lib.lists.foldl': Make strict in the initial accumulator

To maintain backwards compatibility, this can't be changed in the Nix language. We can however ensure that the version Nixpkgs has the more intuitive behavior.
NixOS · Sep 27, 2023 · 3b6169f · 3b6169f
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Showing 4 changed files with 16 additions and 6 deletions.
diff --git a/lib/attrsets.nix b/lib/attrsets.nix
@@ -392,7 +392,7 @@ rec {
       foldlAttrs :: ( a -> String -> b -> a ) -> a -> { ... :: b } -> a
   */
   foldlAttrs = f: init: set:
-    foldl'
+    builtins.foldl'
       (acc: name: f acc name set.${name})
       init
       (attrNames set);

diff --git a/lib/lists.nix b/lib/lists.nix
@@ -90,9 +90,12 @@ rec {
     Reduce a list by applying a binary operator from left to right,
     starting with an initial accumulator.
 
-    After each application of the operator, the resulting value is evaluated.
+    Before each application of the operator, the accumulator value is evaluated.
     This behavior makes this function stricter than [`foldl`](#function-library-lib.lists.foldl).
 
+    Unlike [`builtins.foldl'`](https://nixos.org/manual/nix/unstable/language/builtins.html#builtins-foldl'),
+    the initial accumulator argument is evaluated before the first iteration.
+
     A call like
 
     ```nix
@@ -104,7 +107,7 @@ rec {
 
     ```nix
     let
-      acc₁   =                      op acc₀   x₀   ;
+      acc₁   = builtins.seq acc₀   (op acc₀   x₀  );
       acc₂   = builtins.seq acc₁   (op acc₁   x₁  );
       acc₃   = builtins.seq acc₂   (op acc₂   x₂  );
       ...
@@ -135,7 +138,11 @@ rec {
     # The list to fold
     list:
 
-    builtins.foldl' op acc list;
+    # The builtin `foldl'` is a bit lazier than one might expect.
+    # See https://github.com/NixOS/nix/pull/7158.
+    # In particular, the initial accumulator value is not forced before the first iteration starts.
+    builtins.seq acc
+      (builtins.foldl' op acc list);
 
   /* Map with index starting from 0
 

diff --git a/lib/tests/misc.nix b/lib/tests/misc.nix
@@ -524,10 +524,10 @@ runTests {
     expected = [ 3 2 1 ];
   };
 
-  # The same as builtins.foldl', lib.foldl' doesn't evaluate the first accumulator strictly
+  # Compared to builtins.foldl', lib.foldl' evaluates the first accumulator strictly too
   testFoldl'StrictInitial = {
     expr = (builtins.tryEval (foldl' (acc: el: el) (throw "hello") [])).success;
-    expected = true;
+    expected = false;
   };
 
   # Make sure we don't get a stack overflow for large lists

diff --git a/nixos/doc/manual/release-notes/rl-2311.section.md b/nixos/doc/manual/release-notes/rl-2311.section.md
@@ -226,6 +226,9 @@
 
 - `networking.networkmanager.firewallBackend` was removed as NixOS is now using iptables-nftables-compat even when using iptables, therefore Networkmanager now uses the nftables backend unconditionally.
 
+- [`lib.lists.foldl'`](https://nixos.org/manual/nixpkgs/stable#function-library-lib.lists.foldl-prime) now always evaluates the initial accumulator argument first.
+  If you depend on the lazier behavior, consider using [`lib.lists.foldl`](https://nixos.org/manual/nixpkgs/stable#function-library-lib.lists.foldl) or [`builtins.foldl'`](https://nixos.org/manual/nix/stable/language/builtins.html#builtins-foldl') instead.
+
 - `rome` was removed because it is no longer maintained and is succeeded by `biome`.
 
 - The `services.mtr-exporter.target` has been removed in favor of `services.mtr-exporter.jobs` which allows specifying multiple targets.