Turn back all ye who enter here, for the black magic of {-# LANGUAGE RebindableSyntax #-} is afoot. This is the worst code I've ever written. The f# code is probably more idiomatic?

EvilCode.hs

{-# LANGUAGE RebindableSyntax #-}

import Prelude hiding ((>>=), return)

main = mapM_ (putStrLn . fizzbuzz) [1..100]

fizzbuzz n = do
    fizz <- (3, "Fizz")
    buzz <- (5, "Buzz") 
    return $ fizz ++ buzz
  where (key, val) >>= f | rem n key == 0 = f val
        _          >>= f = f ""
        return "" = show n
        return s  = s

GoodCode.fs

type FizzBuzzBuilder(n) =
    member this.Bind((k, v), f) = 
        if n % k = 0 then v else "" |> f
    member this.Return(x) =
        if x = "" then n.ToString() else x

let fizzbuzz n =
           let b = new FizzBuzzBuilder(n)
           b {
             let! fizz = (3, "Fizz")
             let! buzz = (5, "Buzz")
             return (fizz + buzz)
           }

let main = [1..100] |> List.map (fizzbuzz >> printfn "%s")