(* File mosml/examples/pretty/ppexpr.sml  * Example demonstrating installable prettyprinters. * * Try it out by running  *	mosml ppexpr.sml * from the command line. *) val _ = (load "PP"; load "Int"); datatype expr =     Cst of int   | Neg of expr  | Plus of expr * expr fun ppexpr pps e0 =     let open PP        fun ppe (Cst i)        = add_string pps (Int.toString i)          | ppe (Neg e)        = (add_string pps "~"; ppe e)          | ppe (Plus(e1, e2)) = (begin_block pps CONSISTENT 0;                                  add_string pps "(";                                  ppe e1;                                   add_string pps " + ";                                  add_break pps (0, 1);                                  ppe e2;                                   add_string pps ")";                                  end_block pps)    in        begin_block pps INCONSISTENT 0;         ppe e0;        end_block pps    end val e1 = Cst 1;val e2 = Cst 2;val e3 = Plus(e1, Neg e2);val e4 = Plus(Neg e3, e3);val e5 = Plus(Neg e4, e4);val e6 = Plus(e5, e5);val e7 = Plus(e6, e6);val e8 = Plus(e3, Plus(e3, Plus(e3, Plus(e3, Plus(e3, Plus(e3, e7)))))); val _ = installPP ppexpr; val _ = print     "\nNow a pretty-printer for arithmetic expressions has been installed.\n\     \To see it in action, evaluate e.g.\n\     \  e1;\n  e3;\n  Plus(e1,e3);\n  e5;\n  e6;\n  e7;\n";