Ask HN: Which Functional Language to Implement for Learning?

They say the best way to learn about things is to just go and implement it. That being said, I'm looking to implement an interpreter (or maybe a vm and compiler) for a functional language to enforce the ideas I've learned in the past few months.

In the past, I've written interpreters for a few basic languages and Scheme (not fully r4rs compliant, but close enough to learn what I wanted). Of course, I wrote these in languages that provided garbage collection, and without any type-checking. Basically as AST-walkers.

I want to establish more expertise with these topics and create something that I can actually use, maybe even extend, but to build a new language from scratch and make real design decisions isn't in my cards yet.

So, the question, does anyone know of language specifications for functional languages that aren't entirely crippled that would be interesting to implement as a learning project?

Please don't suggest scheme. I love scheme, but it's been done to death, and is the language I've studied the inner-workings of the most. Thanks.