Metamine was a language that appeared here on HN a few times, but seems to have disappeared. The thing that made it interesting is that it let you mix imperative and declarative code in the same program. It did this with a "magical equals", after executing it once, the left side auto-updated outside the flow of the imperative code.

  =   magical equals, which maintains the relation for the rest of the runtime
           note: once you use it on a variable, you can't redefine it later, that's an error
 :=   normal assignment, which only fires once

The demo was pretty fun to watch.[1]

[1] http://web.archive.org/web/20201014024057/https://www.youtub...