Documentation for the Tcl switch command can be found at
http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/switch.htm 

History corner: First announced by [JO] for Tcl 7.0 in [http://groups.google.com/groups?q=group:comp.lang.tcl+author:ousterhout&start=800&hl=de&lr=&newwindow=1&scoring=d&selm=1t8s4g%24kfs%40agate.berkeley.edu&rnum=840]
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[RS] Make sure you always write "the switch to end all switches", "--", if there is the slightest possibility that faulty or unexpected data (i.e., beginning with a dash) may occur - they raise a data-driven syntax error which does not make a good impression on end-users.
 switch -- $foo {...}  ;# robust for all values of foo
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In the form of the switch command where all patterns and bodies are in the last argument, this argument is technically a string which will be parsed by Tcl as a list. The rules for how a string is parsed as a list can be found on the [lindex] manual page [http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/lindex.htm].
The difference here between a whitespace-delimited substring and a list element parsed from a string is usually negligible, but one must sometimes take them into account to get the right amount of quoting in place. Regexp patterns are particularly easy to overquote.

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Coding style for switch is tricky.  Recently on comp.lang.tcl there was
an exchange about getting switch to work right.  The user had originally
typed:

 switch $::errorCode {
  $NOTFOUND {
        puts "page not found "
  }
  default {
        puts "Program encountered code $::errorCode"
        break
  }
 }

and was frustrated because the application kept taking the default branch.

A well known Tcl pundit suggested:

 switch $::errorCode "
    $NOTFOUND {
        puts "page not found "
    }
    default {
        puts "Program encountered code $::errorCode"
        break
    }
  "

so that Tcl had a chance to substitue the value of $NOTFOUND.
However, the follow up, by [Ken Jones] said it best:

you don't want that (unless you're doing some very, very, very
tricky stuff). Quoting switch's pattern/action block with "" allows
substitutions within the entire argument. So, yes you get the $URL_NOT_FOUND
variable substituted. But you also get the contents of the actions
substituted before switch even gets called. 

Instead, we want the "alternate" syntax of switch, where you provide each
pattern and its corresponding action as separate arguments to switch,
rather than as a monolithic pattern/action quoted argument. Here's a
correctly working example of the above code:

 set NOTFOUND "value"
 set errorCode "nonsense"

 switch -- $::errorCode \
    $NOTFOUND  {
        puts "page not found"
        } \
    default {
        puts "Program encountered code $::errorCode"
        break
    }

[KBK] - I dislike both of these.  I tend to think of a switch as a
selection among fixed alternatives, and substitution in the cases
confuses me.  I'd have just written:

  if { [string equal $::errorcode $NOTFOUND] } {
      puts "page not found"
  } else {
      puts "Program encountered code $::errorCode"
      break
  }

[RHS] - I find that, when I need substitution for switch values, formatting it like the following looks cleaner:

 switch -- $::errorCode $NOTFOUND {
    puts "page not found"
 } $value1 {
    puts "blah"
 } default {
    puts "Program encountered code $::errorCode"
    break
 }
----
The above sample was, of course, intended to be brief, and didn't show the other several dozen switch cases .  For a single comparison, I agree with KBK.  But if I have more than one or two of these types of comparisons, then I know '''I''' [LV] prefer a switch.

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With '''switch -regexp''', you can specify abbreviations of a flag to be switched on like this:

 switch -regexp -- $variable {
  -h(e(lp?)?)?             {...}
  -h|-he|-hel|-help|-ayuda {...}
  -\\? { ... }                     ;$ Match -? flag
  "(?x) - | -h | -help" {do stuff} ;# extended RE syntax
 }

without the -regexp, you would write:
 switch -- $variable {
  -h - -he - -hel - -help { ... }
 }

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[LV] I really find using - as a seperator between switch case alternatives to be counter-intuitive; normally one uses | for that sort of thing .

[glennj]: It might be clearer if more whitespace is used:
 switch -- $variable {
    -h    - 
    -he   - 
    -hel  - 
    -help { ... }
 }


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[SB], 2003-05-21: When I use switch -regexp, Tcl already use regexp once to enter the proper clause. How can the match pattern be accessed from within the clause, if possible, and is switch -regexp able to use () match variables? I think perl store the regexp matches in global variables $1 .. $9 (The following example is not valid, only for illustrating question)
 switch -regexp -- $somevar {
    {^(\S+)\s+(\S+)} {puts "You hit matches $1 and $2"}
    {^(\d+)\s+(\d+)} {puts "You hit number matches $1 and $2"}
 }

[KBK] - You can't.  However, that form of [[switch]] is no slower than the corresponding [[if]]...[[elseif]], so you can do:

   if { [regexp {^(\S+)\s+(\S+)$} $somevar -> part1 part2] } {
        puts "You hit matches $part1 and $part2"
    } elseif { [regexp {^(\d+)\s+(\d+)} -> part1 part2] } {
        puts "You hit number matches $part1 and $part2"
    }

[hkoba], 2005-03-18: Hmm, how about [switch-regexp]?

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How's this for a silly compromise. Nicely aligns the dispatch conditions and removes the confusing barrage of backslashes.

 if {0} {
 } elseif {$state == $INIT_STATE} {

    puts init

 } elseif {$state == $CLEANING_STATE} {

    puts cleaning

 } elseif {$state == $DONE_STATE} {

    puts done

 }


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The silliness continues...this actually works though I'm not sure of the side effects.

proc dispatch {thevar args} {
     set pass1 [uplevel subst $args]
     set pass2 "switch \$$thevar \{ $pass1 \}"
     uplevel $pass2
 }
 
 set INIT_STATE "init"
 set CLEANING_STATE "cleaning"
 set DONE_STATE "done"
 
 set state $INIT_STATE
 
 dispatch state {
     $INIT_STATE {
 
         puts init
         set nextstate $DONE_STATE
 
     }
     $CLEANING_STATE {
 
         puts cleaning
 
     }
     $DONE_STATE {
 
         puts done
 
     }
     default {
     }
 }
 puts "next state $nextstate"


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See also [ranged switch]



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[LES]: So, if it is so good a practice ALWAYS to call switch as "switch --" (the switch to end all switches), why can't the core code already cope with that?
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