Defensive Golang coding to prevent panics, silent data corruption, and subtle runtime bugs. Use whenever writing or reviewing Go code that involves nil-prone types (pointers, interfaces, maps, slices, channels), numeric conversions, resource lifecycle (defer in loops), or defensive copying. Also triggers on questions about nil panics, append aliasing, map concurrent access, float comparison, or zero-value design.
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Defensive Golang coding to prevent panics, silent data corruption, and subtle runtime bugs. Use whenever writing or reviewing Go code that involves nil-prone types (pointers, interfaces, maps, slices, channels), numeric conversions, resource lifecycle (defer in loops), or defensive copying. Also triggers on questions about nil panics, append aliasing, map concurrent access, float comparison, or zero-value design.
user-invocable
true
license
MIT
compatibility
Designed for Claude Code or similar AI coding agents, and for projects using Golang.
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 comma-ok for type assertions — bare assertions panic on mismatch
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
Nil Safety
Nil-related panics are the most common crash in Go.
The nil interface trap
Interfaces store (type, value). An interface is nil only when both are nil. Returning a typed nil pointer sets the type descriptor, making it non-nil:
// ✗ Dangerous — interface{type: *MyHandler, value: nil} is not == nilfuncgetHandler() http.Handler {
var h *MyHandler // nil pointerif !enabled {
return h
}
h
}
http.Handler {
!enabled {
}
&MyHandler{}
}
// interface{type: *MyHandler, value: nil} != nil
return
// ✓ Good — return nil explicitly
funcgetHandler()
if
return
nil
// interface{type: nil, value: nil} == nil
return
Nil map, slice, and channel behavior
Type
Read from nil
Write to nil
Len/Cap of nil
Range over nil
Map
Zero value
panic
0
0 iterations
Slice
panic (index)
panic (index)
0
0 iterations
Channel
Blocks forever
Blocks forever
0
Blocks forever
// ✗ Bad — nil map panics on writevar m map[string]int
m["key"] = 1// ✓ Good — initialize or lazy-init in methods
m := make(map[string]int)
func(r *Registry) Add(name string, val int) {
if r.items == nil { r.items = make(map[string]int) }
r.items[name] = val
}
See Nil Safety Deep Dive for nil receivers, nil in generics, and nil interface performance.
Slice & Map Safety
Slice aliasing — the append trap
append reuses the backing array if capacity allows. Both slices then share memory:
// ✗ Dangerous — a and b share backing array
a := make([]int, 3, 5)
b := append(a, 4)
b[0] = 99// also modifies a[0]// ✓ Good — full slice expression forces new allocation
b := append(a[:len(a):len(a)], 4)
Map concurrent access
Maps MUST NOT be accessed concurrently — → see samber/cc-skills-golang@golang-concurrency for sync primitives.
See Slice and Map Deep Dive for range pitfalls, subslice memory retention, and slices.Clone/maps.Clone.
Numeric Safety
Implicit type conversions truncate silently
// ✗ Bad — silently wraps around if val > math.MaxInt32 (3B becomes -1.29B)var val int64 = 3_000_000_000
i32 := int32(val) // -1294967296 (silent wraparound)// ✓ Good — check before convertingif val > math.MaxInt32 || val < math.MinInt32 {
return fmt.Errorf("value %d overflows int32", val)
}
i32 := int32(val)
Float comparison
// ✗ Bad — floating point arithmetic is not exact0.1+0.2 == 0.3// false// ✓ Good — use epsilon comparisonconst epsilon = 1e-9
math.Abs((0.1+0.2)-0.3) < epsilon // true
Division by zero
Integer division by zero panics. Float division by zero produces +Inf, -Inf, or NaN.
funcavg(total, count int) (int, error) {
if count == 0 {
return0, errors.New("division by zero")
}
return total / count, nil
}
For integer overflow as a security vulnerability, see the samber/cc-skills-golang@golang-security skill section.
