Golang

ABOUTME: Go 1.26 idiomatic development skill with automated quality gates

ABOUTME: Enforces 2025 best practices, pre-commit hooks, and golangci-lint

Go 1.26 Idiomatic Development

Quick Reference

🛑 FILE OPERATION CHECKPOINT (BLOCKING)

Before EVERY Write or Edit tool call on a .go file:

╔══════════════════════════════════════════════════════════════════╗
║  🛑 STOP - GO SKILL CHECK                                        ║
║                                                                  ║
║  You are about to modify a .go file.                             ║
║                                                                  ║
║  QUESTION: Is /golang skill currently active?                    ║
║                                                                  ║
║  If YES → Proceed with the edit                                  ║
║  If NO  → STOP! Invoke /golang FIRST, then edit                  ║
║                                                                  ║
║  This check applies to:                                          ║
║  ✗ Write tool with file_path ending in .go                       ║
║  ✗ Edit tool with file_path ending in .go                        ║
║  ✗ ANY Go file, regardless of conversation topic                 ║
║                                                                  ║
║  Examples that REQUIRE this skill:                               ║
║  - "add error handling" (edits .go file)                         ║
║  - "fix the test" (edits _test.go file)                          ║
║  - "update the handler" (edits handler.go)                       ║
╚══════════════════════════════════════════════════════════════════╝

Why this matters: Go code without proper error wrapping (fmt.Errorf %w) loses context. The skill ensures idiomatic patterns and golangci-lint compliance.

🔄 RESUMED SESSION CHECKPOINT

When a session is resumed from context compaction, verify Go development state:

┌─────────────────────────────────────────────────────────────┐
│  SESSION RESUMED - GO SKILL VERIFICATION                    │
│                                                             │
│  Before continuing Go implementation:                       │
│                                                             │
│  1. Was I in the middle of writing Go code?                 │
│     → Check summary for "implementing", "writing", ".go"    │
│                                                             │
│  2. Did I follow all Go skill guidelines?                   │
│     → Explicit error handling with wrapping                 │
│     → Small interfaces defined by consumer                  │
│     → Functions ≤50 lines, ≤4 params                        │
│     → ABOUTME headers on new files                          │
│                                                             │
│  3. Check code quality before continuing:                   │
│     → Run: go build ./...                                   │
│     → Run: golangci-lint run                                │
│                                                             │
│  If implementation was in progress:                         │
│  → Review the partial code for completeness                 │
│  → Ensure all errors are handled and wrapped                │
│  → Verify golangci-lint passes with no warnings             │
│  → Re-invoke /golang if skill context was lost              │
└─────────────────────────────────────────────────────────────┘

Core Philosophy

The Five Pillars

1. Simplicity over cleverness - Start with the simplest solution
2. Explicit over implicit - Clear code paths, no magic
3. Composition over inheritance - Use interfaces and embedding
4. Small interfaces - "The bigger the interface, the weaker the abstraction"
5. Errors are values - Handle errors explicitly; don't panic

Abstraction Rules

Rule 1: Principle of Least Abstraction

1.1 Default to a Single Function

• Solve the problem within a single function first
• Do NOT create helper functions, new types, or new packages prematurely

1.2 Justify Every Abstraction

• Before creating a new function, struct, or package, justify its existence
• If there's no strong reason to abstract, don't

Rule 2: Function Design

2.1 Single Responsibility

• If you cannot describe what a function does in one simple sentence, it's doing too much

2.2 Return Values

// 1-2 values: return directly
func Parse(s string) (int, error)

// 3+ related values: use struct
type ParseResult struct {
    Value    int
    Consumed int
    Warnings []string
}
func ParseDetailed(s string) (ParseResult, error)

Rule 3: Duplication vs. Abstraction

3.1 The Rule of Three

• Do NOT refactor duplicated code on first or second appearance
• Only consider abstraction on third instance

3.2 Verify True Duplication

• Confirm duplicated code represents the same core logic
• If similar by coincidence but different business rules, keep separate

Rule 4: Package and Interface Philosophy

4.1 Packages Have a Singular Purpose

• Package should represent a single concept
• DO NOT create "utility", "common", or "helpers" packages

4.2 Interfaces Defined by Consumer

• Do NOT define large, monolithic interfaces on producer side
• Consumer defines small interface describing only required behavior

4.3 Keep Interfaces Small

// GOOD: Single-method interface
type Reader interface {
    Read(p []byte) (n int, err error)
}

// BAD: Kitchen-sink interface
type DataManager interface {
    Read() ([]byte, error)
    Write([]byte) error
    Delete() error
    List() ([]string, error)
    // ... more methods
}

