diff: list__foldl2

[Tyson Richard DOWD]

Hi,

Can someone review this small addition?

===================================================================

Estimated hours taken: 0.25

Add a double accumulator version of list__foldl (very useful for 
accumulator based predicates that also use io__states).

library/list.m:
	Add list__foldl2/6.

Index: library/list.m
===================================================================
RCS file: /home/staff/zs/imp/mercury/library/list.m,v
retrieving revision 1.70
diff -u -r1.70 list.m
--- list.m	1997/06/03 01:45:58	1.70
+++ list.m	1997/06/12 02:43:33
@@ -388,6 +388,20 @@
 :- pred list__sort(pred(X, X, comparison_result), list(X), list(X)).
 :- mode list__sort(pred(in, in, out) is det, in, out) is det.
 
+	% list__foldl2(Pred, List, Start, End, Start2, End2) 
+	% calls Pred with each element of List (working left-to-right),
+	% 2 accumulators (with the initial values of Start and Start2),
+	% and returns the final values in End and End2.
+	% (Although no more expressive than list__foldl, this is often
+	% a more convenient format, and a little more efficient).
+:- pred list__foldl2(pred(X, Y, Y, Z, Z), list(X), Y, Y, Z, Z).
+:- mode list__foldl2(pred(in, in, out, in, out) is det,
+		in, in, out, in, out) is det.
+:- mode list__foldl2(pred(in, in, out, di, uo) is det,
+		in, in, out, di, uo) is det.
+
+%-----------------------------------------------------------------------------%
+
 	% list__sort_and_remove_dups(Compare, Unsorted, Sorted) is true iff 
 	% Sorted is a list containing the same elements as Unsorted, but with
 	% any duplicates removed. Where Sorted is a sorted list, with respect  
@@ -871,6 +885,11 @@
 list__foldl(P, [H|T], Acc0, Acc) :-
 	call(P, H, Acc0, Acc1),
 	list__foldl(P, T, Acc1, Acc).
+
+list__foldl2(_, [], FirstAcc, FirstAcc, SecAcc, SecAcc).
+list__foldl2(P, [H|T], FirstAcc0, FirstAcc, SecAcc0, SecAcc) :-
+	call(P, H, FirstAcc0, FirstAcc1, SecAcc0, SecAcc1),
+	list__foldl2(P, T, FirstAcc1, FirstAcc, SecAcc1, SecAcc).
 
 list__foldr(_, [], Acc, Acc).
 list__foldr(P, [H|T], Acc0, Acc) :-

-- 
       Tyson Dowd           # 
                            #  Surreal humour isn't eveyone's cup of
     trd at cs.mu.oz.au        #  fur.
http://www.cs.mu.oz.au/~trd #