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.

1 files changed, 10 insertions, 3 deletions

diff --git a/lib/lists.nix b/lib/lists.nix
index 8c5099084bb..3835e3ba69c 100644
--- 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