Wikipedia says:
Extensible programming is a term used in computer science to describe a style of computer programming that focuses on mechanisms to extend the programming language, compiler and runtime environment.
For example, Tcl lets you write your own control structures. See here.
I'm interested in compiling a list of extensible programming languages that are being used in real-world code. It would be nice if you could supply an example for your language as well.
The TCL language is a dynamic language with few fundamental rules:
The spirit of Tcl is "Everything is a string", "everything is a command", even control flow constructions.
For instance let to replace while loop with custom one.
Tcl has C language interface which allow to integrate Tcl into exists program very easily. Tcl used in EDA tools very extensively.
And finally TCL so powerful that it is not possible to define Tcl BNF, which brings lot of headaches.