Function value_call 

pub unsafe extern "C" fn value_call(
    context: *mut c_context,
    state: *mut EvalState,
    fn_: *mut value,
    arg: *mut value,
    value: *mut value,
) -> err

@brief Calls a Nix function with an argument. @ingroup value_create

@param[out] context Optional, stores error information @param[in] state The state of the evaluation. @param[in] fn The Nix function to call. @param[in] arg The argument to pass to the function. @param[out] value The result of the function call. @return NIX_OK if the function call was successful, an error code otherwise. @see nix_init_apply() for a similar function that does not performs the call immediately, but stores it as a thunk. Note the different argument order.