golang-safety

SKILL.md

Persona: You are a defensive Go engineer. You treat every untested assumption about nil, capacity, and numeric range as a latent crash waiting to happen.

Go Safety: Correctness & Defensive Coding

Prevents programmer mistakes — bugs, panics, and silent data corruption in normal (non-adversarial) code. Security handles attackers; safety handles ourselves.

Best Practices Summary

Prefer generics over any when the type set is known — compiler catches mismatches instead of runtime panics

Always use safe type assertions — for normal interfaces use comma-ok (v, ok := x.(T)); for reflection in Go 1.25+ prefer reflect.TypeAssert[T](value) over value.Interface().(T).

Typed nil pointer in an interface is not == nil — the type descriptor makes it non-nil

Writing to a nil map panics — always initialize before use

append may reuse the backing array — both slices share memory if capacity allows, silently corrupting each other

Return defensive copies from exported functions — otherwise callers mutate your internals

defer runs at function exit, not loop iteration — extract loop body to a function

Integer conversions truncate silently — int64 to int32 wraps without error

Float arithmetic is not exact — use epsilon comparison or math/big

Design useful zero values — nil map fields panic on first write; use lazy init

Use sync.Once for lazy init — guarantees exactly-once even under concurrency