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From: Harald Oehlmann <wortkarg3@yahoo.com>
Newsgroups: comp.lang.tcl
Subject: Re: misunderstaning of switch command
Date: Tue, 24 Jun 2025 10:37:08 +0200
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Am 24.06.2025 um 10:23 schrieb Mark Summerfield:
> I have a switch command which is doing something I don't expect but I
> don't understand what I've done wrong. In this example the default is
> always executed but I expect the case before that to be executed.
> 
>      const UNCOMPRESSED U
>      const ZLIB_COMPRESSED Z
>      const SAME_AS_PREV =
>      set filename somefile.txt
>      set action "added"
>      set kind Z
>      switch $kind {
>          $::SAME_AS_PREV { puts "unchanged \"$filename\"" }
>          $::UNCOMPRESSED { puts "$action \"$filename\"" }
>          $::ZLIB_COMPRESSED { puts "$action \"$filename\" (zlib
> compressed)" }
>          default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }
>      }
> 
> What am I doing wrong?

Nothing wrong. Tcl is just different, sorry for that.

The "{" always avoids expansion of variables and commands. If you want 
to use variables in the switch, you have to avoid the "{".

     switch -exact -- $kind [list\
           $::SAME_AS_PREV { puts "unchanged \"$filename\"" }\
           $::UNCOMPRESSED { puts "$action \"$filename\"" }
           $::ZLIB_COMPRESSED { puts "$action \"$filename\" (zlib 
compressed)" }\
           default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }\
           ]

Nevertheless, this is no fun on the quoting level (backslashes at the 
end etc). In addition, you have to take care, when the variable 
expansion happens. This might be tricky.

I personally would write it like that:

       switch -exact -- $kind {
           = { # SAME_AS_PREV
               puts "unchanged \"$filename\""
           }
           U { # UNCOMPRESSED
               puts "$action \"$filename\""
           }
           Z { # ZLIB_COMPRESSED
               puts "$action \"$filename\" (zlib compressed)"
           }
           default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }
       }

Harald