Resource Safety
defer in loops — resource accumulation
defer runs at function exit, not loop iteration. Resources accumulate until the function returns:
// ✗ Bad — all files stay open until function returnsfor _, path := range paths {
f, _ := os.Open(path)
defer f.Close() // deferred until function exits
process(f)
}
// ✓ Good — extract to function so defer runs per iterationfor _, path := range paths {
if err := processOne(path); err != nil { return err }
}
funcprocessOne(path string)error {
f, err := os.Open(path)
if err != nil { return err }
defer f.Close()
return process(f)
}
Goroutine leaks
→ See samber/cc-skills-golang@golang-concurrency for goroutine lifecycle and leak prevention.
Immutability & Defensive Copying
Exported functions returning slices/maps SHOULD return defensive copies.
Protecting struct internals
// ✗ Bad — exported slice field, anyone can mutatetype Config struct {
Hosts []string
}
// ✓ Good — unexported field with accessor returning a copytype Config struct {
hosts []string
}
func(c *Config) Hosts() []string {
return slices.Clone(c.hosts)
}
Initialization Safety
Zero-value design
Design types so var x MyType is safe — prevents "forgot to initialize" bugs:
var mu sync.Mutex // ✓ usable at zero valuevar buf bytes.Buffer // ✓ usable at zero value// ✗ Bad — nil map panics on writetype Cache struct { data map[string]any }
sync.Once for lazy initialization
type DB struct {
once sync.Once
conn *sql.DB
}
func(db *DB) connection() *sql.DB {
db.once.Do(func() {
db.conn, _ = sql.Open("postgres", connStr)
})
return db.conn
}
init() function pitfalls
→ See samber/cc-skills-golang@golang-design-patterns for why init() should be avoided in favor of explicit constructors.
Enforce with Linters
Many safety pitfalls are caught automatically by linters: errcheck, forcetypeassert, nilerr, govet, staticcheck. See the samber/cc-skills-golang@golang-lint skill for configuration and usage.
Cross-References
→ See samber/cc-skills-golang@golang-concurrency skill for concurrent access patterns and sync primitives
→ See samber/cc-skills-golang@golang-data-structures skill for slice/map internals, capacity growth, and container/ packages
→ See samber/cc-skills-golang@golang-error-handling skill for nil error interface trap
→ See samber/cc-skills-golang@golang-security skill for security-relevant safety issues (memory safety, integer overflow)
→ See samber/cc-skills-golang@golang-troubleshooting skill for debugging panics and race conditions
Common Mistakes
Mistake
Fix
Bare type assertion v := x.(T)
Panics on type mismatch, crashing the program. Use v, ok := x.(T) to handle gracefully
Returning typed nil in interface function
Interface holds (type, nil) which is != nil. Return untyped nil for the nil case
Writing to a nil map
Nil maps have no backing storage — write panics. Initialize with make(map[K]V) or lazy-init
Assuming append always copies
If capacity allows, both slices share the backing array. Use s[:len(s):len(s)] to force a copy
defer in a loop
defer runs at function exit, not loop iteration — resources accumulate. Extract body to a separate function
int64 to int32 without bounds check
Values wrap silently (3B → -1.29B). Check against math.MaxInt32/math.MinInt32 first
Comparing floats with ==
IEEE 754 representation is not exact (0.1+0.2 != 0.3). Use math.Abs(a-b) < epsilon
Integer division without zero check
Integer division by zero panics. Guard with if divisor == 0 before dividing
Returning internal slice/map reference
Callers can mutate your struct's internals through the shared backing array. Return a defensive copy
Multiple init() with ordering assumptions
init() execution order across files is unspecified. → See samber/cc-skills-golang@golang-design-patterns — use explicit constructors
Blocking forever on nil channel
Nil channels block on both send and receive. Always initialize before use
Cross-References
→ See samber/cc-skills-golang@golang-continuous-integration skill for automated AI-driven code review in CI using these guidelines
