← Starting Henceforth

Episode 9

The three stacks

One stack is yours, one is the machine's — and the third belongs to bitcoin.

2:34 · Double Concerto in A minor · Allegro (RV 522 i)

Watch here · code in the app

Henceforth is the real terminal — one purchase ($9.99) for iPhone, iPad and Mac. Type the commands below into the app as you watch.

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Code along

Tap to copy, then type it into Henceforth.

What you’ll learn

  • ✓ >r takes the top of your stack and sets it down on the return stack — the machine's own pile. r> is the exact reverse: it brings the number home, and you never named it once.
  • ✓ .s shows your stack, .rs the return stack, .ds the data stack — three stacks, three owners: yours, the machine's, and bitcoin's.
  • ✓ Reaching into an empty pile is an error, not a mystery: r> on an empty return stack reports return stack underflow — each of the three stacks has its own floor.
Transcript
  • one pile of numbers — that's all there is?
  • no. henceforth runs three stacks. three owners.
  • the first is yours — you've pushed numbers to it since episode two.
  • .s shows your stack — 7, then 8.
  • 8 landed last, so 8 sits on top.
  • but where does the machine itself hold a number for later?
  • i push 42 onto your stack first.
  • then >r takes that top number — the 42 — straight off your stack...
  • ...and sets it down on the return stack — the machine's own pile.
  • watch your stack — back to two. 7 and 8. the 42 is gone.
  • .rs shows the return stack — the machine's own.
  • there's the 42, held for later. no variable needed.
  • and to pull it back onto yours?
  • r> is the exact reverse of >r.
  • it takes the top of the return stack — the 42 — and sets it back on your stack.
  • your stack now — 7, 8, 42. the 42 came home, sitting on top.
  • moved to the machine's pile and back — and you never named it once.
  • and the third stack?
  • .ds prints the data stack. empty for now.
  • not yours. not the machine's.
  • this one is bitcoin's — the consensus stack a script runs on.
  • next episode, we put the first values onto it.
  • and reaching into an empty stack?
  • the return stack's empty — the 42's already home.
  • r> reaches for the top — and there's nothing to take. return stack underflow.
  • one floor of three — each stack has its own.
  • three stacks. one yours, one the machine's, one bitcoin's.
  • you've worked two of them today.
  • the third is the whole point of next time.
  • henceforth.
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