list indexing functions

nib/std/lists.nix

@@ -1,4 +1,4 @@
-rec {
+{}: rec {
   foldl = op: nul: list: let
     foldl' = n:
       if n == -1
@@ -8,4 +8,47 @@ rec {
     foldl' (builtins.length list - 1);
 
   crossLists = f: foldl (fs: args: builtins.concatMap (f: map f args) fs) [f];
+
+  findFirstIndex = pred: default: list: let
+    # A naive recursive implementation would be much simpler, but
+    # would also overflow the evaluator stack. We use `foldl'` as a workaround
+    # because it reuses the same stack space, evaluating the function for one
+    # element after another. We can't return early, so this means that we
+    # sacrifice early cutoff, but that appears to be an acceptable cost. A
+    # clever scheme with "exponential search" is possible, but appears over-
+    # engineered for now. See https://github.com/NixOS/nixpkgs/pull/235267
+    # Invariant:
+    # - if index < 0 then el == elemAt list (- index - 1) and all elements before el didn't satisfy pred
+    # - if index >= 0 then pred (elemAt list index) and all elements before (elemAt list index) didn't satisfy pred
+    #
+    # We start with index -1 and the 0'th element of the list, which satisfies the invariant
+    resultIndex =
+      builtins.foldl' (
+        index: el:
+          if index < 0
+          then
+            # No match yet before the current index, we need to check the element
+            if pred el
+            then
+              # We have a match! Turn it into the actual index to prevent future iterations from modifying it
+              -index - 1
+            else
+              # Still no match, update the index to the next element (we're counting down, so minus one)
+              index - 1
+          else
+            # There's already a match, propagate the index without evaluating anything
+            index
+      ) (-1)
+      list;
+  in
+    if resultIndex < 0
+    then default
+    else resultIndex;
+
+  findFirst = pred: default: list: let
+    index = findFirstIndex pred null list;
+  in
+    if index == null
+    then default
+    else builtins.elemAt list index;
 }