Variation of an older version... Less computing (still roughly the same amount of time needed), but more memory usage.

Combinaties.hs

module Combinaties where

deleteFromHead :: Eq a => a -> [a] -> [a]
deleteFromHead x [] = []
deleteFromHead x (y:ys)
  | x == y     =  ys
  | otherwise  =  (y:ys)


generate :: Eq a => Int -> [a] -> [a] -> [[a]]

generate n values knowns = generate' n knowns
  
  where
    -- base cases
    generate' 0 []      =  [[]]
    generate' 0 knowns  =  []
    
    -- recursive cases
    generate' n []      =  [v : subsolution | subsolution <- generate' (n-1) [], v <- values]

    generate' n knowns@(k:ks)
      | n > m           =  [k : subsolution | subsolution <- generate' (n-1) ks]
                           ++
                           [v : subsolution | subsolution <- generate' (n-1) knowns, v <- values, v /= k]
      | otherwise       =  [knowns]
      where
        m = length knowns




digits = ['0'..'9']

test = generate 9 digits "783659"

-- length test == 64234  -- (0.05 secs, 13067840 bytes)