include XMLPL package.

packages/xml/xml_example/misc.pl

@@ -0,0 +1,95 @@
+% Utility Predicates
+
+% The following predicates are used in the puzzle solutions. 
+% unique_solution( +Goal ) holds when Goal has one ground solution. Operationally,
+% Goal may produce several solutions, ("don't care" non-deterministically), but they
+% must all be identical (==). 
+
+unique_solution( Goal ) :-
+	findall( Goal, Goal, [Solution|Solutions] ),
+	same_solution( Solutions, Solution ),
+	Solution = Goal.
+
+same_solution( [], _Solution ).
+same_solution( [Solution0|Solutions], Solution ) :-
+	Solution0 == Solution,
+	same_solution( Solutions, Solution ).
+
+% forall( +Enumerator, +Test ) is true if Enumerator and Test are goals and Test holds everywhere
+% that Enumerator does. NB: does not further instantiate arguments.
+
+%% forall( Enumerator, Test ) :-
+%% 	\+ (call(Enumerator), \+ call(Test)).
+
+% member( ?Element, ?List ) holds when Element is a member of List. 
+member( H, [H|_] ).
+member( H, [_|T] ):-
+	member( H, T ).
+
+% select( ?Element, ?List0, ?List1 ) is true if List1 is equal to List1 with Element removed.
+
+select( H, [H|T], T ).
+select( Element, [H|T0], [H|T1] ):-
+	select( Element, T0, T1 ).
+
+% memberchk( +Element, +List ) succeeds (once) if Element is a member of List. 
+memberchk( Element, List ):-
+	member( Element, List ),
+	!.
+
+% between( +Lower, +Upper, ?Index ) is true if Lower =< Index =< Upper. Two valid cases are
+% possible:
+% - Index is already instantiated to an integer so the checks on order are applied (test).
+% - Index is a logical variable so a series of alternative solutions is generated as the
+%   monotonic sequence of values between Lower and Upper (non-deterministic generator).
+
+%% between( Lower, Upper, Index ):-
+%% 	integer( Lower ),
+%% 	integer( Upper ),
+%% 	Lower =< Upper,
+%% 	( integer( Index ) ->   % Case 1: "test"
+%% 		Index >= Lower,
+%% 		Index =< Upper
+%% 	; var( Index ) ->	  % Case 2: "generate".
+%% 		generate_between( Lower, Upper, Index )
+%% 	).
+
+generate_between( Lower, Upper, Index ) :-
+	( Lower =:= Upper ->
+		Index = Lower
+	;   Index = Lower
+	;   Next is Lower + 1,
+		Next =< Upper,
+		generate_between( Next, Upper, Index )
+	).
+
+% sum( +List, ?Sum ) holds when the List of numbers sum to Sum.
+
+sum( [H|T], Sum ) :-
+	sum1( T, H, Sum ).
+
+sum1( [], Sum, Sum ).
+sum1( [H|T], Sum0, Sum ):-
+	Sum1 is Sum0 + H,
+	sum1( T, Sum1, Sum ).
+
+% put_chars( +Chars ) if Chars is a (possibly empty) list of character codes and the
+% corresponding characters are written to the current output stream.
+
+put_chars( [] ).
+put_chars( [Char|Chars] ) :-
+	put( Char ),
+	put_chars( Chars ).
+
+% get_chars( ?Chars ) if Chars is a (possibly empty) list of character codes read
+% from the current input stream.
+
+get_chars( Input ) :-
+	get0( Char ),
+	( Char > -1 ->
+		Input = [Char|Chars],
+		get_chars( Chars )
+	; otherwise ->
+		Input = []
+	).
+