Episode 7

Decisions

Give forth a yes-or-no and it picks a path — if, else, then, and decisions inside decisions.

1:37 · March (Nutcracker, Op. 71)

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✓ if … else … then picks a path from a yes-or-no on the stack: : positive? dup 0 > if ." positive!" else ." not positive." then drop ;
✓ Any non-zero is true; zero is the only no. The condition is just a number — even? tests dup 2 mod 0 = and decides from it.
✓ Decisions nest inside decisions — sign-of asks "over zero?", then in the else asks again. Bitcoin Script branches the same way: if a signature checks, spend; else, don't.

Transcript

every word so far just runs. can one... choose?
give forth a yes-or-no, and it picks a path.
if the number is over zero, say so. otherwise, the other branch.
five is positive — the if branch ran.
minus three took the else. same word, two paths.
and what counts as yes?
any non-zero is true. zero is the only no.
the test can be computed — remainder by two, is it zero?
six yes, seven no. the condition is just a number on the stack.
can a decision hold another?
over zero? positive. else, ask again — under zero, or nothing at all.
zero, positive, negative — decisions inside decisions.
why teach the dog to choose?
bitcoin script branches the same way — if a signature checks, spend; else, don't.
every spending rule is a decision.
more in episode ten.
henceforth.