Can a package be an interface in Go? - go

I'd like to do something like the following in Go, where c/c.go contains:
package c
import (
a "github.com/.../a"
b "github.com/.../b"
)
func CallHandler(pkgName string) (err error) {
pkg, err := DereferencePackage(pkgName)
...
method, err := GetMethod(pkg, "Handler")
if err != nil {
return err
}
return method()
}
...
{
err = CallHandler("a")
...
err = CallHandler("b")
...
}
a/a.go contains:
package a
func Handler() (err error) {
// Do whatever
return err
}
and b/b.go contains:
package b
func Handler() (err error) {
// Do whatever
return err
}
An obvious solution would be a switch:
func CallHandler(pkgName string) (err error) {
switch pkgName {
case "a":
return a.Handler()
case "b":
return b.Handler()
default:
return fmt.Errorf("no handler defined for %s", pkgName)
}
}
but I wondered if it's possible to do this a bit more elegantly, perhaps using the reflect package. Another way to ask the same question is, can a package be an interface in Go?

One simple solution is that create a registry package which contain
a map (key as string i.e. name of package and value is function pointer)
a function to fill the data in map.
a function to call the function pointer from map based on package name passed.
In each package, implement init() function that will register the package name with Handler function to register package.

Related

How to create a mock inside the function under test

I have a set of functions, which uses the pool of objects. This pool has been mocked. It works fine in most of the cases. But in some functions i call the methods of objects from the pool. So i need to mock this objects too.
Lets say:
// ObjectGeter is a interface that is mocked
type ObjectGeter interface {
GetObject(id int) ObjectType, error
}
// this function is under test
func SomeFunc(og ObjectGeter,id int, otherArgument SomeType) error {
// some actions with otherArgument
// and may be return an error
obj, err := og.GetObject(id)
if err !=nil {
return errors.New("GetObject error")
}
rezult, err := obj.SomeMethod()
if err !=nil {
return errors.New("One of internal errors")
}
return rezult, nil
}
Is there a way to test whole this function? I can create interface SomeMethoder which wraps the SomeMethod(), but i can't find the way how to assign it to obj inside SomeFunc without changing the signature of GetObject to GetObject(id int) SomeMethoder,error.
Currently i see the one approach - testing by a parts.
The only solution i'v found without of changing of paradigm is a wrapper. It is pretty trivial but may be some one will need it once.
Originally i have some type:
type PoolType struct {...}
func (p *PoolType)GetObject(id int) (ObjectType, error) {...}
and interface, that wraps PoolType.GetObject and that i'v mocked.
Now i have the interface:
type SomeMethoder interface {
SomeMethod() (ResultType, error)
}
to wrap object returned by PoolType.GetObject().
To produce it i have interface:
type ObjectGeter interface {
GetObject(id int) (SomeMethoder, error)
}
and type
type MyObjectGeter struct {
pool *PoolType
}
func New(pool *PoolType) *MyObjectGeter {
return &MyObjectGeter{pool: pool}
}
func (p *MyObjectGeter)GetObject(id int) (SomeMethoder, error) {
return p.pool.GetObject(id)
}
that implements it.
So:
// this function is under test
func SomeFunc(og ObjectGeter,id int, otherArgument SomeType) error {
// some actions with otherArgument
// and may be return an error
iface, err := og.GetObject(id)
if err !=nil {
return errors.New("GetObject error")
}
rezult, err := iface.SomeMethod()
if err !=nil {
return errors.New("One of internal errors")
}
return rezult, nil
}
is called by
og := New(pool)
SomeFunc(og,id,otherArgument)
in real work.
After all to test whole SomeFunc i have to:
func TestSomeFuncSuccess (t *testing.T) {
controller := gomock.NewController(t)
defer controller.Finish()
objectGeter := mocks.NewMockObjectGeter(controller)
someMethoder := mocks.NewMockSomeMethoder(controller)
gomock.InOrder(
args.objectGeter.EXPECT().
GetObject(correctIdCOnst).
Return(someMethoder, nil),
args.someMethoder.EXPECT().
SomeMethod().
Return(NewResultType(...),nil).
Times(args.test.times[1]),
)
result, err := SomeFunc(objectGeter,correctIdCOnst,otherArgumentConst)
// some checks
}
So, the only untested part is MyObjectGeter.GetObject that is enough for me.

