improve '$skip_max_list'/4 and length/2 (#1023, #110)

src/lib/builtins.pl

@@ -329,7 +329,7 @@ comma_dispatch_call_list([G1]) :-
 
 :- non_counted_backtracking univ_errors/3.
 univ_errors(Term, List, N) :-
-    '$skip_max_list'(N, -1, List, R),
+    '$skip_max_list'(N, _, List, R),
     (  var(R)        ->
        (  var(Term),
           throw(error(instantiation_error, (=..)/2))      % 8.5.3.3 a)
@@ -399,7 +399,7 @@ get_args([Arg|Args], Func, I0, N) :-
 :- meta_predicate parse_options_list(?, 0, ?, ?, ?).
 
 parse_options_list(Options, Selector, DefaultPairs, OptionValues, Stub) :-
-    '$skip_max_list'(_, -1, Options, Tail),
+    '$skip_max_list'(_, _, Options, Tail),
     (  Tail == [] ->
        true
     ;  var(Tail) ->
@@ -452,7 +452,7 @@ parse_write_options_(max_depth(MaxDepth), max_depth-MaxDepth) :-
     ).
 
 must_be_var_names_list(VarNames) :-
-    '$skip_max_list'(_, -1, VarNames, Tail),
+    '$skip_max_list'(_, _, VarNames, Tail),
     (  Tail == [] ->
        must_be_var_names_list_(VarNames, VarNames)
     ;  var(Tail)  ->
@@ -549,7 +549,7 @@ can_be_list(List, _)  :-
     var(List),
     !.
 can_be_list(List, _)  :-
-    '$skip_max_list'(_, -1, List, Tail),
+    '$skip_max_list'(_, _, List, Tail),
     (  var(Tail) ->
        true
     ;  Tail == []
@@ -1196,7 +1196,7 @@ atom_length(Atom, Length) :-
     ).
 
 atom_chars(Atom, List) :-
-    '$skip_max_list'(_, -1, List, Tail),
+    '$skip_max_list'(_, _, List, Tail),
     (  ( Tail == [] ; var(Tail) ) ->
        true
     ;  throw(error(type_error(list, List), atom_chars/2))
@@ -1214,7 +1214,7 @@ atom_chars(Atom, List) :-
     ).
 
 atom_codes(Atom, List) :-
-    '$skip_max_list'(_, -1, List, Tail),
+    '$skip_max_list'(_, _, List, Tail),
     (  ( Tail == [] ; var(Tail) ) ->
        true
     ;  throw(error(type_error(list, List), atom_codes/2))
@@ -1271,9 +1271,9 @@ sub_atom(Atom, Before, Length, After, Sub_atom) :-
     ;  atom_chars(Atom, AtomChars),
        lists:append(BeforeChars, LengthAndAfterChars, AtomChars),
        lists:append(LengthChars, AfterChars, LengthAndAfterChars),
-       '$skip_max_list'(Before, -1, BeforeChars, []),
-       '$skip_max_list'(Length, -1, LengthChars, []),
-       '$skip_max_list'(After, -1, AfterChars, []),
+       '$skip_max_list'(Before, _, BeforeChars, []),
+       '$skip_max_list'(Length, _, LengthChars, []),
+       '$skip_max_list'(After, _, AfterChars, []),
        atom_chars(Sub_atom, LengthChars)
     ).
 

src/lib/charsio.pl

@@ -118,7 +118,7 @@ read_from_chars(Chars, Term) :-
        instantiation_error(read_from_chars/2)
     ;  nonvar(Term) ->
        throw(error(uninstantiation_error(Term), read_from_chars/2))
-    ;  '$skip_max_list'(_, -1, Chars, Chars0),
+    ;  '$skip_max_list'(_, _, Chars, Chars0),
        Chars0 == [],
        partial_string(Chars) ->
        true

src/lib/error.pl

@@ -80,7 +80,7 @@ character(C) :-
         atom_length(C, 1).
 
 ilist(Ls) :-
-        '$skip_max_list'(_, -1, Ls, Rs),
+        '$skip_max_list'(_, _, Ls, Rs),
         (   var(Rs) ->
             instantiation_error(must_be/2)
         ;   Rs == []
@@ -124,7 +124,7 @@ can_(list, Term)    :- list_or_partial_list(Term).
 can_(boolean, Term) :- boolean(Term).
 
 list_or_partial_list(Ls) :-
-        '$skip_max_list'(_, -1, Ls, Rs),
+        '$skip_max_list'(_, _, Ls, Rs),
         (   var(Rs) -> true
         ;   Rs == []
         ).

src/lib/lists.pl

@@ -50,27 +50,20 @@
 :- meta_predicate foldl(3, ?, ?, ?).
 :- meta_predicate foldl(4, ?, ?, ?, ?).
 
-
-length(Xs, N) :-
-    var(N),
-    !,
-    '$skip_max_list'(M, -1, Xs, Xs0),
-    (  Xs0 == [] -> N = M
-    ;  var(Xs0)  -> length_addendum(Xs0, N, M)
-    ).
-length(Xs, N) :-
-    integer(N),
-    N >= 0,
-    !,
-    '$skip_max_list'(M, N, Xs, Xs0),
-    (  Xs0 == [] -> N = M
-    ;  var(Xs0)  -> R is N-M, length_rundown(Xs0, R)
-    ).
+length(Xs0, N) :-
+   '$skip_max_list'(M, N, Xs0,Xs),
+   !,
+   (  Xs == [] -> N = M
+   ;  nonvar(Xs) -> var(N), throw(error(resource_error(finite_memory),_))
+   ;  nonvar(N) -> R is N-M, length_rundown(Xs, R)
+   ;  N == Xs -> throw(error(resource_error(finite_memory),_))
+   ;  length_addendum(Xs, N, M)
+   ).
 length(_, N) :-
-    integer(N), !,
-    domain_error(not_less_than_zero, N, length/2).
+   integer(N), !,
+   domain_error(not_less_than_zero, N, length/2).
 length(_, N) :-
-    type_error(integer, N, length/2).
+   type_error(integer, N, length/2).
 
 length_addendum([], N, N).
 length_addendum([_|Xs], N, M) :-
@@ -288,8 +281,8 @@ list_min_(N, Min0, Min) :-
 %           or partial list.
 
 permutation(Xs, Ys) :-
-    '$skip_max_list'(Xlen, -1, Xs, XTail),
-    '$skip_max_list'(Ylen, -1, Ys, YTail),
+    '$skip_max_list'(Xlen, _, Xs, XTail),
+    '$skip_max_list'(Ylen, _, Ys, YTail),
     (   XTail == [], YTail == []            % both proper lists
     ->  Xlen == Ylen
     ;   var(XTail), YTail == []             % partial, proper

src/lib/ordsets.pl

@@ -89,7 +89,7 @@ because the order it relies on may have been changed.
 %   setof/3.
 
 is_ordset(Term) :-
-    '$skip_max_list'(_, -1, Term, Tail), Tail == [], %% is_list(Term),
+    '$skip_max_list'(_, _, Term, Tail), Tail == [], %% is_list(Term),
     is_ordset2(Term).
 
 is_ordset2([]).