I've got a little problem i don't understand. I need to calculate an index for a list (by use of !!). It should look like this: floor(j * (n / p2n)) where j is an Integer, n is the length of a list, therefore Integer too, and p2n is 2^n. When i use the former line in my Haskell code (pasted at the end of this mail), i get the following error message: qc.lhs:464:16: No instance for (RealFrac Int) arising from use of `floor' at qc.lhs:464:16-36 Possible fix: add an instance declaration for (RealFrac Int) In the expression: floor (j * (n / p2n)) In the definition of `i': i = floor (j * (n / p2n)) In the definition of `genUf'': genUf' f j n | j == p2n = [] | otherwise = [(replicate (j + (y1 - y0)) 0) ++ ([1] ++ (replicate (p2n - ((j + (y1 - y0)) + 1)) 0))] ++ (genUf' f (j + 1) n) where y0 = mod j 2 y1 = xor y0 (f !! i) xor a b = mod (a + b) 2 p2n = (2 ^ n) i = floor (j * (n / p2n)) Failed, modules loaded: none. ------------------------------------------------------------------------ genUf :: [Int] -> QOp genUf f = (QOp(genUf' f 0 (length f))) where p2n = 2 ^ (length f) {- ok -} genUf' f j n | j == p2n = [] {- ok -} | otherwise = [(replicate (j+(y1-y0)) 0) ++ [1] ++ (replicate (p2n-(j+(y1-y0)+1)) 0)] ++ (genUf' f (j+1) n) where y0 = mod j 2 y1 = xor y0 (f!!i) xor a b = mod (a+b) 2 p2n = (2 ^ n) i = floor (j * (n / p2n))