Commit 3b6169f8 authored by Silvan Mosberger's avatar Silvan Mosberger
Browse files

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.
parent 857a844e

lib/attrsets.nix

+1 −1
Original line number Diff line number Diff line
@@ -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);

lib/lists.nix

+10 −3
Original line number Diff line number Diff line
@@ -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

lib/tests/misc.nix

+2 −2
Original line number Diff line number Diff line
@@ -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

nixos/doc/manual/release-notes/rl-2311.section.md

+3 −0
Original line number Diff line number Diff line
@@ -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.