$ ghci
   ___         ___ _
  / _ \ /\  /\/ __(_)
 / /_\// /_/ / /  | |      GHC Interactive, version 6.4.2, for Haskell 98.
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type:? for help.

Loading package base-1.0 ... linking ... done.
Prelude> let f as bs sep = as ++ sep ++ sep ++ bs
Prelude> f "Rosetta" "Code" ":"
"Rosetta::Code"