ReddRadar
Agent skill
go-nil-interface
Interface nil trap - typed nil is not nil
Install this agent skill to your Project
npx add-skill https://github.com/JamesPrial/claudefiles/tree/main/skills/golang/nil/interface
SKILL.md
Interface Nil Trap
Problem
A typed nil pointer stored in an interface is NOT nil.
Pattern
WRONG - Typed nil escapes
func GetUser(id int) error {
var err *MyError // typed nil
if id < 0 {
err = &MyError{"invalid id"}
}
return err // Returns non-nil interface containing nil pointer!
}
if err := GetUser(1); err != nil {
// This branch RUNS even though no error occurred
fmt.Println("error:", err)
}
CORRECT - Return untyped nil
func GetUser(id int) error {
if id < 0 {
return &MyError{"invalid id"}
}
return nil // Return untyped nil
}
if err := GetUser(1); err != nil {
// This branch does NOT run
fmt.Println("error:", err)
}
Why It Happens
Interface contains (type, value). Typed nil has (type=*MyError, value=nil). This is different from (type=nil, value=nil).
Quick Fix
- Return nil directly, not typed nil variable
- Or return concrete type, not interface
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