Golang Uber Dig Agent Skill

Persona: You are a Go architect wiring an application graph with dig. You keep the container at the composition root, depend on interfaces not concrete types, and treat constructor errors as first-class failures.

Using uber-go/dig for Dependency Injection in Go

Reflection-based DI toolkit, designed to power application frameworks (it is the engine behind uber-go/fx) and resolve object graphs during startup.

Official Resources:

• pkg.go.dev/go.uber.org/dig
• github.com/uber-go/dig

This skill is not exhaustive. Please refer to library documentation and code examples for more information. For Go package docs, symbols, versions, importers, and known vulnerabilities, → See samber/cc-skills-golang@golang-pkg-go-dev skill (godig) — prefer it over Context7 for Go package facts. To navigate this library's usage in your own code (definitions, call sites, diagnostics), → See samber/cc-skills-golang@golang-gopls skill (gopls). Context7 remains a fallback for docs not indexed on pkg.go.dev.

go get go.uber.org/dig

dig vs. fx

fx is built on dig and shares the same container engine — the DI primitives (Provide, Invoke, In/Out structs, named values, value groups) are identical. fx.In/fx.Out are re-exports of dig.In/dig.Out.

What fx adds on top of dig:

| Concern | dig | fx |

| --- | --- | --- |

| DI container | ✅ dig.New() | ✅ (embedded) |

| Lifecycle hooks | ❌ | ✅ fx.Lifecycle OnStart/OnStop |

| Module system | ❌ | ✅ fx.Module with scoped decorators |

| Signal-aware run loop | ❌ | ✅ app.Run() blocks on SIGINT/SIGTERM |

| Structured event logging | ❌ | ✅ fx.WithLogger / fxevent |

| Startup/shutdown timeout | ❌ | ✅ fx.StartTimeout / fx.StopTimeout |

Choose dig when you need the wiring graph only: CLI tools, libraries exposing a container to callers, test harnesses, or embedding DI into an existing app that manages its own lifecycle.

Choose fx for long-running services (HTTP servers, workers, daemons) — lifecycle and signal handling are non-negotiable there. See samber/cc-skills-golang@golang-uber-fx skill.

Container

import "go.uber.org/dig"

c := dig.New()

Useful options: dig.DeferAcyclicVerification() (faster startup), dig.RecoverFromPanics() (turn panics into dig.PanicError), dig.DryRun(true) (validate without invoking).

Provide and Invoke

// Register a constructor — lazy, only runs when its output is needed
err := c.Provide(func(cfg *Config) (*sql.DB, error) {
    return sql.Open("postgres", cfg.DSN)
})

// Pull a service out of the container by asking for it as a function parameter
err = c.Invoke(func(db *sql.DB) error {
    return db.Ping()
})

Constructors are lazy and memoized: each output type is built once and shared (singleton per container). Provide errors at registration if the constructor is malformed; Invoke returns the constructor's error wrapped with the dependency path that triggered it.

A dig constructor is any function. Inputs are dependencies, outputs are provided types. error (last return) signals construction failure. Follow "accept interfaces, return structs".

Parameter Objects with dig.In

Once a constructor has 4+ dependencies, embed dig.In to group them as struct fields and tag fields:

type HandlerParams struct {
    dig.In

    Logger *zap.Logger
    DB     *sql.DB
    Cache  *redis.Client `optional:"true"`           // zero value if not provided
    DBRO   *sql.DB       `name:"readonly"`           // named dependency
    Routes []http.Handler `group:"routes"`           // value group
}

func NewHandler(p HandlerParams) *Handler { /* ... */ }

Tags: name:"...", optional:"true", group:"...".

Result Objects with dig.Out

Return several values from one constructor and attach name/group tags to results:

type ConnResult struct {
    dig.Out

    ReadWrite *sql.DB `name:"primary"`
    ReadOnly  *sql.DB `name:"readonly"`
}

func NewConnections(cfg *Config) (ConnResult, error) { /* ... */ }

Named Values

Two providers of the same type collide. Disambiguate with dig.Name:

c.Provide(NewPrimaryDB,  dig.Name("primary"))
c.Provide(NewReadOnlyDB, dig.Name("readonly"))

Consume by adding name:"primary" / name:"readonly" to a dig.In field.

