Best way to set up a dict of defaults?

I have a preferences class that acts something like a persistent
dict. One of parameters to initialise the class is a list of name/
value pairs representing the configuration defaults.

But some of the default values are calculated/derived rather than
static (say based on the name of the application, $argv0). What is
the best way for me to set up these defaults?

I current use list, which is ugly because of line continuation:
set app_path [file dirname $argv0]
set app_base_name [file tail [file rootname $argv0]]
set prefs_defaults [list \
log.file [file join $app_path "data" "${app_base_name}.log"] \
log.level WARN \
]

Subst appears to be a solution, but can trick the unwary:
set prefs_defaults [subst {
log.file [file join $app_path "data" "${app_base_name}.log"]
log.level WARN
}]

Notice that this will work correctly only so long as the resulting
file path contains no spaces, otherwise you will get e.g.
log.file C:/Program Files/MyApp/data/myapp.log
which is 3 elements instead of 2 ;(

Am I missing something obvious; is there a better way?

Regards, Twylite

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