Let me offer you an example: proc maybe {onno onjust val} { if {[llength $val] == 2 && [string equal [lindex $val 0] Just]} { lap

Let me offer you an example: proc maybe {onno onjust val} { if {[llength $val] == 2 && [string equal [lindex $val 0] Just]} { lappend onjust [lrange $val 1 end] return [uplevel $onjust] } # I cannot extract value return [uplevel $onno] } maybe {puts "---"} {puts } {Just "huh?"} maybe {puts "---"} {puts } {} This is an example of custom control statement, modeled after Haskell's maybe [1]. [1] https://hackage.haskell.org/package/base-4.16.1.0/docs/Data-Maybe.html#v:maybe https://hackage.haskell.org/package/base-4.16.1.0/docs/Data-... It is easy to use and write. You can follow algebraic data types convention and prefix values with tags and use something like control structure drafted above. I did that, it is relatively comfortable. Note that special case of empty list also handled as if it is Nothing. So it is more useful than would appear. ;)