SKILL.md
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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 == nil
func getHandler() http.Handler {
var h *MyHandler // nil pointer
if !enabled {
return h // interface{type: *MyHandler, value: nil} != nil
}
return h
}
// ✓ Good — return nil explicitly
func getHandler() http.Handler {
if !enabled {
return nil // interface{type: nil, value: nil} == nil
}
return &MyHandler{}
}Nil map, slice, and channel behavior
Type
Index into nil
Write to nil
Len/Cap of nil
Range over nil
Map
Zero value
panic
0
0 iterations
Slice
panic
panic
0
0 iterations
Channel
Blocks forever
Blocks forever
0
Blocks forever
// ✗ Bad — nil map panics on write
var 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 converting
if 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 exact
var a, b, c float64 = 0.1, 0.2, 0.3
a+b == c // false
// ✓ Good — use epsilon comparison
const epsilon = 1e-9
math.Abs((a+b)-c) < epsilon // trueDivision by zero
Integer division by zero panics. Float division by zero produces +Inf, -Inf, or NaN.
func avg(total, count int) (int, error) {
if count == 0 {
return 0, 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 returns
for _, path := range paths {
f, _ := os.Open(path)
defer f.Close() // deferred until function exits
process(f)
}
// ✓ Good — extract to function so defer runs per iteration
for _, path := range paths {
if err := processOne(path); err != nil { return err }
}
func processOne(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 mutate
type Config struct {
Hosts []string
}
// ✓ Good — unexported field with accessor returning a copy
type 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 value
var buf bytes.Buffer // ✓ usable at zero value
// ✗ Bad — nil map panics on write
type 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.
Go 1.25+ reflection type assertions
For reflection code, prefer reflect.TypeAssert[T] over value.Interface().(T).
v := reflect.ValueOf(x)
if s, ok := reflect.TypeAssert[string](v); ok {
use(s)
}Cross-References
- → See
samber/cc-skills-golang@golang-concurrencyskill for concurrent access patterns and sync primitives
- → See
samber/cc-skills-golang@golang-data-structuresskill for slice/map internals, capacity growth, and container/ packages
- → See
samber/cc-skills-golang@golang-error-handlingskill for nil error interface trap
- → See
samber/cc-skills-golang@golang-securityskill for security-relevant safety issues (memory safety, integer overflow)
- → See
samber/cc-skills-golang@golang-troubleshootingskill 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-integrationskill for automated AI-driven code review in CI using these guidelines