Go 1.26 Features

Generic Type Constraints

// Use generics when type safety matters across multiple types
func Min[T cmp.Ordered](a, b T) T {
    if a < b {
        return a
    }
    return b
}

// Use constraints for custom types
type Number interface {
    ~int | ~int64 | ~float64
}

When to use generics:

• Collection operations (slices, maps)
• Type-safe data structures
• Avoiding interface{} with type assertions

When NOT to use generics:

• Single concrete type suffices
• Interface polymorphism is clearer
• Adding complexity without benefit

Standard Library Packages

import (
    "cmp"
    "maps"
    "slices"
)

// slices package
sorted := slices.Clone(items)
slices.Sort(sorted)
idx, found := slices.BinarySearch(sorted, target)
slices.Reverse(items)

// maps package
maps.Clone(m)
maps.DeleteFunc(m, func(k, v string) bool {
    return v == ""
})

// cmp package
result := cmp.Compare(a, b)  // -1, 0, or 1
cmp.Or(a, b, c)  // first non-zero value

Structured Logging (log/slog)

import "log/slog"

// Create logger with handler
logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
    Level: slog.LevelInfo,
}))

// Structured logging
logger.Info("request processed",
    slog.String("method", r.Method),
    slog.String("path", r.URL.Path),
    slog.Duration("latency", elapsed),
    slog.Int("status", status),
)

// With context
logger.InfoContext(ctx, "operation completed",
    slog.String("operation", op),
)

// Group related attributes
logger.Info("user action",
    slog.Group("user",
        slog.String("id", user.ID),
        slog.String("role", user.Role),
    ),
)

Context Patterns

// context.AfterFunc for cleanup
ctx, cancel := context.WithCancel(parentCtx)
stop := context.AfterFunc(ctx, func() {
    cleanup()
})
// stop() to prevent callback if not needed

// Always propagate context
func ProcessItem(ctx context.Context, item Item) error {
    select {
    case <-ctx.Done():
        return ctx.Err()
    default:
    }
    // process...
}

Range-over-func Iterators

// Define an iterator
func (s *Set[T]) All() iter.Seq[T] {
    return func(yield func(T) bool) {
        for v := range s.items {
            if !yield(v) {
                return
            }
        }
    }
}

// Use with range
for item := range mySet.All() {
    process(item)
}

Error Handling

Wrapping Errors

// ALWAYS add context when wrapping
if err != nil {
    return fmt.Errorf("failed to process user %s: %w", userID, err)
}

// Sentinel errors for expected conditions
var (
    ErrNotFound     = errors.New("resource not found")
    ErrUnauthorized = errors.New("unauthorized")
)

// Custom error types for rich context
type ValidationError struct {
    Field   string
    Message string
}

func (e *ValidationError) Error() string {
    return fmt.Sprintf("validation failed on %s: %s", e.Field, e.Message)
}

Error Checking

// errors.Is for sentinel errors
if errors.Is(err, ErrNotFound) {
    return http.StatusNotFound, nil
}

// errors.As for error types
var validErr *ValidationError
if errors.As(err, &validErr) {
    return http.StatusBadRequest, validErr
}

// Multiple wrapped errors (Go 1.20+)
err := errors.Join(err1, err2, err3)

Anti-Patterns to Avoid

// BAD: No context
if err != nil {
    return err
}

// BAD: Logging and returning (double handling)
if err != nil {
    log.Printf("error: %v", err)
    return err
}

// BAD: Ignoring errors
result, _ := riskyOperation()

// BAD: Panic in library code
func ParseConfig(s string) Config {
    // Don't do this in libraries!
    if s == "" {
        panic("empty config")
    }
}

Concurrency Patterns

Worker Pool with Bounded Concurrency

func Process(ctx context.Context, items []Item, workers int) error {
    g, ctx := errgroup.WithContext(ctx)
    sem := make(chan struct{}, workers)

    for _, item := range items {
        item := item  // capture for Go < 1.22; still good practice
        g.Go(func() error {
            select {
            case sem <- struct{}{}:
                defer func() { <-sem }()
            case <-ctx.Done():
                return ctx.Err()
            }
            return processItem(ctx, item)
        })
    }
    return g.Wait()
}

Channel Ownership

// Producer owns the channel; producer closes
func Generate(ctx context.Context) <-chan int {
    ch := make(chan int)
    go func() {
        defer close(ch)  // Producer closes
        for i := 0; ; i++ {
            select {
            case ch <- i:
            case <-ctx.Done():
                return
            }
        }
    }()
    return ch
}

sync.Once for Lazy Initialization

type Client struct {
    once   sync.Once
    conn   *Connection
    connErr error
}

func (c *Client) getConn() (*Connection, error) {
    c.once.Do(func() {
        c.conn, c.connErr = dial()
    })
    return c.conn, c.connErr
}

