Documentation can be found for various (tcllib) data structures.Provided structures, so far:
How is the tcllib struct::list going to work - I presume that it can't be namespace import'd to replace the core's list command? Is there anything that can be done to prevent this from accidentally happening?
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 [L1 ] that covers one nice implementation.
The first exchange in that thread is also here at C Struct is Tcl!