Ironically, one of Lua's direct ancestors was SOL (Simple Object Language). I guess we've come full circle.

There's also Typed Lua: https://github.com/andremm/typedlua

The lead author gave a good presentation at Lua Workshop 2017 in SF: https://www.lua.org/wshop17/Maidl.pdf

Both Sol and Typed Lua are no longer maintained. The Typed Lua author is working on a statically compiled cousin to Lua: https://github.com/titan-lang/titan

I haven't used Sol or Typed Lua (too lazy), but I use the Typed Lua syntax for my recent API documentation as it reads well and allows me to minimally document an API before I get around to adding proper descriptions.

Titan looks great! But it seems far from being usable.