Ok, I've turned streams into a tagged type. I had to just support them in car and cdr and carif to get common list operations to work. However, existing operations still return regular (eager) lists when you pass them lazy streams, so I followed malisper's idea of creating lazy variants that preserve laziness in the result.
I've also added unit tests for them in lib/streams.arc.t, which is my first serious attempt at using zck's nice unit-test harness with suites.
malisper, I took some liberties with your code, such as renaming the 's' prefix to 'lazy-'. I'm not attached to these things, so feel free to revert any of my changes you don't like. Thanks for a fun exercise!
I'm not sure how you're measuring overhead, but let me know if it seems slower than before.
Well, you need it for cons and the generator lazy-gen and other functions that don't take a stream. Maybe the others don't, but it seemed to me that sometimes it makes sense to return a list and sometimes not. So especially since functions like firstn do something useful without changing anything, maybe we should keep that behavior around. But it's just an idea. What do you think? Since you're using them for project Euler, it'll be good to hear your experiences.
