# Go Nil Interface

> Interface nil trap - typed nil is not nil Use when this capability is needed.

- Skill: `tomevault-io/go-nil-interface` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add tomevault-io/go-nil-interface`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/go-nil-interface/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: tomevault-io (https://skillmd.com/u/tomevault-io)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/tomevault-io/go-nil-interface

---


# Interface Nil Trap

## Problem
A typed nil pointer stored in an interface is NOT nil.

## Pattern

### WRONG - Typed nil escapes
```go
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
```go
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

---
> Converted and distributed by [TomeVault](https://tomevault.io/claim/jamesprial) — claim your Tome and manage your conversions.
<!-- tomevault:4.0:skill_md:2026-04-13 -->