golang-uber-dig

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 — 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), preferred 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

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:

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.

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).

// 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 whose inputs are dependencies and whose outputs are provided types. error (last return) signals construction failure. Follow "accept interfaces, return structs".

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:"...".

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) { /* ... */ }

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.

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.

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.

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.

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. Return errors from constructors instead of panicking — dig wraps them with the dependency path, which makes the failure point obvious.

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.

• 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

• → 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.