From Tcl 8.5 [http://wiki.tcl.tk/10630], [expr] operator for list inclusion test.
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 if {$el in $list} ...
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is equivalent to, but much nicer than
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 if {[lsearch -exact $list $el] >= 0} ...
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<<discussion>>
The negation ("not in") is [ni]. So 8.5 Tcl really makes us knights who say 'ni'...
For users of earlier versions, the following little helper comes in handy:
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 proc in {list el} {expr [lsearch -exact $list $el] >= 0}}
 ...
 if [in $list $el] ...
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- [rs] 2007-04-26

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Let's see - so it would be something like:

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 set el Mon
 if {$el in {Mon Tue Wed Thu Fri Sat Sun}} {
   puts "Valid day abbreviation"
 } else {
   puts "Invalid day abbreviation"
 }
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[Lordmundi] 2011-03022

How come these aren't equivalent?

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% set item foo
foo
% set list "1 2 3 4 5 foo"
1 2 3 4 5 foo
%
% if { $item in $list } { puts "Yep" }                                    
Yep
% expr ($item in $list)
invalid bareword "foo"
in expression "(foo in 1 2 3 4 5 foo)";
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it seems when using expr, you have to do some funky explicit casting:

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% expr ("$item" in [list $list])
1
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<<categories>> Operator