catAttrs

lib.catAttrs

Primop

Docs pulled from | This Revision | 18 minutes ago

Nixpkgs manual

Collect each attribute named attr from a list of attribute sets. Sets that don't contain the named attribute are ignored.

Inputs

attr: The attribute name to get out of the sets.
list: The list of attribute sets to go through

Type

catAttrs :: String -> [{ [String] :: a }] -> [a]

Examples

`lib.attrsets.catAttrs` usage example

catAttrs "a" [{a = 1;} {b = 0;} {a = 2;}]
=> [1 2]

(lib.attrsets.catAttrs)

Nix manual

Takes 2 arguments

attr, list

Collect each attribute named attr from a list of attribute sets. Attrsets that don't contain the named attribute are ignored. For example,

builtins.catAttrs "a" [{a = 1;} {b = 0;} {a = 2;}]

evaluates to [1 2].

Time Complexity

O(n * log m) where:

n = list length m = number of attributes per set

Noogle detected

Aliases

Implementation

This function is implemented in c++ and is part of the native nix runtime.

src/libexpr/primops.cc:3479

static void prim_catAttrs(EvalState & state, const PosIdx pos, Value ** args, Value & v)
{
    auto attrName = state.symbols.create(
        state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.catAttrs"));
    state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.catAttrs");

    SmallValueVector<nonRecursiveStackReservation> res(args[1]->listSize());
    size_t found = 0;

    for (auto v2 : args[1]->listView()) {
        state.forceAttrs(
            *v2, pos, "while evaluating an element in the list passed as second argument to builtins.catAttrs");
        if (auto i = v2->attrs()->get(attrName))
            res[found++] = i->value;
    }

    auto list = state.buildList(found);
    for (size_t n = 0; n < found; ++n)
        list[n] = res[n];
    v.mkList(list);
}