listToAttrs
lib.attrsets.listToAttrs
Primop
Docs pulled from | This Revision | about 23 hours ago
Nix manual
Takes 1 arguments
e
Construct a set from a list specifying the names and values of each
attribute. Each element of the list should be a set consisting of a
string-valued attribute name specifying the name of the attribute,
and an attribute value specifying its value.
In case of duplicate occurrences of the same name, the first takes precedence.
Example:
builtins.listToAttrs
[ { name = "foo"; value = 123; }
{ name = "bar"; value = 456; }
{ name = "bar"; value = 420; }
]
evaluates to
{ foo = 123; bar = 456; }
Time Complexity
O(n log n) where n = number of list elements
Noogle detected
Detected Type
listToAttrs :: [{name :: String; value :: a}] -> AttrSet
Implementation
This function is implemented in c++ and is part of the native nix runtime.
static void prim_listToAttrs(EvalState & state, const PosIdx pos, Value ** args, Value & v)
{
state.forceList(*args[0], pos, "while evaluating the argument passed to builtins.listToAttrs");
// Step 1. Sort the name-value attrsets in place using the memory we allocate for the result
auto listView = args[0]->listView();
size_t listSize = listView.size();
auto & bindings = *state.mem.allocBindings(listSize);
using ElemPtr = decltype(&bindings[0].value);
for (const auto & [n, v2] : enumerate(listView)) {
state.forceAttrs(*v2, pos, "while evaluating an element of the list passed to builtins.listToAttrs");
auto j = state.getAttr(state.s.name, v2->attrs(), "in a {name=...; value=...;} pair");
auto name = state.forceStringNoCtx(
*j->value,
j->pos,
"while evaluating the `name` attribute of an element of the list passed to builtins.listToAttrs");
auto sym = state.symbols.create(name);
// (ab)use Attr to store a Value * * instead of a Value *, so that we can stabilize the sort using the Value * *
bindings[n] = Attr(sym, std::bit_cast<Value *>(&v2));
}
std::sort(&bindings[0], &bindings[listSize], [](const Attr & a, const Attr & b) {
// Note that .value is actually a Value * * that corresponds to the position in the list
return a < b || (!(a > b) && std::bit_cast<ElemPtr>(a.value) < std::bit_cast<ElemPtr>(b.value));
});
// Step 2. Unpack the bindings in place and skip name-value pairs with duplicate names
Symbol prev;
for (size_t n = 0; n < listSize; n++) {
auto attr = bindings[n];
if (prev == attr.name) {
continue;
}
// Note that .value is actually a Value * *; see earlier comments
Value * v2 = *std::bit_cast<ElemPtr>(attr.value);
auto j = state.getAttr(state.s.value, v2->attrs(), "in a {name=...; value=...;} pair");
prev = attr.name;
bindings.push_back({prev, j->value, j->pos});
}
// help GC and clear end of allocated array
for (size_t n = bindings.size(); n < listSize; n++) {
bindings[n] = Attr{};
}
v.mkAttrs(&bindings);
}