trace

builtins.trace

Primop

Docs pulled from | This Revision | 3 days ago

Nix manual

Takes 2 arguments

e1, e2

Evaluate e1 and print its abstract syntax representation on standard error. Then return e2. This function is useful for debugging.

If the debugger-on-trace option is set to true and the --debugger flag is given, the interactive debugger is started when trace is called (like break).

Noogle detected

Aliases

Detected Type

trace :: a -> b -> b

Implementation

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

src/libexpr/primops.cc:1308

static void prim_trace(EvalState & state, const PosIdx pos, Value ** args, Value & v)
{
    state.forceValue(*args[0], pos);
    if (args[0]->type() == nString)
        printError("trace: %1%", args[0]->string_view());
    else
        printError("trace: %1%", ValuePrinter(state, *args[0]));
    if (state.settings.builtinsTraceDebugger) {
        state.runDebugRepl(nullptr);
    }
    state.forceValue(*args[1], pos);
    v = *args[1];
}