golint-sl

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emptyinterface

About 308 words

Flags problematic usage of interface{} or any.

Category

Clean Code

What It Checks

This analyzer detects uses of empty interface (interface{} or any) that could be replaced with concrete types or proper interfaces:

  • Function results of type interface{} or any, except for functions whose names mark them as decoders, getters or wrappers (Marshal, Unmarshal, Decode, Encode, Get, Load, Read, Parse, Convert, Wrap, Value)
  • Parameters that are maps with interface{} values, such as map[string]any
  • Struct fields that are maps with interface{} values, or slices or arrays of interface{}

An alias of any counts as the empty interface; a defined type such as type Value interface{} doesn't. The analyzer doesn't check type assertions: errcheck's check-type-assertions setting reports assertions without an ok check.

Why It Matters

Empty interface:

  • Bypasses type safety
  • Requires type assertions at runtime
  • Makes code harder to understand
  • Hides API contracts

Examples

Bad: Empty Interface Parameter

func Process(data interface{}) error {
    // What types are valid? Unknown!
    switch v := data.(type) {
    case string:
        return processString(v)
    case int:
        return processInt(v)
    default:
        return errors.New("unsupported type")  // Runtime error!
    }
}

Good: Specific Interface

type Processable interface {
    Process() error
}

func Process(data Processable) error {
    return data.Process()  // Type-safe!
}

Good: Generics (Go 1.18+)

func Process[T Processable](data T) error {
    return data.Process()
}

Acceptable: JSON/Reflection

// JSON unmarshaling needs interface{}
func parseJSON(data []byte) (map[string]interface{}, error) {
    var result map[string]interface{}
    err := json.Unmarshal(data, &result)
    return result, err
}

Acceptable: Logging

// Logging accepts any value
logger.Info("event", zap.Any("data", complexStruct))

Configuration

# .golint-sl.yaml
analyzers:
  emptyinterface: true  # enabled by default

When to Disable

  • Heavy use of reflection
  • JSON processing code
  • Plugin systems
analyzers:
  emptyinterface: false