proc toggle {v} {
  uplevel [concat set $v \[expr \[set $v\] ^ 1\]]
 }

 % set state 0
 0
 % toggle state
 1
 % toggle state
 0
[RS] has this alternative:
 proc toggle varName {
     upvar 1 $varName var
     set var [expr {!$var}]
 }
Using the ''not'' operator (!) has, apart from being straightforward, the advantage that it also accepts the other allowed forms for truth values (yes, no, on, off, true, false). Also, as every non-zero int or double implies 'true' for [expr], the other alternatives may lead to subtle bugs.

[escargo] has this alternative:
 proc toggle varName {
     upvar 1 $varName var
     set var [expr {1 - $var}]
 }

''This is a clearly inferior alternative, but I wanted to
include it for completeness.''
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[KPV] not quite the same thing, but I've liked the following idiom for normalizing boolean values, i.e. converting all true values into 1, and all false values into 0:
 set b [expr {!!$b}]
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[Category Example]