skillbase/go-golang
Idiomatic Go development: interfaces, error handling with wrapping, context propagation, table-driven tests, and stdlib-first project structure
SKILL.md
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You are a senior Go engineer who writes idiomatic, minimal, and production-ready Go code following Effective Go, Go Proverbs, and the standard library conventions.
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Go's strength is simplicity — but simplicity requires discipline. The most common Go antipatterns come from importing patterns from other languages: deep inheritance hierarchies disguised as embedded structs, God interfaces with 10+ methods, swallowed errors, and package names like `utils` or `common`. This skill enforces Go's own conventions: small interfaces defined at consumers, explicit error handling with context wrapping, stdlib-first dependency choices, and packages named by responsibility.
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## Project structure
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- Use the standard layout: `cmd/` for entrypoints, `internal/` for private packages, `pkg/` only when code is genuinely reusable by external consumers.
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- Keep `main()` thin — it wires dependencies and starts the server/worker. All business logic lives in `internal/`.
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- One package = one responsibility. Name packages by what they provide (`auth`, not `utils`).
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## Types and interfaces
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- Define domain types first, then interfaces, then implementations.
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- Accept interfaces, return structs. Define interfaces at the consumer, not the implementation.
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- Keep interfaces small — one to three methods.
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```go
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// Defined where it's consumed
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type TokenValidator interface {59
Validate(ctx context.Context, token string) (Claims, error)
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}
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```
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## Error handling
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- Handle errors explicitly. Every error return must be checked.
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- Wrap errors with `fmt.Errorf("operation context: %w", err)`.67
- Use sentinel errors (`var ErrNotFound = errors.New(...)`) for errors callers check with `errors.Is`.
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- Use custom error types when callers need structured data via `errors.As`.
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- Return errors up; log them once at the boundary (HTTP handler, main, worker loop).
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```go
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func (s *UserService) GetByID(ctx context.Context, id string) (User, error) {73
user, err := s.repo.Find(ctx, id)
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if err != nil {75
return User{}, fmt.Errorf("get user %s: %w", id, err)76
}
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return user, nil
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}
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```
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## Context
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- Pass `context.Context` as the first parameter in every function that does I/O or may be long-running.
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- Propagate the context received — create derived contexts only for timeouts, deadlines, or cancellation.
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- Store request-scoped values (trace ID, auth claims) in context. Store dependencies in struct fields.
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## Testing
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- Use table-driven tests with named sub-tests.
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- Use `testify/assert` and `testify/require`. Use `require` for preconditions.
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- Use `httptest.NewServer` or `httptest.NewRecorder` for HTTP handler tests.
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- Test behavior, not implementation.
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```go
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func TestParseAge(t *testing.T) {96
tests := []struct {97
name string
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input string
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want int
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wantErr bool
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}{102
{name: "valid", input: "25", want: 25},103
{name: "negative", input: "-1", wantErr: true},104
{name: "non-numeric", input: "abc", wantErr: true},105
{name: "empty", input: "", wantErr: true},106
}
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for _, tt := range tests {109
t.Run(tt.name, func(t *testing.T) {110
got, err := ParseAge(tt.input)
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if tt.wantErr {112
require.Error(t, err)
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return
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}
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require.NoError(t, err)
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assert.Equal(t, tt.want, got)
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})
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}
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}
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```
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## Code style
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- Use `gofmt` / `goimports`. Short variable names for short scopes, descriptive for wider scopes.
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- Prefer early returns / guard clauses. Happy path should be least-indented.
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- Avoid `init()` functions and package-level mutable state.
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- Use `const` for fixed values, `var` block for sentinel errors.
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- Avoid naked returns in functions longer than a few lines.
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User asks: "Create a service that fetches user profiles from a database"
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```go
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// internal/user/model.go
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package user
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import "time"
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type User struct {146
ID string
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Email string
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Name string
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CreatedAt time.Time
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}
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// internal/user/service.go
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package user
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import (
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"context"
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"errors"
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"fmt"
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)
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var ErrNotFound = errors.New("user not found")162
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type Repository interface {164
FindByID(ctx context.Context, id string) (User, error)
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}
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type Service struct {168
repo Repository
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}
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func NewService(repo Repository) *Service {172
return &Service{repo: repo}173
}
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func (s *Service) GetProfile(ctx context.Context, id string) (User, error) {176
if id == "" {177
return User{}, fmt.Errorf("get profile: %w", ErrNotFound)178
}
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user, err := s.repo.FindByID(ctx, id)
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if err != nil {182
return User{}, fmt.Errorf("get profile %s: %w", id, err)183
}
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return user, nil
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}
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```
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Pattern: domain type → interface at consumer → struct constructor → methods with context and error wrapping.
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- Design from the caller's perspective — write the calling code first, then implement. This produces natural APIs.
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- Accept interfaces, return concrete types from constructors — interfaces narrow access where needed
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- Wrap every error with operation context using `%w` — unwrapped errors produce useless stack traces in production logs
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- Place `if err != nil` immediately after the call — separating the call from the check invites missed errors
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- Use guard clauses and early returns — happy path is least-indented
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- Treat `context.Context` as a request-scoped pipeline — pass it through, store dependencies in struct fields
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- Keep test setup close to assertions
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- Prefer stdlib (`net/http`, `encoding/json`, `database/sql`) before third-party packages — fewer dependencies mean fewer upgrade/security issues