It's a toy language created for, and used by, the book "Writing An Interpreter In Go"¹ (presumably, also by the subsequent book, "Writing A Compiler In Go") It

It's a toy language created for, and used by, the book "Writing An Interpreter In Go"¹ (presumably, also by the subsequent book, "Writing A Compiler In Go") It's a good book, very easy to follow, so I think it had a large following of people writing their own dialects, in many languages. ¹=https://interpreterbook.com https://interpreterbook.com