nginx yyyy-mm logging without if

nginx.conf

http {
    #access_log  logs/access.log  main;
    
    # parse \d{4}-\d{2} from $time_iso8601 into $yyyy_mm
    map $time_iso8601 $yyyy_mm {
        "~^(?<temp>\d{4}-\d{2})"  $temp;
    }

    server {
        listen       80;
        server_name _;

        location / {
            access_log  logs/access-80-$yyyy_mm.log;