---

name: go-options-gen description: Expert in generating functional options for Go structs using the options-gen library. metadata: short-description: Functional options generation for Go.

go-options-gen

You are an expert in using the options-gen library (https://github.com/kazhuravlev/options-gen) to create robust, type-safe functional options for Go components. You prioritize unexported option fields to maintain encapsulation while providing a clean, exported API for configuration.

Core Mandates

• File Naming:
  • Single Option Set: Struct definition MUST be in options.go, and generated code MUST be in options_generated.go.
  • Multiple Option Sets: For a component named MyService, the struct (e.g., MyServiceOptions) MUST be in myservice_options.go, and generated code MUST be in myservice_options_generated.go.
• Encapsulation:
  • Options fields within the struct SHOULD be unexported (start with a lowercase letter) to prevent direct modification from outside the package.
• Tooling:
  • Always run the tool using go tool options-gen.
  • Install and track the tool in go.mod using:

    Copygo get -tool github.com/kazhuravlev/options-gen/cmd/options-gen@latest
    

• Validation:
  • Always include validation tags (using go-playground/validator syntax) for configuration fields.
  • ALWAYS call the generated Validate() method within the component's constructor.
• Component Integration:
  • Store the resulting options struct in an unexported field named opts within your component struct.

Developer Workflow

1. Installation: Ensure the tool is tracked in your project:

   Copygo get -tool github.com/kazhuravlev/options-gen/cmd/options-gen@latest
   

2. Define Options (options.go): Define your options struct with unexported fields. Use the //go:generate directive to specify the output filename and the target struct.

   Copypackage mypackage
   
   import "time"
   
   //go:generate go tool options-gen -from-struct=Options -out-filename=options_generated.go
   type Options struct {
       timeout    time.Duration `option:"mandatory" validate:"required"`
       maxRetries int           `default:"3" validate:"min=1"`
       endpoints  []string      `option:"variadic=true"`
   }
   

3. Generate: Run the generator:

   Copygo generate ./options.go
   

4. Integration: Use the generated types in your component's constructor and store them in an opts field.

   Copytype Component struct {
       opts Options
   }
   
   func New(setters ...OptionOptionsSetter) (*Component, error) {
       opts := NewOptions(setters...)
       if err := opts.Validate(); err != nil {
           return nil, fmt.Errorf("invalid options: %w", err)
       }
       return &Component{opts: opts}, nil
   }
   

Expert Guidance

Mandatory vs. Default

• Use option:"mandatory" for fields that have no safe default (e.g., API keys, target URLs). These become required arguments in NewOptions().
• Use default:"value" for sensible defaults. options-gen supports basic types and time.Duration.

Advanced Defaults

For complex types (like maps or nested structs), use -defaults-from=func in the generate directive and define a provider function:

Copy//go:generate go tool options-gen -from-struct=Options -out-filename=options_generated.go -defaults-from=func
func defaultOptions() Options {
    return Options{
        headers: map[string]string{"User-Agent": "my-client"},
    }
}

Validation Best Practices

• Use validate:"required" for any field that must not be zero-valued.
• Use validate:"oneof=tcp udp" for enum-like string fields.
• Use validate:"min=1" for counters or sizes.

Variadic Setters

For slice fields, use option:"variadic=true" to generate a setter that accepts multiple arguments (e.g., WithEndpoints("a", "b")) instead of a single slice (e.g., WithEndpoints([]string{"a", "b"})).

Avoiding Exported Fields

By keeping fields unexported in options.go, you ensure that the only way to configure the component is through the generated With* setters, which can include validation logic.

Multiple Options in One Package

When a package contains multiple components (e.g., Client and Server), use prefixes to avoid name collisions in generated types and functions.

1. Filenames: Use <prefix>_options.go and <prefix>_options_generated.go.
2. Generator Flag: Use -out-prefix to prefix the generated NewOptions and Option...Setter types.

Example for MyService (myservice_options.go):

Copy//go:generate go tool options-gen -from-struct=MyServiceOptions -out-filename=myservice_options_generated.go -out-prefix=MyService
type MyServiceOptions struct {
    timeout time.Duration `option:"mandatory"`
}

This will generate NewMyServiceOptions and OptionMyServiceOptionsSetter, allowing them to coexist with other options in the same package.

Resources

• Examples: Complete implementations showing unexported fields, validation, and component integration can be found in the assets directory.