  = r( div n 10, s+1) ++ r( mod n 10, s)
  in r(n,0)            

main = print(toRoman 3896 )