lib.lists.findFirstIndex: init

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

2 files changed, 64 insertions, 28 deletions

diff --git a/lib/lists.nix b/lib/lists.nix
index 5d9af0cf711..c3e889644b3 100644
--- a/lib/lists.nix
+++ b/lib/lists.nix
@@ -180,18 +180,18 @@ rec {
       else if len != 1 then multiple
       else head found;
 
-  /* Find the first element in the list matching the specified
+  /* Find the first index in the list matching the specified
      predicate or return `default` if no such element exists.
 
-     Type: findFirst :: (a -> bool) -> a -> [a] -> a
+     Type: findFirstIndex :: (a -> Bool) -> b -> [a] -> (Int | b)
 
      Example:
-       findFirst (x: x > 3) 7 [ 1 6 4 ]
-       => 6
-       findFirst (x: x > 9) 7 [ 1 6 4 ]
-       => 7
+       findFirstIndex (x: x > 3) null [ 0 6 4 ]
+       => 1
+       findFirstIndex (x: x > 9) null [ 0 6 4 ]
+       => null
   */
-  findFirst =
+  findFirstIndex =
     # Predicate
     pred:
     # Default value to return
@@ -229,7 +229,33 @@ rec {
     if resultIndex < 0 then
       default
     else
-      elemAt list resultIndex;
+      resultIndex;
+
+  /* Find the first element in the list matching the specified
+     predicate or return `default` if no such element exists.
+
+     Type: findFirst :: (a -> bool) -> a -> [a] -> a
+
+     Example:
+       findFirst (x: x > 3) 7 [ 1 6 4 ]
+       => 6
+       findFirst (x: x > 9) 7 [ 1 6 4 ]
+       => 7
+  */
+  findFirst =
+    # Predicate
+    pred:
+    # Default value to return
+    default:
+    # Input list
+    list:
+    let
+      index = findFirstIndex pred null list;
+    in
+    if index == null then
+      default
+    else
+      elemAt list index;
 
   /* Return true if function `pred` returns true for at least one
      element of `list`.
diff --git a/lib/tests/misc.nix b/lib/tests/misc.nix
index ce980436c1b..dfa6a94ee48 100644
--- a/lib/tests/misc.nix
+++ b/lib/tests/misc.nix
@@ -518,45 +518,55 @@ runTests {
     expected = false;
   };
 
-  testFindFirstExample1 = {
-    expr = findFirst (x: x > 3) 7 [ 1 6 4 ];
-    expected = 6;
+  testFindFirstIndexExample1 = {
+    expr = lists.findFirstIndex (x: x > 3) (abort "index found, so a default must not be evaluated") [ 1 6 4 ];
+    expected = 1;
   };
 
-  testFindFirstExample2 = {
-    expr = findFirst (x: x > 9) 7 [ 1 6 4 ];
-    expected = 7;
+  testFindFirstIndexExample2 = {
+    expr = lists.findFirstIndex (x: x > 9) "a very specific default" [ 1 6 4 ];
+    expected = "a very specific default";
   };
 
-  testFindFirstEmpty = {
-    expr = findFirst (abort "when the list is empty, the predicate is not needed") null [];
+  testFindFirstIndexEmpty = {
+    expr = lists.findFirstIndex (abort "when the list is empty, the predicate is not needed") null [];
     expected = null;
   };
 
-  testFindFirstSingleMatch = {
-    expr = findFirst (x: x == 5) null [ 5 ];
-    expected = 5;
+  testFindFirstIndexSingleMatch = {
+    expr = lists.findFirstIndex (x: x == 5) null [ 5 ];
+    expected = 0;
   };
 
-  testFindFirstSingleDefault = {
-    expr = findFirst (x: false) null [ (abort "if the predicate doesn't access the value, it must not be evaluated") ];
+  testFindFirstIndexSingleDefault = {
+    expr = lists.findFirstIndex (x: false) null [ (abort "if the predicate doesn't access the value, it must not be evaluated") ];
     expected = null;
   };
 
-  testFindFirstNone = {
-    expr = builtins.tryEval (findFirst (x: x == 2) null [ 1 (throw "the last element must be evaluated when there's no match") ]);
+  testFindFirstIndexNone = {
+    expr = builtins.tryEval (lists.findFirstIndex (x: x == 2) null [ 1 (throw "the last element must be evaluated when there's no match") ]);
     expected = { success = false; value = false; };
   };
 
   # Makes sure that the implementation doesn't cause a stack overflow
-  testFindFirstBig = {
-    expr = findFirst (x: x == 1000000) null (range 0 1000000);
+  testFindFirstIndexBig = {
+    expr = lists.findFirstIndex (x: x == 1000000) null (range 0 1000000);
     expected = 1000000;
   };
 
-  testFindFirstLazy = {
-    expr = findFirst (x: x == 1) 7 [ 1 (abort "list elements after the match must not be evaluated") ];
-    expected = 1;
+  testFindFirstIndexLazy = {
+    expr = lists.findFirstIndex (x: x == 1) null [ 1 (abort "list elements after the match must not be evaluated") ];
+    expected = 0;
+  };
+
+  testFindFirstExample1 = {
+    expr = lists.findFirst (x: x > 3) 7 [ 1 6 4 ];
+    expected = 6;
+  };
+
+  testFindFirstExample2 = {
+    expr = lists.findFirst (x: x > 9) 7 [ 1 6 4 ];
+    expected = 7;
   };
 
 # ATTRSETS