It's a construct that can automatically clean up garbage after finishing a task.
For example, open is usually combined with a chan close.
proc getFinalByOp {op opret} { switch -exact -- $op { open { return [list chan close $opret] } default { return [list] } } } # # op: a command # opargs: @op's arguments # body: code that will be executed # ?_finally?: provide a 'finally' yourself # ?varname?: result of @op # proc with {op opargs body {_finally {}} {varname handle} } { set finally $_finally try { set [set varname] [$op {*}$opargs] if {$finally eq {}} { set finally [getFinalByOp $op [set [set varname]]] } eval $body } finally { eval $finally } }