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.

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 :-)

Seems like something where you want to illustrate the concepts of integrating with a scripting language:

* Calling out from your C program to the scripting language.

* Setting variables in the scripting language from C.

* Creating functions that can be called from the scripting language.

* Making those available by creating a loadable shared lib that the scripting language can utilize.

Stuff like that is not that different across different languages and if you've figured one out, others won't be that hard.

My take: Tcl's C API is pretty cool. I wrote about it some here: http://www.amazon.com/dp/032133633X/?tag=dedasys-20