removeAttrs

lib.removeAttrs

Primop

Docs pulled from | This Revision | about 1 hour ago

Nix manual

Takes 2 arguments

set, list

Remove the attributes listed in list from set. The attributes don’t have to exist in set. For instance,

removeAttrs { x = 1; y = 2; z = 3; } [ "a" "x" "z" ]

evaluates to { y = 2; }.

Time Complexity

O(n + k log k) where:

n = number of attributes in input set k = number of attribute names to remove

Noogle detected

Aliases

Detected Type

removeAttrs :: AttrSet -> [String] -> AttrSet

Implementation

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

src/libexpr/primops.cc:3256

static void prim_removeAttrs(EvalState & state, const PosIdx pos, Value ** args, Value & v)
{
    state.forceAttrs(*args[0], pos, "while evaluating the first argument passed to builtins.removeAttrs");
    state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.removeAttrs");

    /* Get the attribute names to be removed.
       We keep them as Attrs instead of Symbols so std::set_difference
       can be used to remove them from attrs[0]. */
    // 64: large enough to fit the attributes of a derivation
    boost::container::small_vector<Attr, 64> names;
    names.reserve(args[1]->listSize());
    for (auto elem : args[1]->listView()) {
        state.forceStringNoCtx(
            *elem, pos, "while evaluating the values of the second argument passed to builtins.removeAttrs");
        names.emplace_back(state.symbols.create(elem->string_view()), nullptr);
    }
    std::sort(names.begin(), names.end());

    /* Copy all attributes not in that set.  Note that we don't need
       to sort v.attrs because it's a subset of an already sorted
       vector. */
    auto attrs = state.buildBindings(args[0]->attrs()->size());
    std::set_difference(
        args[0]->attrs()->begin(), args[0]->attrs()->end(), names.begin(), names.end(), std::back_inserter(attrs));
    v.mkAttrs(attrs.alreadySorted());
}