Merge branch 'hsv/add_droplast_to_lists'

* hsv/add_droplast_to_lists:
  Added documentation of lists:droplast/1
  Added tests for lists:droplast/1 to stdlib/lists_SUITE
  stdlib/lists: Add function droplast/1 This functions drops the last element of a non-empty list.

3 files changed, 32 insertions, 3 deletions

diff --git a/lib/stdlib/doc/src/lists.xml b/lib/stdlib/doc/src/lists.xml
index 251a383cf8..ee3c51c62c 100644
--- a/lib/stdlib/doc/src/lists.xml
+++ b/lib/stdlib/doc/src/lists.xml
@@ -123,6 +123,14 @@
       </desc>
     </func>
     <func>
+      <name name="droplast" arity="1"/>
+      <fsummary>Drop the last element of a list</fsummary>
+      <desc>
+        <p>Drops the last element of a <c><anno>List</anno></c>. The list should
+           be non-empty, otherwise the function will crash with a <c>function_clause</c></p>
+      </desc>
+    </func>
+    <func>
       <name name="dropwhile" arity="2"/>
       <fsummary>Drop elements from a list while a predicate is true</fsummary>
       <desc>
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
diff --git a/lib/stdlib/test/lists_SUITE.erl b/lib/stdlib/test/lists_SUITE.erl
index 92253ef5b9..f4589a8e24 100644
--- a/lib/stdlib/test/lists_SUITE.erl
+++ b/lib/stdlib/test/lists_SUITE.erl
@@ -61,7 +61,7 @@
 	 zip_unzip/1, zip_unzip3/1, zipwith/1, zipwith3/1,
 	 filter_partition/1, 
 	 otp_5939/1, otp_6023/1, otp_6606/1, otp_7230/1,
-	 suffix/1, subtract/1]).
+	 suffix/1, subtract/1, droplast/1]).
 
 %% Sort randomized lists until stopped.
 %%
@@ -2641,4 +2641,12 @@ sub_non_matching(A, B) ->
 sub(A, B) ->
     Res = A -- B,
     Res = lists:subtract(A, B).
-	
+
+%% Test lists:droplast/1
+droplast(Config) when is_list(Config) ->
+    ?line [] = lists:droplast([x]),
+    ?line [x] = lists:droplast([x, y]),
+    ?line {'EXIT', {function_clause, _}} = (catch lists:droplast([])),
+    ?line {'EXIT', {function_clause, _}} = (catch lists:droplast(x)),
+
+    ok.