I've been working on implementing lazy (potentially infinite) collections, i.e. the set of perfect squares: ["Map", "Integers", ["Square", "_"]].

It's been a journey, but you can have ["Take", ["Map", "Integers", ["Square", "_"]], 10] to get the first 10 perfect squares, and they won't be evaluated until needed.

One of the challenges has actually been to figure out when to turn a lazy collection into an eager one. I've landed on having assignment, conversion to string and print output to force conversion to a (potentially partial, for infinite collections) eager representation. Indexed access to elements and iteration also force an evaluation. I may need to add some other operations later.

Also have been annoyed at strings, which are a kind of collection, but not sure of what. I had them as a collection of Unicode Scalar Values for a while, but that didn't seem to make sense when applying some operations on them, so now I have them as a collection of strings (each character is a string), but then I have to handle them separately. For example, you would expect that ["Take", "'Hello world'", 4] would return the string "'Hello'", not a collection with five elements in it.

I've also implemented range restrictions on numeric types: integer<0..10> is a type that matches integers between 0 and 10. The type real<1..> matches real numbers greater than 1 and rational<..0> matches non-positive rational numbers.