diff options
Diffstat (limited to 'nixpkgs/lib/lists.nix')
-rw-r--r-- | nixpkgs/lib/lists.nix | 62 |
1 files changed, 55 insertions, 7 deletions
diff --git a/nixpkgs/lib/lists.nix b/nixpkgs/lib/lists.nix index 0800aeb65451..3835e3ba69cb 100644 --- a/nixpkgs/lib/lists.nix +++ b/nixpkgs/lib/lists.nix @@ -86,15 +86,63 @@ rec { else op (foldl' (n - 1)) (elemAt list n); in foldl' (length list - 1); - /* Strict version of `foldl`. + /* + Reduce a list by applying a binary operator from left to right, + starting with an initial accumulator. - The difference is that evaluation is forced upon access. Usually used - with small whole results (in contrast with lazily-generated list or large - lists where only a part is consumed.) + Before each application of the operator, the accumulator value is evaluated. + This behavior makes this function stricter than [`foldl`](#function-library-lib.lists.foldl). - Type: foldl' :: (b -> a -> b) -> b -> [a] -> b - */ - foldl' = builtins.foldl' or 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 + foldl' op acc₀ [ x₀ x₁ x₂ ... xₙ₋₁ xₙ ] + ``` + + is (denotationally) equivalent to the following, + but with the added benefit that `foldl'` itself will never overflow the stack. + + ```nix + let + acc₁ = builtins.seq acc₀ (op acc₀ x₀ ); + acc₂ = builtins.seq acc₁ (op acc₁ x₁ ); + acc₃ = builtins.seq acc₂ (op acc₂ x₂ ); + ... + accₙ = builtins.seq accₙ₋₁ (op accₙ₋₁ xₙ₋₁); + accₙ₊₁ = builtins.seq accₙ (op accₙ xₙ ); + in + accₙ₊₁ + + # Or ignoring builtins.seq + op (op (... (op (op (op acc₀ x₀) x₁) x₂) ...) xₙ₋₁) xₙ + ``` + + Type: foldl' :: (acc -> x -> acc) -> acc -> [x] -> acc + + Example: + foldl' (acc: x: acc + x) 0 [1 2 3] + => 6 + */ + foldl' = + /* The binary operation to run, where the two arguments are: + + 1. `acc`: The current accumulator value: Either the initial one for the first iteration, or the result of the previous iteration + 2. `x`: The corresponding list element for this iteration + */ + op: + # The initial accumulator value + acc: + # The list to fold + 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 |