[m-rev.] diff: list__split_last

[Zoltan Somogyi]

library/list.m:
	Add three new predicates. list__split_last is like list__last but also
	returns the rest of the list. list__last_det and list__split_last_det
	are deterministic versions of list__last and list__split_last
	respectively, which abort instead of failing if the input list is
	non-empty.

NEWS:
	Mention the new predicates, and consolidate all the announcements
	about list.m in a single group.

Zoltan.

cvs diff: Diffing .
Index: NEWS
===================================================================
RCS file: /home/mercury1/repository/mercury/NEWS,v
retrieving revision 1.233
diff -u -b -r1.233 NEWS
--- NEWS	2001/12/19 06:44:43	1.233
+++ NEWS	2001/12/29 04:38:07
@@ -64,12 +64,13 @@
   extensions which are unlikely to be implemented.
 
 Changes to the Mercury standard library:
-* We've added two new list functions to list.m, namely
-  list__filter_map_corresponding/3 and list__filter_map_corresponding3/4.
-* We've added a func version of error/1, called func_error/1, to require.m.
-* We've added two functions to list.m for mapping functions over
-  corresponding members of lists (list__map_corresponding/3 and
-  list__map_corresponding3/4).
+
+* 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
+  and list__filter_map_corresponding3/4.
+* We've added some other new functions to list.m, namely
+  list__last_det/2, list__split_last/3 and list__split_last_det/3.
 * We've added cc_multi modes to list__foldl/4 and list__foldr/4.
 * As mentioned above, the constructor for lists has changed from './2'
   to `[|]/2'. This change affects the behaviour of the term manipulation
@@ -80,6 +81,8 @@
   Also beware that std_util__functor and std_util__deconstruct now
   return `[|]' rather than `.' for lists, and calls to std_util__construct
   which construct lists may need to be updated.
+
+* We've added a func version of error/1, called func_error/1, to require.m.
 
 * ops.m now defines a typeclass which can be used to define operator
   precedence tables for use by parser.m and term_io.m. See
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Index: library/list.m
===================================================================
RCS file: /home/mercury1/repository/mercury/library/list.m,v
retrieving revision 1.102
diff -u -b -r1.102 list.m
--- library/list.m	2001/12/17 00:28:05	1.102
+++ library/list.m	2001/12/29 04:32:49
@@ -401,6 +401,22 @@
 :- pred list__last(list(T), T).
 :- mode list__last(in, out) is semidet.
 
+	% A deterministic version of list__last, which aborts instead of
+	% failing if the input list is empty.
+:- pred list__last_det(list(T), T).
+:- mode list__last_det(in, out) is det.
+
+	% list__split_last(List, AllButLast, Last) is true
+	%	if Last is the last element of List and AllButLast is the list
+	%	of elements before it.
+:- pred list__split_last(list(T), list(T), T).
+:- mode list__split_last(in, out, out) is semidet.
+
+	% A deterministic version of list__split_last, which aborts instead of
+	% failing if the input list is empty.
+:- pred list__split_last_det(list(T), list(T), T).
+:- mode list__split_last_det(in, out, out) is det.
+
 %-----------------------------------------------------------------------------%
 %
 % The following group of predicates use higher-order terms to simplify
@@ -1186,6 +1202,32 @@
 	;
 		T = [_|_],
 		list__last(T, Last)
+	).
+
+list__last_det(List, Last) :-
+	( list__last(List, LastPrime) ->
+		Last = LastPrime
+	;
+		error("list__last_det: empty list")
+	).
+
+list__split_last([H | T], AllButLast, Last) :-
+	(
+		T = [],
+		AllButLast = [],
+		Last = H
+	;
+		T = [_ | _],
+		list__split_last(T, AllButLast1, Last),
+		AllButLast = [H | AllButLast1]
+	).
+
+list__split_last_det(List, AllButLast, Last) :-
+	( list__split_last(List, AllButLastPrime, LastPrime) ->
+		AllButLast = AllButLastPrime,
+		Last = LastPrime
+	;
+		error("list__split_last_det: empty list")
 	).
 
 %-----------------------------------------------------------------------------%
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