Построить список

nxt :: String -> String
nxt (x:xs) | (x=='1')  = '0' : (nxt xs)
           | otherwise = '1' : xs
 
isU :: String -> Bool
isU "" = True
isU (x:xs) | x == '0'  = False
           | otherwise = isU xs
 
next :: String -> String
next s | s=="" = "0"
       | isU s = take (n+1) $ repeat '0'
       | otherwise = reverse $ nxt $ reverse s
         where n = length s
 
make = iterate next ""
 
Main> take 20 make
["","0","1","00","01","10","11","000","001","010","011","100","101","110","111","0000","0001","0010","0011","0100"
]

bits 0 = [""]
bits n = [b:bs | b <- ['0', '1'], bs <- bits (n-1) ]
 
make = concatMap bits [0..]

bits n = replicateM n "01"
make = concatMap bits [0..]

make = concat [replicateM n "01" | n <- [0..]]