Documentation can be found for various (tcllib) data structures.
Provided structures, so far:

   * [tree] - http://tcllib.sourceforge.net/doc/tree.html
   * [graph] - http://tcllib.sourceforge.net/doc/graph.html
   * [queue] - http://tcllib.sourceforge.net/doc/queue.html
   * [prioqueue] - http://tcllib.sourceforge.net/doc/prioqueue.html
   * [skiplist] - http://tcllib.sourceforge.net/doc/skiplist.html
   * [stack] - http://tcllib.sourceforge.net/doc/stack.html
   * [matrix] - http://tcllib.sourceforge.net/doc/matrix.html

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Often what people want from "struct" is a C-like datatype.  This is much
different from the tcllib data structures described above.  Various people have coded Tcl extensions for this kind of "struct";
[Iain B. Findleton]'s [Containers] appears to be a very nice data structure extension that includes one.

Many people have written small procs that implement a C like structure
facility in Tcl.  Check google nearly monthly for discussions.  Here's a thread [http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&selm=a4ee9c%24mvi%40gap.cco.caltech.edu] that covers one nice implementation.

The first exchange in that thread is also here at [C Struct is Tcl!]
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[Category Package], subset [Tcllib]