Advanced — uber-go/dig

Detail topics that are referenced from SKILL.md. Each section is self-contained.

Decorate

Decorate modifies a value already provided in the container — the decorator receives the original instance and returns a replacement. Common uses: enriching a logger with context, wrapping a metrics scope with tags, swapping a real client for a recording one in a child scope.

c.Decorate(func(log *zap.Logger) *zap.Logger {
    return log.Named("worker")
})

Decorators apply to the scope they were registered in and to that scope's descendants. Use them at scope boundaries or package wiring boundaries, not in main(), so changes stay local.

Scopes

A Scope is a child container that inherits providers from its parent and can add or override its own. Scopes let request-, tenant-, or module-level dependencies coexist with shared singletons:

root := dig.New()
root.Provide(NewLogger)
root.Provide(NewDatabase)

requestScope := root.Scope("request")
requestScope.Provide(NewRequestContext)            // only visible inside requestScope
requestScope.Decorate(func(l *zap.Logger) *zap.Logger {
    return l.With(zap.String("scope", "request"))
})

By default, providers registered to a scope are private to that scope and its children. Pass dig.Export(true) to Provide inside a scope to make the type visible from the parent:

requestScope.Provide(NewSharedCache, dig.Export(true))

Optional Dependencies

optional:"true" lets a consumer compile and run when a provider is missing. Use it sparingly — optional dependencies hide configuration mistakes. They make sense for genuinely optional features (a tracing exporter, an in-memory cache) but not for core services like a database.

type Params struct {
    dig.In

    Logger *zap.Logger
    Tracer trace.Tracer `optional:"true"`
}

Error Handling

dig wraps the constructor error with the dependency path so you can see which graph edge failed:

if err := c.Invoke(run); err != nil {
    // err describes the chain: "could not build *http.Server: ..."
}

Useful helpers:

errors.As(err, &dig.Error{}) — true if the error originated inside dig
dig.RootCause(err) — unwrap to the original constructor error returned by user code
dig.IsCycleDetected(err) — true if the graph contains a cycle (typically reported at first Invoke unless DeferAcyclicVerification is set)
errors.As(err, &dig.PanicError{}) — when RecoverFromPanics is enabled, a panicking constructor surfaces as this typed error

Visualization

dig can emit the dependency graph in DOT format — useful when wiring becomes too tangled to reason about by reading code:

f, _ := os.Create("graph.dot")
_ = dig.Visualize(c, f)
// then: dot -Tpng graph.dot -o graph.png

dig.VisualizeError(err) highlights the failed edges when an Invoke returns an error — invaluable for debugging "missing type" failures in deep graphs.

Quick Reference

Container

Function/Method	Purpose
`dig.New(opts...)`	Create a root container
`c.Provide(ctor, opts...)`	Register a constructor
`c.Invoke(fn, opts...)`	Run a function with injected dependencies
`c.Decorate(fn, opts...)`	Modify a previously-provided value within a scope
`c.Scope(name, opts...)`	Create a child scope (private providers by default)
`c.String()`	Human-readable text summary of providers (not DOT; use `dig.Visualize` for DOT)

Provide options

Option	Purpose
`dig.Name("...")`	Disambiguate same-typed providers
`dig.Group("...")`	Add the result to a value group
`dig.As(new(I))`	Provide the concrete value as one or more interfaces
`dig.Export(true)`	Make a scope-level provider visible from the root
`dig.FillProvideInfo(&info)`	Capture metadata for tooling

Container options

Option	Purpose
`dig.DeferAcyclicVerification()`	Defer cycle check to first `Invoke`
`dig.RecoverFromPanics()`	Convert constructor panics into `dig.PanicError`
`dig.DryRun(true)`	Validate without invoking constructors

Errors

Helper	Purpose
`dig.RootCause(err)`	Unwrap to the user-returned error
`dig.IsCycleDetected(err)`	True if the graph has a cycle
`errors.As(err, &dig.PanicError{})`	Detect a recovered panic
`dig.Visualize(c, w, opts...)`	Write the graph in DOT format

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