For certain reason, I had to modify Prolog interpreter written in Standard ML. I used Moscow ML implementation. Interpreter had structures which look like:

...
datatype Term = Var of string * pos

              | Fun of string * Term list * pos

type Atom = string * Term list

datatype Query = And of Query * Query * pos
               | Or of Query * Query * pos
...

So in order to print those structure, one can do that in the top level mode of Moscow ML. But this may be somewhat inconvenient. So I ended up writing my own PrettyPrinter module where I created separate data type for types I wanted to print:

datatype PrintType =
         Clause of Prolog.Clause
       | ClauseList of Prolog.Clause list
       | Atom of Prolog.Atom
       | Query of Prolog.Query
       | Term of Prolog.Term
       ...

And had following print and show functions:

fun show (Clause (atom, query)) =
		(show (Atom atom)) ^ " " ^ (show (Query query))
  | show (ClauseList cl) =
		"[" ^ (showList (ClauseList cl))  ^ "]"
  | show (Atom (atomName, terms)) =
  ...
and showList(Clause _) = ""             (* shouldn't happen *)
  | showList(ClauseList []) = ""
  | showList(ClauseList (c::cs)) =
		(show (Clause c)) ^ (getListSeparator cs) ^ (showList (ClauseList cs))
  ...

fun print pt = BasicIO.print (show pt ^ "\n")

Annoying.