Can golang plugins be used for factory functions?

I have the following code in a golang plugin module:
plug.go
package main
import "fmt"
var (
Thing = New("first thing")
ThingFactory = thingFactory{}
)
type thing struct {
i int
s string
}
func New(s string) thing {
return thing{s: s}
}
func (t *thing) Say() string {
t.i++
return fmt.Sprintf("%s - %d", t.s, t.i)
}
type thingFactory struct{}
func (t thingFactory) Make(s string) thing {
return New(s)
}
it is compiled as a .so object and used in another program:
main.go
package main
import (
"fmt"
"plugin"
)
func main() {
p, err := plugin.Open("../plug/plug.so")
if err != nil {
panic(err)
}
symbol, err := p.Lookup("Thing")
if err != nil {
panic(err)
}
thing := symbol.(Sayer)
fmt.Println(thing.Say())
symbol, err = p.Lookup("ThingFactory") // <-problems start here
if err != nil {
panic(err)
}
factory := symbol.(GetSayer)
madeThing := factory.Make("how about me?")
fmt.Println(madeThing.Say())
fmt.Println(madeThing.Say())
}
type Sayer interface {
Say() string
}
type GetSayer interface {
Make(string) Sayer
}
I'm able to lookup the Thing, and call Say() on it, but the second interface conversion panics:
first thing - 1
panic: interface conversion: *main.thingFactory is not main.GetSayer: missing method Make
even though the runtime recognizes the first symbol as a Sayer it doesn't recognize that thingFactory obviously has a Make() method, which should return something that is also a Sayer.
Am I missing something obvious here?
The first problem is that in your plugin thingFactory (more precicely *thingfactory) does not have a method described in your main app's GetSayer interface:
Make(string) Sayer
You have:
Make(string) thing
So (first) you have to change thingFactory.Make() to this:
type Sayer interface {
Say() string
}
func (t thingFactory) Make(s string) Sayer {
th := New(s)
return &th
}
After this it still won't work. And the reason for this is because the plugin's Sayer type is not identical to your main app's Sayer type. But they must be the same in order to implement your main app's GetSayer interface.
One solution is to "outsource" the Sayer interface to its own package, and use this common, shared package both in the plugin and in the main app.
Let's create a new package, call it subplay:
package subplay
type Sayer interface {
Say() string
}
Import this package and use it in the plugin:
package main
import (
"fmt"
"path/to/subplay"
)
var (
Thing = New("first thing")
ThingFactory = thingFactory{}
)
type thing struct {
i int
s string
}
func New(s string) thing {
return thing{s: s}
}
func (t *thing) Say() string {
t.i++
return fmt.Sprintf("%s - %d", t.s, t.i)
}
type thingFactory struct{}
func (t thingFactory) Make(s string) subplay.Sayer {
th := New(s)
return &th
}
And also import and use it in the main app:
package main
import (
"fmt"
"path/to/subplay"
"plugin"
)
func main() {
p, err := plugin.Open("../plug/plug.so")
if err != nil {
panic(err)
}
symbol, err := p.Lookup("Thing")
if err != nil {
panic(err)
}
thing := symbol.(subplay.Sayer)
fmt.Println(thing.Say())
symbol, err = p.Lookup("ThingFactory")
if err != nil {
panic(err)
}
factory := symbol.(GetSayer)
madeThing := factory.Make("how about me?")
fmt.Println(madeThing.Say())
fmt.Println(madeThing.Say())
}
type GetSayer interface {
Make(string) subplay.Sayer
}
Now it will work, and output will be:
first thing - 1
how about me? - 1
how about me? - 2
See related questions:
go 1.8 plugin use custom interface
How do Go plugin dependencies work?
Your plugin Make method should return a Sayer object not thing
type Sayer interface {
Say() string
}
func (t *thingFactory) Make(s string) Sayer {
return New(s)
}

