elem

builtins.elem

Primop

Docs pulled from | This Revision | about 4 hours ago

Nix manual

Takes 2 arguments

x, xs

Return true if a value equal to x occurs in the list xs, and false otherwise.

Time Complexity

O(n * T) (worst case) where:

n = list length T = time to compare two elements

returns early if the elements is found

Noogle detected

Aliases

lib.elem
lib.lists.elem
lib.strings.elem

Detected Type

elem :: a -> [b] -> Bool

Implementation

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

src/libexpr/primops.cc:3910

static void prim_elem(EvalState & state, const PosIdx pos, Value ** args, Value & v)
{
    bool res = false;
    state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.elem");
    for (auto elem : args[1]->listView())
        if (state.eqValues(*args[0], *elem, pos, "while searching for the presence of the given element in the list")) {
            res = true;
            break;
        }
    v.mkBool(res);
}