Effects, propagated: the blast radius before you edit

Ask a coding agent to refactor processOrder and the first honest answer should be a question: what does this actually touch? Does it write to the database? Post to an external API? Read a config file off disk? And not just directly — through everything it calls, and everything those call.

Without that map, an agent is refactoring blind. It might turn a synchronous write into a batched one without realizing a downstream caller depends on the write having already happened. The blast radius is invisible until something breaks.

Effects are declarations, propagated

ASD models side effects as 17 categories — io.fs.read, io.db.write, io.net.out, io.queue.publish, and so on. Each is declared on the symbol that performs it. Then the important part: effects propagate transitively through the call graph. A function that calls something with io.db.write inherits io.db.write, annotated with where it came from.

asd effects_of processOrder

io.db.write      via orders.persist        — writes orders table in a txn
io.net.out       via payments.chargeCard    — HTTPS POST to Stripe
io.fs.read        via config.load           — reads payments.toml
io.queue.publish  via notify.send           — order.placed → NATS
4 effect categories · 3 transitive · policy: pass

The agent now knows: this “simple” order function reaches the database, an external payment gateway, the filesystem, and a message queue — and it knows which callee introduces each one. That’s the blast radius, computed rather than guessed.

Declared vs. transitive is the useful distinction

Every effect in the set carries its provenance. A declared effect is one this symbol performs directly; a transitive effect is inherited from something it calls. That distinction is what lets an agent reason about a change: if you’re editing processOrder itself, the declared effects are yours to change; the transitive ones are contracts with your callees.

It’s also what makes policy meaningful. A gate that says “any change touching io.net.out in the payments path needs review” can only fire if the effect set is complete — including the transitive io.net.out three calls deep. Propagation is what turns a pile of per-symbol annotations into an answer you can actually gate on.

Next week: a decision ledger that outlives the author.