stdlib/lists: Add function droplast/1

This functions drops the last element of a non-empty list.

lists:last/1 and lists:droplast/1 are the dual of hd/1 and tl/1 but
for the end of a list.

1 files changed, 14 insertions, 1 deletions

diff --git a/lib/stdlib/src/lists.erl b/lib/stdlib/src/lists.erl
index d6a9f4645d..6303465d3d 100644
--- a/lib/stdlib/src/lists.erl
+++ b/lib/stdlib/src/lists.erl
@@ -22,7 +22,7 @@
 -compile({no_auto_import,[min/2]}).
 
 -export([append/2, append/1, subtract/2, reverse/1,
-	 nth/2, nthtail/2, prefix/2, suffix/2, last/1, 
+	 nth/2, nthtail/2, prefix/2, suffix/2, droplast/1, last/1,
 	 seq/2, seq/3, sum/1, duplicate/2, min/1, max/1, sublist/2, sublist/3,
 	 delete/2,
 	 unzip/1, unzip3/1, zip/2, zip3/3, zipwith/3, zipwith3/4,
@@ -203,6 +203,19 @@ suffix(Suffix, List) ->
     Delta = length(List) - length(Suffix),
     Delta >= 0 andalso nthtail(Delta, List) =:= Suffix.
 
+%% droplast(List) returns the list dropping its last element
+
+-spec droplast(List) -> InitList when
+      List :: [T, ...],
+      InitList :: [T],
+      T :: term().
+
+%% This is the simple recursive implementation
+%% reverse(tl(reverse(L))) is faster on average,
+%% but creates more garbage.
+droplast([_T])  -> [];
+droplast([H|T]) -> [H|droplast(T)].
+
 %% last(List) returns the last element in a list.
 
 -spec last(List) -> Last when