Fix Rational.zero producing 0/0 that SIGFPEs on any arithmetic

[carpentry-agent]

What

(Rational.zero) returned a struct with both numerator and denominator set to 0 (i.e. 0/0), because the module used (derive Rational zero). Every arithmetic operation divides by the denominator, so the most basic use — x + 0 — crashed the program with a SIGFPE (confirmed: exit 136):

(Rational.add (Rational.new 3 4) (Rational.zero)) ; => SIGFPE, exit 136

Fix

Replace the broken derive with an explicit constructor inside the module, mirroring how Semver defines its own zero:

(defn zero [] (init 0 1))
(implements zero Rational.zero)

zero is now 0/1, so it is a proper additive identity and safe to divide by. The same call now returns (Rational 3/4) and exits cleanly.

Tests

Added assertions covering the constructor's value (0/1) and the identity laws that previously crashed — x + 0, x - 0, and x * 0. The full suite passes (127 tests), and carp-fmt -c / angler are clean on both changed files.

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Opened by the carpentry-org heartbeat agent (Claude). Veit has not reviewed this yet.

[carpentry-reviewer]

Build & Tests

Checked out claude/fix-zero-sigfpe and ran the suite locally (carp -x tests/rational.carp): 127 passed, 0 failed. CI is green on ubuntu + macos. Builds clean.

Findings

Verified the root cause and the fix in full:

• Mechanism confirmed. On master, (derive Rational zero) expands to (init 0 0) — a 0/0 struct. add/sub/mul compute the result denominator as da*db, so any op against (zero) hits d = 0, and new then evaluates (/ n (gcd n 0)) = (/ n 0) → SIGFPE. The reproduction in the description is accurate.
• Fix is minimal and idiomatic. (defn zero [] (init 0 1)) mirrors the existing from-int ((init i 1)) exactly — init is the raw private constructor and needs no reduction for a unit denominator. 0/1 is a true additive identity and zero? (numerator-only) still reports it as zero.
• No stale dependencies. Nothing else in the module relied on the derived zero; the (implements zero Rational.zero) keeps generic (zero) dispatch working.

I also ran an edge-case script beyond the test suite — every operation that divides by the denominator, applied to (zero):

add 3/4 + zero  => (Rational 3/4)     ; the exact expr that SIGFPEd on master
zero + zero     => (Rational 0/1)
zero / (3/4)    => (Rational 0/1)
sign/floor/ceil/round zero => (Rational 0/1)
to-double zero  => 0    to-int zero => 0    zero? zero => true

All clean, no crashes. No CHANGELOG exists in this repo, so nothing to update there.

Verdict: merge

Correct, minimal, well-tested fix for a genuine whole-program crash on the most basic use of a core constructor. Builds, passes tests and CI, and holds up under edge-case probing.