[Haskell-cafe] Markov chain algorithm (from "The Practice of Programming", Kernighan/Pike)

It's hard to improve on a 20 line Awk program for generating text but I thought it would be fun to investigate a Haskell solution. Why can't I cons an element onto an existing list? Michael Prelude Data.List Data.Map> insertWith (:) ("Moby", "Dick") "will" (fromList [(("Joe", "Blow"),["is"]), (("Moby", "Dick"),["may"])]) <interactive>:1:11:    Occurs check: cannot construct the infinite type: a = [a]      Expected type: a      Inferred type: [a]    In the first argument of `insertWith', namely `(:)'    In the expression:        insertWith          (:)          ("Moby", "Dick")          "will"          (fromList [(("Joe", "Blow"), ["is"]), (("Moby", "Dick"), ["may"])])Prelude Data.List Data.Map>