I worked on a toy programming language (that compile down to golang), which is a fork of the go lexer/parser, but it changes how functions can only return one value allowing the use of Result[T]/Option[T] and error propagation operators `!` and `?`.

It has enums (sum type), tuple, built-in Set[T], and good Iterator methods. It has very nice type inferred lambda function (heavily inspired by the swift syntax)... lots of good stuff!

https://github.com/alaingilbert/agl