Looks good, you might compare to this implementation of Monkey with a bunch of extensions (such as "objects", regular expressions, and a standard-library written in the language itself):

https://github.com/skx/monkey/

A similar idea allows embedded usage of a scripting language, as a filter:

https://github.com/skx/evalfilter/

Of course it gets addictive writing little languages, so you'll probably want to have fun and try making more! I went with BASIC:

https://github.com/skx/gobasic

But then you start thinking about assembly language:

https://github.com/skx/math-compiler

I recently took time to dive into compilers and programming languages by reading Thorsten Bell's excellent books, and this is what came out !