Local Helper Closure

Local Helper Function

• A technique of assigning an anonymous function that is only used inside a function to a variable.
• Minimizes argument passing by capturing variables from the parent function and removes repetitive logic.

e.g.

 1func BuildControllers(ctx context.Context, controllerCtx ControllerContext, controllerDescriptors map[string]*ControllerDescriptor,
 2	unsecuredMux *mux.PathRecorderMux, healthzHandler HealthCheckAdder) ([]Controller, error) {
 3	...
 4	buildController := func(controllerDesc *ControllerDescriptor) error {
 5		controllerName := controllerDesc.Name()
 6		ctrl, err := controllerDesc.BuildController(ctx, controllerCtx)
 7		...
 8		controllers = append(controllers, ctrl)
 9		checks = append(checks, check)
10		return nil
11	}
12	...
13	for _, controllerDesc := range controllerDescriptors {
14		...
15		if err := buildController(controllerDesc); err != nil {
16			return nil, err
17		}
18	}
19	...
20}

Advantages

• Can eliminate duplicate code
• Simplified Function Arguments
  • Uses captured external variables instead of function arguments (closure)
• Share state without global variables
  • Shares local variables within the scope
• Encapsulation/Isolation
  • Usable only within its scope

Caution

• Cannot write unit tests
• From a functional programming perspective, it's an impure function
• Avoid capturing too many external variables
• It's best to keep it short for readability
• Loop Variable Capture Issue
  • Go 1.21 and below: The loop variable (v) is not newly created with each iteration; instead, it continuously overwrites a single memory address
  • Go 1.22 and above: A new variable is created (shadowing) for each iteration of the loop