golang-samber-do

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Core Concepts

The Injector (Container)

import "github.com/samber/do/v2"

injector := do.New()

Service Types

• Lazy (default): Created when first requested

• Eager: Created immediately when the container starts

• Transient: New instance created on every request

• Value: Pre-created value, no instantiation

Provider Functions

Services MUST be registered via provider functions:

type Provider[T any] func(i Injector) (T, error)

Basic Usage

1. Define and Register Services

Follow "Accept Interfaces, Return Structs":

// Register a service (lazy by default)

do.Provide(injector, func(i do.Injector) (Database, error) {

    return &PostgreSQLDatabase{connString: "postgres://..."}, nil

})

// Register a pre-created value

do.ProvideValue(injector, &Config{Port: 8080})

// Register a transient service (new instance each time)

do.ProvideTransient(injector, func(i do.Injector) (*Logger, error) {

    return &Logger{}, nil

})

// Register an eager service (created immediately at startup)

do.ProvideValue(injector, &Config{Port: 8080})

2. Invoke Services

The container MUST only be accessed at the composition root:

// Invoke with error handling

db, err := do.Invoke[Database](injector)

// MustInvoke panics on error (use when confident service exists)

db := do.MustInvoke[Database](injector)

3. Service Dependencies

func NewUserService(i do.Injector) (UserService, error) {

    db := do.MustInvoke[Database](i)

    cache := do.MustInvoke[Cache](i)

    return &userService{db: db, cache: cache}, nil

}

do.Provide(injector, NewUserService)

4. Implicit Aliasing (Preferred)

Register a concrete type and invoke as an interface without explicit aliasing:

// Register concrete type

do.Provide(injector, func(i do.Injector) (*PostgreSQLDatabase, error) {

    return &PostgreSQLDatabase{}, nil

})

// Invoke directly as interface (implicit aliasing)

db := do.MustInvokeAs[Database](injector)

5. Named Services

Register multiple services of the same type:

do.ProvideNamed(injector, "primary-db", func(i do.Injector) (*Database, error) {

    return &Database{URL: "postgres://primary..."}, nil

})

mainDB := do.MustInvokeNamed[*Database](injector, "primary-db")

Package Organization

Use do.Package() to organize service registration by module:

// infrastructure/package.go

var Package = do.Package(

    do.Lazy(func(i do.Injector) (*postgres.DB, error) {

        cfg := do.MustInvoke[*Config](i)

        return postgres.Connect(cfg.DatabaseURL)

    }),

    do.Lazy(func(i do.Injector) (*redis.Client, error) {

        cfg := do.MustInvoke[*Config](i)

        return redis.NewClient(cfg.RedisURL), nil

    }),

)

// main.go

injector := do.New(infrastructure.Package, service.Package)

Full Application Setup

func main() {

    injector := do.New(

        infrastructure.Package,

        repository.Package,

        service.Package,

        transport.Package,

    )

    server := do.MustInvoke[*http.Server](injector)

    go server.ListenAndServe()

    _ = injector.ShutdownOnSignalsWithContext(context.Background(), os.Interrupt)

}

Best Practices

• Depend on interfaces, not concrete types — lets you swap implementations in tests without touching production code

• Each service should have one job — services with multiple responsibilities are harder to test and harder to replace

• Keep dependency trees shallow — chains beyond 3-4 levels make initialization order fragile and errors harder to trace

• Handle errors in provider functions — a silently failing provider creates a broken service that crashes later in unexpected places

• Use scopes to organize services by lifecycle — request-scoped services prevent leaks, global services prevent redundant initialization

For scopes, lifecycle management, struct injection, and debugging, see Advanced Usage.

For testing patterns (cloning, overrides, mocks), see Testing.

Quick Reference

Registration

Function

Purpose

do.Provide[T]()

Register lazy service (default)

do.ProvideNamed[T]()

Register named lazy service

do.ProvideValue[T]()

Register pre-created value

do.ProvideNamedValue[T]()

Register named value

do.ProvideTransient[T]()

Register new instance each time

do.ProvideNamedTransient[T]()

Register named transient service

do.Package()

Group service registrations

Invocation

Function

Purpose

do.Invoke[T]()

Get service (with error)

do.InvokeNamed[T]()

Get named service

do.InvokeAs[T]()

Get first service matching interface

do.InvokeStruct[T]()

Inject into struct fields using tags

do.MustInvoke[T]()

Get service (panic on error)

do.MustInvokeNamed[T]()

Get named service (panic on error)

do.MustInvokeAs[T]()

Get service by interface (panic on error)

do.MustInvokeStruct[T]()

Inject into struct (panic on error)

Cross-References

• → See samber/cc-skills-golang@golang-dependency-injection skill for DI concepts, comparison, and when to adopt a DI library

• → 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