Map nil safety - read OK, write panics
Scanned 2/12/2026
Skills Directorynpx -y skills add JamesPrial/claudefiles --skill map --agent claude-codeInstalls into .claude/skills of the current project.
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---
name: go-nil-map
description: Map nil safety - read OK, write panics
---
# Map Nil Safety
## Problem
Reading from nil map returns zero value. Writing to nil map panics.
## Pattern
### WRONG - Write to nil map
```go
var m map[string]int // nil map
m["key"] = 42 // PANIC: assignment to entry in nil map
```
### CORRECT - Initialize with make
```go
m := make(map[string]int)
m["key"] = 42 // OK
```
### Reading is Safe
```go
var m map[string]int // nil map
v := m["key"] // OK, v = 0 (zero value)
v, ok := m["key"] // OK, v = 0, ok = false
```
## Quick Fix
- [ ] Always initialize maps with make() before writing
- [ ] Check if map is nil before range/write
- [ ] Reading from nil map is safe but returns zero values
## Defensive Pattern
```go
if m == nil {
m = make(map[string]int)
}
m["key"] = 42
```
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