lib.dropEnd (NixOS#370558)

lib/default.nix

@@ -94,8 +94,9 @@ let
     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

@@ -6,6 +6,7 @@ let
   inherit (lib.strings) toInt;
   inherit (lib.trivial) compare min id warn pipe;
   inherit (lib.attrsets) mapAttrs;
+  inherit (lib) max;
 in
 rec {
 
@@ -1497,6 +1498,46 @@ rec {
     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

@@ -854,6 +854,30 @@ runTests {
     ([ 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;