lib.attrsets.foldAttrs': init #427473
lib.attrsets.foldAttrs': init #427473
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Same as `foldAttrs`, but passing attribute name to the function.
This is helpful in cases when the function needs to log or throw errors
with attribute names included in the message. For example:
```
assertUniqueAttrs = foldAttrs' (
name: v: acc:
assert (assertMsg (acc == null) "${name} is defined in multiple sets");
v
) null
```
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Most of the functionality can already be achieved with zipAttrsWith. If you just want one accumulator per attribute name.
You can use zipAttrsWith and do one fold operation with an accumulator like this
builtins.zipAttrsWith (name: values: foldl' (acc: v: ..) null values) [ { a = 1; } { a = 2; } ] For your assertUniqueAttrs you could even skip the inner fold and directly throw (or assert) the error
builtins.zipAttrsWith (name: values: if builtins.length values > 1 then throw "this" else lib.head values) [ { a = 1; } { a = 2; } ]I'd open considering this if more convincing use-cases would be presented but as is this function doesn't really introduce a newer or cleaner way to do zipping and folding
| ## `lib.attrsets.foldAttrs'` usage example | ||
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| ```nix | ||
| foldAttrs' (name: item: acc: ["${name}${toString item}"] ++ acc) [] [{ a = 2; b = 3; } { a = 4; }] |
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This example is equivalent to the existing
zipAttrsWith (name: values: map (v: name + toString v) values) [{ a = 2; b = 3; } { a = 4; }]Only missing feature would be the accumulator. But the mentioned use-cases didn't really convince the need for that.
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@hsjobeki Thanks for the suggestion, we'll try running with it for now. We'll see if any use cases that would require |
Same as
foldAttrs, but passing attribute name to the function.This is helpful in cases when the function needs to log or throw errors with attribute names included in the message. For example:
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