-- 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 )

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