diff options
| -rw-r--r-- | lib/attrsets.nix | 2 | ||||
| -rw-r--r-- | lib/lists.nix | 13 | ||||
| -rw-r--r-- | lib/tests/misc.nix | 4 | ||||
| -rw-r--r-- | nixos/doc/manual/release-notes/rl-2311.section.md | 3 |
4 files changed, 16 insertions, 6 deletions
diff --git a/lib/attrsets.nix b/lib/attrsets.nix index 77e36d3271f7..11932c05dd29 100644 --- 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 index 8c5099084bb5..3835e3ba69cb 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 diff --git a/lib/tests/misc.nix b/lib/tests/misc.nix index 50d615c5be38..d40d92049880 100644 --- 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 index cdb73fb49fa8..011fa84c96af 100644 --- 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. |