Matching streams

Take one apart with when, and build one lazily.

main (streams, print)

# when takes a stream apart: empty, or a head followed by a tail.
# Building a stream from a recursive call keeps it lazy.
runningTotal(s |int|, acc int) |int|
    when s is
        |int:| |int:|
        |int: head, *tail| |int: acc + head, *runningTotal(tail, acc + head)|

main()
    sums = streams.collect(runningTotal(streams.range(1, 6), 0), [int:])
    print.line("{sums[0]} {sums[4]}")

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