writing-go

name	writing-go
description	Idiomatic Go 1.25+ development. Use when writing Go code, designing APIs, discussing Go patterns, or reviewing Go implementations. Emphasizes stdlib, concrete types, simple error handling, and minimal dependencies.
allowed-tools	Read, Bash, Grep, Glob

Go Development (1.25+)

Core Philosophy

1. Stdlib and Mature Libraries First

   • Always prefer Go stdlib solutions
   • External deps only when stdlib is insufficient
   • Choose mature, well-maintained libs when needed
   • Don't reinvent the wheel—use existing solutions

2. Concrete Types Over any

   • Never use interface{} or any when concrete type works
   • Generics for reusable utilities, concrete types for business logic
   • Accept interfaces, return structs

3. Private Interfaces at Consumer

   • Define interfaces private (lowercase) where used
   • Decouples code, enables testing
   • Implementation returns concrete types

4. Flat Control Flow

   • Early returns, guard clauses
   • No nested IFs—max 2 levels
   • Switch for multi-case logic

5. Explicit Error Handling

   • Always wrap with context
   • Use errors.Is()/errors.As()
   • No bare return err

Quick Patterns

Private Interface at Consumer

// service/user.go - private interface where it's USED
type userStore interface {
    Get(ctx context.Context, id string) (*User, error)
}

type Service struct {
    store userStore  // accepts interface
}

// repo/postgres.go - returns concrete type
func NewPostgresStore(db *sql.DB) *PostgresStore {
    return &PostgresStore{db: db}
}

Flat Control Flow (No Nesting)

// GOOD: guard clauses, early returns
func process(user *User) error {
    if user == nil {
        return ErrNilUser
    }
    if user.Email == "" {
        return ErrMissingEmail
    }
    if !isValidEmail(user.Email) {
        return ErrInvalidEmail
    }
    return doWork(user)
}

// BAD: nested conditions
func process(user *User) error {
    if user != nil {
        if user.Email != "" {
            if isValidEmail(user.Email) {
                return doWork(user)
            }
        }
    }
    return nil
}

Error Handling

if err := doThing(); err != nil {
    return fmt.Errorf("do thing: %w", err)  // always wrap
}

// Sentinel errors
if errors.Is(err, ErrNotFound) {
    return http.StatusNotFound
}

Concrete Types (Avoid any)

// GOOD: concrete types
func ProcessUsers(users []User) error { ... }
func GetUserByID(id string) (*User, error) { ... }

// BAD: unnecessary any
func ProcessItems(items []any) error { ... }
func GetByID(id any) (any, error) { ... }

Table-Driven Tests

tests := []struct {
    name    string
    input   string
    want    string
    wantErr bool
}{
    {"valid", "hello", "HELLO", false},
    {"empty", "", "", true},
}
for _, tt := range tests {
    t.Run(tt.name, func(t *testing.T) {
        got, err := Process(tt.input)
        if tt.wantErr {
            require.Error(t, err)
            return
        }
        require.NoError(t, err)
        assert.Equal(t, tt.want, got)
    })
}

Go 1.25 Features

• testing/synctest: Deterministic concurrent testing
• encoding/json/v2: 3-10x faster (GOEXPERIMENT=jsonv2)
• runtime/trace.FlightRecorder: Production trace capture
• Container-aware GOMAXPROCS: Auto-detects cgroup limits

References

• PATTERNS.md - Detailed code patterns
• TESTING.md - Testing with testify/mockery
• CLI.md - CLI application patterns

Tooling

go build ./...           # Build
go test -race ./...      # Test with race detector
golangci-lint run        # Lint
mockery --all            # Generate mocks