Sol is both a lightweight code-interchange format, and a functional programming language based on the Calculus of Constructions.
It is a port of Haskell-Morte-Library, with addition of Fix, floating-point primitives, a lighter syntax and a substitution-free typechecker.
The core is implemented as a ~200 LOC, dependency-free JS file (sol.js).
It also includes a fast parser/serializer, an observable-based runtime and a compiler to/from native functions, making use of JS's JIT engines to achieve good performances.
I'll be using it as both an experiment lab to explore/learn dependent types, and as a minimal runtime to validate the proposal of writing actual programs with a small core language.
For a similar language without floating points, check out calculus-of-constructions.