How to avoid a long switch-case statement in Go

I'm writing a chat bot in Go and wondering how can I avoid a long switch-case statement similar to this one:
switch {
// #bot search me HMAC
case strings.Contains(message, "search me"):
query := strings.Split(message, "search me ")[1]
return webSearch(query), "html"
// #bot thesaurus me challenge
case strings.Contains(message, "thesaurus me"):
query := strings.Split(message, "thesaurus me ")[1]
return synonyms(query), "html"
Should I define those handlers each in a separate package or should I just use structs and interfaces? Which method will allow me to have a good structure, avoid switch-case and let external developers to easier create handlers?
I think packages will be a better choice but I'm not sure how to register the handlers with the main bot. Would appreciate an example.
You could use a map[string]command similar to how the net/http package registers handlers. Something akin to this:
https://play.golang.org/p/9YzHyLodAQ
package main
import (
"fmt"
"errors"
)
type BotFunc func(string) (string, error)
type BotMap map[string]BotFunc
var Bot = BotMap{}
func (b BotMap) RegisterCommand(command string, f BotFunc) error {
if _, exists := b[command]; exists {
return errors.New("command already exists")
}
b[command] = f
return nil
}
func (b BotMap) Execute(statement string) (string, error) {
// parse out command and query however you choose (not this way obviously)
command := statement[:9]
query := statement[10:]
return b.ExecuteQuery(command, query)
}
func (b BotMap) ExecuteQuery(command, query string) (string, error) {
if com, exists := b[command]; exists {
return com(query)
}
return "", errors.New("command doesn't exist")
}
func main() {
err := Bot.RegisterCommand("search me", func(query string) (string, error) {
fmt.Println("search", query)
return "searched", nil
})
if err != nil {
fmt.Println(err)
return
}
err = Bot.RegisterCommand("thesaurus me", func(query string) (string, error) {
fmt.Println("thesaurus", query)
return "thesaurused", nil
})
if err != nil {
fmt.Println(err)
return
}
result, err := Bot.Execute("search me please")
if err != nil {
fmt.Println(err)
return
}
fmt.Println(result)
}
Obviously there's a lot of checks missing here, but this is the basic idea.

How To Call Method of another package from string method name in Go

Suppose I have two package like
-a
-b
a have some methods like this
func TestOne() { //something }
func TestTwo() { //something }
I need to call package a's methods from package b but by only string method name. Like i get the string "TestOne" and calls for the method TestOne(). How can i get that done.
Edit :
I have Read about reflect. but reflect needs an struct and functions be a member of that struct. What if My functions are not member of a struct? just plain methods in a package. and the calling methods and called methods are in different package. Then?
NB. There could be some methods that have parameters as well.
Like LightWeight said in his answer, you can use reflection.
You use the reflect.ValueOf method to get the value of the type. Then you can use the MethodByName method to get the function value. Once you have the function value you can call the Call method to execute it.
Code Sample
package main
import (
"fmt"
"reflect"
)
type TypeOne struct {
}
func (t *TypeOne) FuncOne() {
fmt.Println("FuncOne")
}
func (t *TypeOne) FuncTwo(name string) {
fmt.Println("Hello", name)
}
func CallFuncByName(myClass interface{}, funcName string, params ...interface{}) (out []reflect.Value, err error) {
myClassValue := reflect.ValueOf(myClass)
m := myClassValue.MethodByName(funcName)
if !m.IsValid() {
return make([]reflect.Value, 0), fmt.Errorf("Method not found \"%s\"", funcName)
}
in := make([]reflect.Value, len(params))
for i, param := range params {
in[i] = reflect.ValueOf(param)
}
out = m.Call(in)
return
}
func main() {
t1 := &TypeOne{}
out, err := CallFuncByName(t1, "FuncOne")
if err != nil {
panic(err)
}
//Return value
_ = out
out, err = CallFuncByName(t1, "FuncTwo", "monkey")
if err != nil {
panic(err)
}
//Return value
_ = out
}
You can try to use reflect in go. This link may be help you
https://golang.org/pkg/reflect/
and http://mikespook.com/2012/07/function-call-by-name-in-golang/
func foo() {
// bla...bla...bla...
}
func bar(a, b, c int) {
// bla...bla...bla...
}
funcs := map[string]interface{}{"foo":foo, "bar":bar}

