Source: registry.go in package github.com/lann/builder

package builder

import (
	"reflect"
	"sync"
)

var (
	registry = make(map[reflect.Type]reflect.Type)
	registryMux sync.RWMutex
)

RegisterType maps the given builderType to a structType. This mapping affects the type of slices returned by Get and is required for GetStruct to work. Returns a Value containing an empty instance of the registered builderType. RegisterType will panic if builderType's underlying type is not Builder or if structType's Kind is not Struct.

func RegisterType(builderType reflect.Type, structType reflect.Type) *reflect.Value {
	registryMux.Lock()
	defer registryMux.Unlock()
	structType.NumField() // Panic if structType is not a struct
	registry[builderType] = structType
	emptyValue := emptyBuilderValue.Convert(builderType)
	return &emptyValue
}

Register wraps RegisterType, taking instances instead of Types. Returns an empty instance of the registered builder type which can be used as the initial value for builder expressions. See example.

func Register(builderProto, structProto interface{}) interface{} {
	empty := RegisterType(
		reflect.TypeOf(builderProto),
		reflect.TypeOf(structProto),
	).Interface()
	return empty
}

func getBuilderStructType(builderType reflect.Type) *reflect.Type {
	registryMux.RLock()
	defer registryMux.RUnlock()
	structType, ok := registry[builderType]
	if !ok {
		return nil
	}
	return &structType
}

func newBuilderStruct(builderType reflect.Type) *reflect.Value {
	structType := getBuilderStructType(builderType)
	if structType == nil {
		return nil
	}
	newStruct := reflect.New(*structType).Elem()
	return &newStruct