Haskell : how to insert test code (for a newbie)

I am asking some questions about haskell : how can I add some testing code inside my functions?

e.g. I tried to change:

peuple i =
  let parcours = ...

by:

peuple i =
  do putStrLn $ "entering peuple " ++ i
  let parcours = ...

but I get a parse error...