Golang downcasting list of structs

I want to be able to unmarshal yaml files less rigidly. That is, my library has a predefined number of options the yaml file must have. Then, the user should be able to extend this to include any custom options.
Here is what I have
package main
import (
"net/http"
"yamlcms"
"github.com/julienschmidt/httprouter"
)
type Page struct {
*yamlcms.Page
Title string
Date string
}
func getBlogRoutes() {
pages := []*Page{}
yamlcms.ReadDir("html", pages)
}
// This section is a work in progress, I only include it for loose context
func main() {
router := httprouter.New()
//blogRoutes := getBlogRoutes()
//for _, blogRoute := range *blogRoutes {
// router.Handle(blogRoute.Method, blogRoute.Pattern,
// func(w http.ResponseWriter, r *http.Request, _ httprouter.Params) {})
//}
http.ListenAndServe(":8080", router)
}
Here is the yamlcms package:
package yamlcms
import (
"io/ioutil"
"os"
"strings"
"gopkg.in/yaml.v2"
)
type Page struct {
Slug string `yaml:"slug"`
File string `yaml:"file"`
}
func (page *Page) ReadFile(file string) (err error) {
fileContents, err := ioutil.ReadFile(file)
if err != nil {
return
}
err = yaml.Unmarshal(fileContents, &page)
return
}
func isYamlFile(fileInfo os.FileInfo) bool {
return !fileInfo.IsDir() && strings.HasSuffix(fileInfo.Name(), ".yaml")
}
func ReadDir(dir string, pages []*Page) (err error) {
filesInfo, err := ioutil.ReadDir(dir)
if err != nil {
return
}
for i, fileInfo := range filesInfo {
if isYamlFile(fileInfo) {
pages[i].ReadFile(fileInfo.Name())
}
}
return
}
There is a compiler issue here:
src/main.go:19: cannot use pages (type []*Page) as type []*yamlcms.Page in argument to yamlcms.ReadDir
My main intent in this question is to learn the idiomatic way of doing this kind of thing in Go. Other 3rd-party solutions may exist but I am not immediately interested in them because I have problems like this frequently in Go having to do with inheritance, etc. So along the lines of what I've presented, how can I best (idiomatically) accomplish what I am going for?
EDIT:
So I've made some changes as suggested. Now I have this:
type FileReader interface {
ReadFile(file string) error
}
func ReadDir(dir string, pages []*FileReader) (err error) {
filesInfo, err := ioutil.ReadDir(dir)
if err != nil {
return
}
for i, fileInfo := range filesInfo {
if isYamlFile(fileInfo) {
(*pages[i]).ReadFile(fileInfo.Name())
}
}
return
}
However, I still get a similar compiler error:
src/main.go:19: cannot use pages (type []*Page) as type []*yamlcms.FileReader in argument to yamlcms.ReadDir
Even though main.Page should be a FileReader because it embeds yamlcms.Page.
EDIT: I forgot that slices of interfaces don't work like that. You'd need to allocate a new slice, convert all pages to FileReaders, call the function, and convert them back.
Another possible solution is refactoring yamlcms.ReadDir to return the contents of the files, so that they could be unmarshaled later:
// In yamlcms.
func ReadYAMLFilesInDir(dir string) ([][]byte, error) { ... }
// In client code.
files := yamlcms.ReadYAMLFilesInDir("dir")
for i := range pages {
if err := yaml.Unmarshal(files[i], &pages[i]); err != nil { return err }
}
The original answer:
There are no such things as inheritance or casting in Go. Prefer composition and interfaces in your designs. In your case, you can redefine your yamlcms.ReadDir to accept an interface, FileReader.
type FileReader interface {
ReadFile(file string) error
}
Both yamlcms.Page and main.Page will implement this, as the latter embeds the former.

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