Not sure what qualifies for a modern library. But you will find out that there are libraries for tcl for basically any purpose you might need. The most fameous one certainly is the tk library for creating GUIs. So doing a small gui might be a nice start, or any shell utility. The asynchroneus IO functions are great for creating small internet based servers and utilities. The most fundamental resource on tcl is probably the book by the creator, its worth a read: https://www.amazon.com/Tcl-Toolkit-2nd-John-Ousterhout/dp/03...

It not only covers the language and its usage but goes into depth of both extending tcl in C as well as embedding it into an application for scripting it.

Tcl's C API is very nice, and makes it easy to either integrate Tcl into a larger C app, or write bits and pieces of C as Tcl commands.

I helped work on the 2nd edition of this book, which I think is still fairly relevant: https://amzn.to/2IB9OyR

There is also an updated version out (with my name on the cover, to boot:-)

http://amzn.to/1LQrLTK

Although even that is not super recent.

Tcl's C API is very pleasant to work with, and its C code is a good read - except for the regex stuff, which are the first few files you'd see in an alphabetical listing.

Actually, internally, everything is not a string, and has not been that way since... 10+ years. Everything is representable as a string, though.

I actually like Tcl quite a bit even if I don't use it much these days:

http://www.amazon.com/Tcl-Toolkit-2nd-John-Ousterhout/dp/032... (See the cover for all of the authors' names :-)