[m-rev.] For review: additions to list.m

[Ralph Becket]

Estimated hours taken: 0.5
Branches: main

library/list.m:
	Added predicates list__map_foldl/5 and list__map_foldr/5.

NEWS:
	Recorded addition of new predicates.

Index: list.m
===================================================================
RCS file: /home/mercury1/repository/mercury/library/list.m,v
retrieving revision 1.103
diff -u -r1.103 list.m
--- list.m	29 Dec 2001 04:41:01 -0000	1.103
+++ list.m	16 Jan 2002 02:06:31 -0000
@@ -607,6 +607,30 @@
 :- pred list__filter_map(pred(X, Y), list(X), list(Y), list(X)).
 :- mode list__filter_map(pred(in, out) is semidet, in, out, out) is det.
 
+	% list__map_foldl(P, [X1, X2, .., Xn], [Y1, Y2, .., Yn], A0, A)
+	% is equivalent to
+	%
+	% 	P(X1, Y1, A0, A1),
+	% 	P(X2, Y2, A1, A2),
+	% 	...
+	% 	P(Xn, Yn, An-1, A)
+	%
+:- pred list__map_foldl(pred(T1, T1, T2, T2), list(T1), list(T1), T2, T2).
+:- mode list__map_foldl(pred(in, out, in, out) is det, in, out, in, out) is det.
+:- mode list__map_foldl(pred(in, out, di, uo) is det, in, out, di, uo) is det.
+
+	% list__map_foldr(P, [X1, X2, .., Xn], [Y1, Y2, .., Yn], A0, A)
+	% is equivalent to
+	%
+	% 	P(Xn, Yn, A1, A0),
+	% 	...
+	% 	P(X2, Y2, An-2, An-1),
+	% 	P(X1, Y1, An-1, A)
+	%
+:- pred list__map_foldr(pred(T1, T1, T2, T2), list(T1), list(T1), T2, T2).
+:- mode list__map_foldr(pred(in, out, in, out) is det, in, out, in, out) is det.
+:- mode list__map_foldr(pred(in, out, di, uo) is det, in, out, di, uo) is det.
+
 	% list__takewhile(Predicate, List, UptoList, AfterList) takes a
 	% closure with one input argument, and calls it on successive members
 	% of List as long as the calls succeed. The elements for which
@@ -1602,6 +1626,22 @@
 
 Lo `..` Hi =
 	list__series(Lo, ( pred(I::in) is semidet :- I =< Hi ), plus(1)).
+
+%------------------------------------------------------------------------------%
+
+map_foldr(_, [], []) --> [].
+
+map_foldr(P, [X | Xs], [Y | Ys]) -->
+	map_foldr(P, Xs, Ys),
+	P(X, Y).
+
+%------------------------------------------------------------------------------%
+
+map_foldl(_, [], []) --> [].
+
+map_foldl(P, [X | Xs], [Y | Ys]) -->
+	P(X, Y),
+	map_foldl(P, Xs, Ys).
 
 % ---------------------------------------------------------------------------- %
 % ---------------------------------------------------------------------------- %


Index: NEWS
===================================================================
RCS file: /home/mercury1/repository/mercury/NEWS,v
retrieving revision 1.237
diff -u -r1.237 NEWS
--- NEWS	15 Jan 2002 07:19:04 -0000	1.237
+++ NEWS	16 Jan 2002 02:09:09 -0000
@@ -65,6 +65,8 @@
 
 Changes to the Mercury standard library:
 
+* We've added two predicates to list.m for mapping predicates with
+  accumulators: list__map_foldl/5 and list__map_foldr/5.
 * We've added four functions to list.m for mapping functions over
   corresponding members of lists: list__map_corresponding/3,
   list__map_corresponding3/4, list__filter_map_corresponding/3
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