> The following function does not compile, since the compiler thinks it has
> the wrong return type: 

You've already received plenty of explanations on what's going on.
Let me just add one easy way to work around this: just have your "try"
construct return () in all cases.  Like this:

>     let read_res file : ('a * 'b) list =
>       let final_result = ref [] in
>       let fd = open_in file in
>       begin try 
>         let lexbuf = Lexing.from_channel fd in
>         while true do
>           let result = Parseres.main Lexres.token lexbuf in
>           final_result := result :: !final_result 
>         done
>       with             
>         Lexres.Eof ->  ()             (* this is the "normal" loop exit *)
>       | Parsing.Parse_error ->          
>           close_in fd; printf "ERROR: Ressourcen Datei fehlerhaft\n";
>           exit (-1)
>       end;
>       close_in fd; !final_result

- Xavier Leroy