Effects And Purity

effects-and-purity — isolate decisions from I/O; don't ban side effects

Purpose: write code where the decision logic can be tested instantly, in isolation, with no infrastructure — not by eliminating side effects, but by moving them to the edges. Bernhardt's Functional Core, Imperative Shell: pure functions hold the logic; a thin shell does the I/O. The payoff is testability, not academic purity.

There are two failure modes, both harmful. (a) Logic tangled with I/O — nothing is testable without mocking a database, a clock, or a network call. (b) Over-correction — purity dogma, IO monads, Result-chaining, or refusing reasonable mutation, which is its own FOLD (Fear Of Looking Dumb in Haskell). This skill addresses (a) without inducing (b). When you feel the pull toward (b), stop and apply craft:right-sized-design instead.

Side effects are necessary and fine. The skill is about where they live, not whether they exist.

Reflex card (the heuristics)

1. Keep decision logic pure and deterministic. A function that takes inputs and returns outputs — no reads, no writes, no time, no randomness — is testable with a single call and an assertEqual. That's the goal for the logic layer.
2. Push I/O to the edges. DB reads/writes, network calls, filesystem, the clock, randomness, environment variables — these belong in a thin shell that calls into the pure core, not scattered through it.
3. Inject the clock, RNG, and clients; don't call them inline. Pass now as a parameter. Pass the HTTP client as a dependency. This is what makes the core testable without patching globals or using freezegun/jest.useFakeTimers.
4. Don't mutate caller-owned data. Return new values unless mutation is the documented contract. Hidden mutation is how "pure-looking" functions become time bombs.
5. A pure core needs no mocks to test. If you're still reaching for five mocks after extraction, the boundary is in the wrong place.
6. The mock-count tell. One or two mocks for true boundaries (network, external service) is fine. Five mocks to test a calculation is the smell.

Red flags (run these on your own diff)

• A function that both computes a result and writes to DB / logs / network in the same body.
• datetime.now(), random(), Date.now(), Math.random(), fetch() called inline inside business logic — the function is now nondeterministic and untestable without patching.
• A "pure-looking" function that mutates a global or its argument as a secret side channel.
• Mutable default argument (Python): def f(x, acc=[]) — shares state across calls silently. Use acc=None then acc = acc or [].
• A unit test that spins up five mocks to test a calculation — the logic wasn't extracted from the I/O.
• Hidden state across calls: anything that behaves differently on the second call than the first for the same inputs.

The procedure

1. Identify the decision — what is the function actually computing or deciding? Write that in one sentence.
2. Identify the I/O — what does it read from / write to? (DB, clock, network, filesystem, env, RNG.)
3. Extract the decision into a pure function. It takes the data it needs as parameters (including any values previously fetched inline) and returns a result. No I/O inside.
4. Inject effectful dependencies. If the decision needs now, pass it in. If it needs a computed value from an API call, fetch it in the shell and pass the value down.
5. Leave a thin shell that does the I/O: fetch inputs, call the pure core, write outputs. The shell is boring by design.
6. Verify the core is testable without mocks. If you can write assert decide(inputs) == expected with no patching, the boundary is right.

Before / after

# BEFORE — logic + I/O tangled; nondeterministic; untestable without freezegun + DB mock
def maybe_expire_subscription(user_id: str) -> None:
    user = db.get_user(user_id)                 # I/O
    if datetime.now() > user.trial_ends_at:     # nondeterministic
        user.status = "expired"
        db.save_user(user)                      # I/O
        analytics.track("trial_expired", user_id)  # I/O

# AFTER — pure core + thin shell
def is_trial_expired(trial_ends_at: datetime, now: datetime) -> bool:
    return now > trial_ends_at                  # pure; trivially testable

def maybe_expire_subscription(user_id: str, now: datetime) -> None:  # shell
    user = db.get_user(user_id)
    if is_trial_expired(user.trial_ends_at, now):
        user.status = "expired"
        db.save_user(user)
        analytics.track("trial_expired", user_id)

# Unit test: no mocks needed
assert is_trial_expired(trial_ends_at=t0, now=t0 + timedelta(days=1)) is True
assert is_trial_expired(trial_ends_at=t0, now=t0 - timedelta(days=1)) is False

The shell still has I/O — that's correct. The decision is now a one-liner that anyone can test in 100ms with no infrastructure.

Anti-over-correction

Don't introduce Result/Either monads, IO wrappers, or effect-system machinery where injecting a dependency suffices. Don't refuse reasonable mutation of locally-owned data. Don't make the code harder to read in the name of purity — clear beats clever (RULES §8). If the "pure" version requires three new types and a bind chain to avoid one datetime.now() call, you've traded one complexity for another. A function parameter is enough.

The right question is: "Can I test the decision without standing up infrastructure?" If yes, stop there.

Critique mode

When auditing, for each function with both logic and I/O: name what it computes, name what it reads/writes, and judge whether the decision layer is extractable. Tag findings [effects-and-purity · tangled-io · SEV] for logic mixed with I/O, [effects-and-purity · inline-nondeterminism · SEV] for clock/RNG/env called inline, [effects-and-purity · hidden-mutation · SEV] for secret argument mutation, and [effects-and-purity · mock-sprawl · SEV] when a unit test needs more than two mocks to exercise a calculation. Resist tagging the absence of a monad as a finding — that's not a defect.

References

• Functional core, imperative shell (Bernhardt): ../../references/PRINCIPLES.md §D
• Python purity and mutable-default pitfalls: ../../references/lang/python.md
• TypeScript: inject Date.now/fetch; don't mutate props: ../../references/lang/typescript.md
• Go: ../../references/lang/go.md
• Rule 7 (keep decision logic pure; quarantine effects at edges): ../../RULES.md
• Over-engineering the solution: ../../skills/right-sized-design/SKILL.md