I don't know what is wrong with the code. I have just started coding in Haskell so sorry for the trouble.
tobase :: Int -> Int -> [Int]
tobase b x = tobase b (x `quot` b) : [x `mod` b]
tobase b x | x `mod` b == x = [x]
Couldn't match expected type ‘[Int]’ with actual type ‘Int’
In the expression: x `mod` b
In the second argument of ‘(:)’, namely ‘[x `mod` b]’
In the expression: tobase b (x `quot` b) : [x `mod` b]
As Willem has already indicated, the reason for the error is that you are trying to combine values of the wrong type via the "cons" operator (:).
This operator's signature is (:) :: a -> [a] -> [a] , not (:) :: [a] -> [a] -> [a], which is what your call is attempting.
Your tobase function returns type [Int], and it's the first argument passed to the : operator. Since the type variable a in (:) :: a -> [a] -> [a] has this [Int] type in your invocation, the compiler expects the second argument to be [[Int]]. However you are passing in [x `mod` b] which is of type [Int].
Instead of using : you can use ++ to combine the two lists.
As also noted, the pattern matching order in your function definition should be reversed because the second case tobase b x | x `mod` b == x = [x] will never be reached. See http://learnyouahaskell.com/syntax-in-functions
tobase b (x `quot` b)will generate a list, not anIntitself, hence the type error.