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Implemented in K! And not very much code either. Certainly shorter than some other language definitions I've read:

That's actually the most verbose k code I've ever seen (but probably also the most reasonably readable)...

And yet still very short and very unreadable.

"The language is purely functional. There are no side-effects."

This can be interpreted very efficiently by skipping evaluation.

It's also Turing complete.

Not to be confused with the other F language, which tried to remove all the cruft from Fortran but didn't really catch on:

Relation to F#?

F – a pure functional concatenative language (2006)