Tcl Source Code

% set x {}           
% trace add variable x read {apply {args {puts $args}}}
% lappend x a
x {} read
a
% apply {{} {
set x {}  
trace add variable x read {apply {args {puts $args}}}
lappend x a
}}
a
% apply {{} {
set x {}  
trace add variable x read {apply {args {puts $args}}}
[subst lappend] x a
}}
x {} read
a
% apply {{} {
set x {}  
trace add variable x read {apply {args {puts $args}}}
lappend x a b
}}
x {} read
a b

This mess is tied up with Bug 3057639, and

http://core.tcl.tk/tcl/info/21b6b23

where the loss of read traces was apparently intentionally
done, at least kinda sorta, with a claim that new tests
append*-9.* demand the changed behavior -- no read
traces from lappend

Trouble is that other tests including append-7.[234]
and trace-3.[12] demand that the traces be there.

This continuing inconsistency is left hanging in 3057639,
which is in fact still open.

Now the new wrinkle is that a whole new collection
of bytecodes INST_LAPPEND_LIST_*& have been invented
and they adopt the "traces? YES!" side of the dilemma.
So we have yet a new dimension of inconsistency, and
this one appears to be wrapped up in performance
differences too (the only reason I'm down this rabbit hole).