Value Groups

Many providers, one consumer slice — typical for HTTP handlers, health checks, migrations:

type RouteResult struct {
    dig.Out
    Handler http.Handler `group:"routes"`
}

func NewUserHandler(db *sql.DB) RouteResult { /* ... */ }
func NewPostHandler(db *sql.DB) RouteResult { /* ... */ }

type ServerParams struct {
    dig.In
    Routes []http.Handler `group:"routes"`
}

Flatten — append ,flatten (e.g. group:"routes,flatten") to unwrap a slice instead of nesting it. Group order is not guaranteed; if order matters, provide an explicit ordered slice from a single constructor.

Provide as Interface (dig.As)

Register a concrete constructor and expose it under one or more interfaces without a separate adapter:

c.Provide(NewPostgresDB, dig.As(new(Database), new(io.Closer)))
// Consumers ask for Database or io.Closer; *PostgresDB stays hidden.

Full Application Example

func main() {
    c := dig.New()

    must(c.Provide(NewConfig))
    must(c.Provide(NewLogger))
    must(c.Provide(NewDatabase))
    must(c.Provide(NewServer))

    err := c.Invoke(func(srv *http.Server) error {
        return srv.ListenAndServe()
    })
    if err != nil {
        log.Fatal(err)
    }
}

func must(err error) { if err != nil { panic(err) } }

dig has no built-in lifecycle. If you need OnStart/OnStop hooks, signal handling, and graceful shutdown, use fx — see samber/cc-skills-golang@golang-uber-fx skill.

For Decorate, Scopes, optional deps, error helpers, and Visualize, see advanced.md.

Best Practices

• Keep the container at the composition root — never pass *dig.Container as a parameter; treat it like a plumbing detail of main(). Service-locator patterns defeat the testability gains of DI.
• Depend on interfaces, not concrete types — lets you swap implementations in tests without touching production code, and lets you use dig.As to expose narrow interfaces from wide structs.
• Prefer parameter objects (dig.In structs) once a constructor has 4+ dependencies — call sites stay readable and adding a new dependency is a one-line change instead of a signature break.
• Group registration by module (one file per module that calls c.Provide for its types) — review and refactoring become a per-module concern, and you can extract a module into a fx.Module later without rewriting wiring.
• Validate the graph eagerly in tests — call c.Invoke against the composition root in CI to surface missing providers at boot time, not at first request. DryRun(true) skips constructor execution.
• Return errors from constructors instead of panicking — dig wraps them with the dependency path, which makes the failure point obvious.

Common Mistakes

| Mistake | Fix |

| --- | --- |

| Passing the container into services | The container belongs to main(). Inject the typed dependencies a service needs; otherwise tests need to build a real container. |

| Two providers for the same type without Name | dig errors at Provide time. Either name them, or merge into a single provider that returns a dig.Out result struct. |

| Ignoring Provide errors | Wrap each Provide with a must helper. A silent registration error becomes a missing-type error far later. |

| Using groups when ordering matters | Groups are unordered. If order matters (middleware chain, migration sequence), provide an explicit ordered slice with one constructor. |

| Constructors with side effects on import | Keep init() empty — start work only inside the constructor, after the graph is built. |

Testing

dig containers are cheap — build a fresh one per test, override providers with Decorate, and call Invoke to drive the system. For full patterns (per-test wiring, shared helpers, graph validation in CI, asserting wire-time errors, recovering from constructor panics), see testing.md.

Further Reading

• advanced.md — Decorate, Scopes, optional deps, error helpers, Visualize, full Quick Reference
• recipes.md — end-to-end examples: HTTP server with route group, two databases, request scopes, decorators, dry-run validation
• testing.md — testing patterns and graph validation

Cross-References

• → See samber/cc-skills-golang@golang-uber-fx skill for application lifecycle, modules, and signal-aware Run() built on top of dig
• → See samber/cc-skills-golang@golang-dependency-injection skill for DI concepts and library comparison
• → See samber/cc-skills-golang@golang-samber-do skill for a generics-based alternative without reflection
• → See samber/cc-skills-golang@golang-google-wire skill for compile-time DI (no runtime container)
• → See samber/cc-skills-golang@golang-structs-interfaces skill for interface design patterns
• → See samber/cc-skills-golang@golang-testing skill for general testing patterns

If you encounter a bug or unexpected behavior in uber-go/dig, open an issue at <https://github.com/uber-go/dig/issues>.