head

lib.strings.head

Primop

Docs pulled from | This Revision | about 1 hour ago

Nix manual

Takes 1 arguments

list

Return the first element of a list; abort evaluation if the argument isn’t a list or is an empty list. You can test whether a list is empty by comparing it with [].

Time Complexity

O(1)

Noogle detected

Aliases

builtins.head
lib.head
lib.lists.head

Detected Type

head :: [a] -> a

Implementation

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

src/libexpr/primops.cc:3752

static void prim_head(EvalState & state, const PosIdx pos, Value ** args, Value & v)
{
    state.forceList(*args[0], pos, "while evaluating the first argument passed to 'builtins.head'");
    if (args[0]->listSize() == 0)
        state.error<EvalError>("'builtins.head' called on an empty list").atPos(pos).debugThrow();
    state.forceValue(*args[0]->listView()[0], pos);
    v = *args[0]->listView()[0];
}