A minor comment, but I’d reformat this a bit to make its structure clearer:

    fizzbuzz n = go id (show n)
      where
        go = test 3 "fizz" . test 5 "buzz"

        test d s x =
          if n `mod` d == 0 
          then const (s ++ x "")
          else x

Explanation:

If both tests fail, return the 'id' function which when applied to (show n) will return the number as a string.

If one or both tests succeed, return a 'const' function which will simply return the appropriate string and ignore the (show n).

This example was illuminating. It helped me realize I'm wasting too much time with Haskell.