diff options
Diffstat (limited to 'lib/lists.nix')
| -rw-r--r-- | lib/lists.nix | 13 |
1 files changed, 10 insertions, 3 deletions
diff --git a/lib/lists.nix b/lib/lists.nix index 8c5099084bb55..3835e3ba69cb8 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 |