/*************************************************************************

         name: sformat_as_lisp.pl
      version: 0.5
  description: Format lists as Lisp lists, usually for output to Lisp.
       author: Tom Breton
 
*************************************************************************/

:- module(sformat_as_lisp,[sformat_as_lisp/2]).

/*========================================================================
    Format lists as lisp lists, recursively.
========================================================================*/
%%This works under SWI prolog and should work under Sicstus.  It will
%%probably NOT work under any strictly Edinburgh prolog, because it
%%uses (Sisctus style) sformat, not (Edinburgh style) swritef.

%%Handle heterogeneous list-structures.

%Single elements except [] are simply formatted with ~w.  
sformat_as_lisp(S,Element):-
	(atomic(Element), \+ Element=[] ; var(Element)),
	sformat(S,"~w",[Element]),!.

%Lists including [] are surrounded by "()"
sformat_as_lisp(S,List):-
	is_list(List),
	sformat_list_innards(Innards,List),
	sformat(S,"(~s)~n",[Innards]),!.


%%Handle the innards of a single list.	

%No elements left gives the empty string
sformat_list_innards(S,[]):-
	S="",!.

%Real elements are concatenated.
sformat_list_innards(S,[Element|R]):-
	sformat_as_lisp(S1,Element),
	( 
	    R=[],
	    S=S1
	;
            %A cdr that isn't a list is printed in dot notation
	    atom(R),
	    sformat_as_lisp(S2,R),
	    sformat(S, "~s . ~s", [S1,S2])
	;
	    sformat_list_innards(S2,R),
	    sformat(S, "~s ~s", [S1,S2])
	),
	!.

%
%Local Variables: 
%mode: prolog
%End: 
% sformat_as_lisp.pl ends here