`lib.dropEnd` (#370558)

lib/default.nix

···
     inherit (self.lists) singleton forEach map foldr fold foldl foldl' imap0 imap1
       filter ifilter0 concatMap flatten remove findSingle findFirst any all count
       optional optionals toList range replicate partition zipListsWith zipLists
-      reverseList listDfs toposort sort sortOn naturalSort compareLists take
-      drop sublist last init crossLists unique allUnique intersectLists
+      reverseList listDfs toposort sort sortOn naturalSort compareLists
+      take drop dropEnd sublist last init
+      crossLists unique allUnique intersectLists
       subtractLists mutuallyExclusive groupBy groupBy' concatLists genList
       length head tail elem elemAt isList;
     inherit (self.strings) concatStrings concatMapStrings concatImapStrings

lib/lists.nix

···
   inherit (lib.strings) toInt;
   inherit (lib.trivial) compare min id warn pipe;
   inherit (lib.attrsets) mapAttrs;
+  inherit (lib) max;
 in
 rec {
 
···
   drop =
     count:
     list: sublist count (length list) list;
+
+  /**
+    Remove the last (at most) N elements of a list.
+
+
+    # Inputs
+
+    `count`
+
+    : Number of elements to drop
+
+    `list`
+
+    : Input list
+
+    # Type
+
+    ```
+    dropEnd :: Int -> [a] -> [a]
+    ```
+
+    # Examples
+
+    :::{.example}
+    ## `lib.lists.dropEnd` usage example
+
+    ```nix
+      dropEnd 2 [ "a" "b" "c" "d" ]
+      => [ "a" "b" ]
+      dropEnd 2 [ ]
+      => [ ]
+    ```
+    :::
+
+   */
+  dropEnd =
+    n: xs:
+      take
+        (max 0 (length xs - n))
+        xs;
 
   /**
     Whether the first list is a prefix of the second list.

lib/tests/misc.nix

···
     ([ 1 2 3 ] == (take 4 [  1 2 3 ]))
   ];
 
+  testDrop = let inherit (lib) drop; in testAllTrue [
+    # list index -1 is out of bounds
+    # ([ 1 2 3 ] == (drop (-1) [  1 2 3 ]))
+    (drop 0 [ 1 2 3 ] == [ 1 2 3 ])
+    (drop 1 [ 1 2 3 ] == [ 2 3 ])
+    (drop 2 [ 1 2 3 ] == [ 3 ])
+    (drop 3 [ 1 2 3 ] == [ ])
+    (drop 4 [ 1 2 3 ] == [ ])
+    (drop 0 [ ] == [ ])
+    (drop 1 [ ] == [ ])
+  ];
+
+  testDropEnd = let inherit (lib) dropEnd; in testAllTrue [
+    (dropEnd 0 [ 1 2 3 ] == [ 1 2 3 ])
+    (dropEnd 1 [ 1 2 3 ] == [ 1 2 ])
+    (dropEnd 2 [ 1 2 3 ] == [ 1 ])
+    (dropEnd 3 [ 1 2 3 ] == [ ])
+    (dropEnd 4 [ 1 2 3 ] == [ ])
+    (dropEnd 0 [ ] == [ ])
+    (dropEnd 1 [ ] == [ ])
+    (dropEnd (-1) [ 1 2 3 ] == [ 1 2 3 ])
+    (dropEnd (-1) [ ] == [ ])
+  ];
+
   testListHasPrefixExample1 = {
     expr = lists.hasPrefix [ 1 2 ] [ 1 2 3 4 ];
     expected = true;