get key value from tcl dict with a default value - tcl

Question

Very new/rusty with TCL here :-(. I am stuck with tcl 8.6 and can't take advantage of tcl 8.7 feature of getwithdefault on a dict.
Tried the following and get error saying "frameLen" is not part of the dict. But I thought the ternary operator should have skiped the part on [dict get $pktBlock frameLen]. What did I do wrong?
Thanks!
package require json
set pktBlock [json::json2dict {{"frameLenn": 253}}]
set frameLen [expr [dict exists $pktBlock framelen] ? [dict get $pktBlock frameLen]: 256 ]

Answer 1

My version (Different syntax than what's slated for 8.7; the default value is optional here and defaults to an empty string):
# dict getdef dictValue -default arg path...
if {[::info commands ::tcl::dict::getdef] eq {}} {
proc ::tcl::dict::getdef {dictionary args} {
::set def {}
if {[lindex $args 0] eq "-default"} {
::set args [::lassign $args _ def]
}
if {[exists $dictionary {*}$args]} {
get $dictionary {*}$args
} else {
::set def
}
}
namespace ensemble configure \
dict -map [dict replace [namespace ensemble configure dict -map] \
getdef ::tcl::dict::getdef]
}

Answer 2

Turned out I had a wrong spelling. I intended to have dict exists $pktBlock frameLen but ended up using framelen.
Case is VERY important. Hope this could be a lesson for those who are stuck

Answer 3

It's not too difficult to implement this feature in plain Tcl:
# dict getdef dictionaryValue ?key ...? key default
#
# This will be in Tcl 8.7 -- https://tcl.tk/man/tcl8.7/TclCmd/dict.htm
proc dict_getdef {dictValue args} {
if {[llength $args] < 2} {
error {wrong # args: should be "dict getdef dictValue ?key ...? key default"}
}
set default [lindex $args end]
set keys [lrange $args 0 end-1]
if {[dict exists $dictValue {*}$keys]} {
return [dict get $dictValue {*}$keys]
} else {
return $default
}
}
This can be added as a dict subcommand like this:
set map [namespace ensemble configure dict -map]
dict set map getdef [namespace which dict_getdef]
namespace ensemble configure dict -map $map
So that:
set frameLen [dict getdef $pktBlock frameLen 256]