Formating issue: default value for undefined variable

Hello.

We're using an irule to log http access to a remote syslog server. I know there is also a dedicated profile for this, but we prefer to use a consistent implementation for all our logging needs, even those not covered by a profile. In order to ease parsing on SIEM side, I'm emulating standard Combined Log format, which use a dash symbol as a default value when a field is empty, for instance when there is no user name, ie:

foo.domain.tld 127.0.0.1 - - [12/Aug/2019:17:41:02 +0200]

I tried to use a ternary operator directly while formatting the message, ie:

set message  [format \
    "%s - %s \[%s\]..." \
    ... \
    [[info exists [HTTP::username]] ? [HTTP::username] : '-' ] \
]

However, this syntax triggers a TCL error:

invalid command name "0"  while executing "[info exists [HTTP::username]] ? [HTTP::username] : '-' "

I searched TCL documentation for conditional expansion, such as in bash, or default value in the format string, without success.

The only workaround I found was to explicitly define a variable:

if { [info exists [HTTP::username]] } {
    set user [HTTP::username]
} else {
    set user -
} 

This is quite cumbersome, as I have another variable with the same formating issue (the size header in the response).

Is there any other way to achieve the same result in a more compact way ?