When NOT to Use Channels

// Use mutex for simple shared state
type Counter struct {
    mu    sync.Mutex
    value int
}

func (c *Counter) Inc() {
    c.mu.Lock()
    c.value++
    c.mu.Unlock()
}

// Use atomic for single values
var counter atomic.Int64
counter.Add(1)

Testing

Table-Driven Tests

func TestParseConfig(t *testing.T) {
    tests := []struct {
        name    string
        input   string
        want    Config
        wantErr bool
    }{
        {
            name:  "valid config",
            input: `{"port": 8080}`,
            want:  Config{Port: 8080},
        },
        {
            name:    "invalid json",
            input:   `{invalid}`,
            wantErr: true,
        },
        {
            name:    "empty input",
            input:   "",
            wantErr: true,
        },
    }

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            got, err := ParseConfig(tt.input)
            if (err != nil) != tt.wantErr {
                t.Errorf("ParseConfig() error = %v, wantErr %v", err, tt.wantErr)
                return
            }
            if !tt.wantErr && got != tt.want {
                t.Errorf("ParseConfig() = %v, want %v", got, tt.want)
            }
        })
    }
}

Parallel Tests

func TestIndependentOperations(t *testing.T) {
    t.Parallel()  // Mark test as parallel-safe

    tests := []struct{...}

    for _, tt := range tests {
        tt := tt  // capture
        t.Run(tt.name, func(t *testing.T) {
            t.Parallel()  // Subtests can also be parallel
            // ...
        })
    }
}

Test Cleanup

func TestWithResource(t *testing.T) {
    db := setupTestDB(t)
    t.Cleanup(func() {
        db.Close()  // Runs after test completes
    })
    // Use db...
}

// Helper that registers cleanup
func setupTestDB(t *testing.T) *DB {
    t.Helper()
    db, err := NewDB(":memory:")
    if err != nil {
        t.Fatal(err)
    }
    t.Cleanup(func() { db.Close() })
    return db
}

AST-Grep Anti-Pattern Detection

Problematic Defer

// WRONG: Arguments evaluated immediately
defer require.NoError(t, failpoint.Disable("path"))

// CORRECT: Wrap in anonymous function
defer func() {
    require.NoError(t, failpoint.Disable("path"))
}()

Detection rule:

id: problematic-defer
language: go
rule:
  kind: defer_statement
  pattern: 'defer $A.$B(t, failpoint.$M($$))'

JSON Tag Security Issue

// WRONG: Field can still be unmarshaled with "-" key
type User struct {
    IsAdmin bool `json:"-,omitempty"`
}

// CORRECT: Properly omits field
type User struct {
    IsAdmin bool `json:"-"`
}

Detection rule:

id: unmarshal-tag-is-dash
language: go
rule:
  pattern: '`$TAG`'
  inside:
    kind: field_declaration
constraints:
  TAG:
    regex: 'json:"-,.*"'

Documentation Standards

// Package user provides user management functionality.
// It handles user creation, authentication, and profile management.
package user

// User represents a registered user in the system.
// Zero value is not useful; use NewUser to create instances.
type User struct {
    ID        string    // Unique identifier
    Email     string    // Validated email address
    CreatedAt time.Time // UTC timestamp
}

// NewUser creates a new User with the given email.
// It returns an error if the email format is invalid.
//
// Example:
//
//	user, err := NewUser("alice@example.com")
//	if err != nil {
//	    return fmt.Errorf("creating user: %w", err)
//	}
func NewUser(email string) (*User, error) {
    if !isValidEmail(email) {
        return nil, &ValidationError{Field: "email", Message: "invalid format"}
    }
    return &User{
        ID:        generateID(),
        Email:     email,
        CreatedAt: time.Now().UTC(),
    }, nil
}

Project Setup

# Initialize new Go 1.26 module
go mod init github.com/org/project

# Verify go.mod contains: go 1.26

# Install development tools
go install github.com/golangci/golangci-lint/cmd/golangci-lint@v1.62.0
go install golang.org/x/tools/cmd/goimports@latest

# Set up pre-commit hook
cp ~/.claude/skills/golang/scripts/pre-commit .git/hooks/pre-commit
chmod +x .git/hooks/pre-commit

# Copy linter config
cp ~/.claude/skills/golang/resources/.golangci.yml .golangci.yml
cp ~/.claude/skills/golang/resources/.editorconfig .editorconfig

Anti-Patterns Checklist

Scripts

Run setup:

~/.claude/skills/golang/scripts/lint.sh --help