I have a package named "seeder":
package seeder
import "fmt"
func MyFunc1() {
fmt.Println("I am Masood")
}
func MyFunc2() {
fmt.Println("I am a programmer")
}
func MyFunc3() {
fmt.Println("I want to buy a car")
}
Now I want to call all functions with MyFunc prefix
package main
import "./seeder"
func main() {
for k := 1; k <= 3; k++ {
seeder.MyFunc1() // This calls MyFunc1 three times
}
}
I want something like this:
for k := 1; k <= 3; k++ {
seeder.MyFunc + k ()
}
and this output:
I am Masood
I am a programmer
I want to buy a car
EDIT1:
In this example, parentKey is a string variable which changed in a loop
for parentKey, _ := range uRLSjson{
pppp := seeder + "." + strings.ToUpper(parentKey)
gorilla.HandleFunc("/", pppp).Name(parentKey)
}
But GC said:
use of package seeder without selector
You can't get a function by its name, and that is what you're trying to do. The reason is that if the Go tool can detect that a function is not referred to explicitly (and thus unreachable), it may not even get compiled into the executable binary. For details see Splitting client/server code.
With a function registry
One way to do what you want is to build a "function registry" prior to calling them:
registry := map[string]func(){
"MyFunc1": MyFunc1,
"MyFunc2": MyFunc2,
"MyFunc3": MyFunc3,
}
for k := 1; k <= 3; k++ {
registry[fmt.Sprintf("MyFunc%d", k)]()
}
Output (try it on the Go Playground):
Hello MyFunc1
Hello MyFunc2
Hello MyFunc3
Manual "routing"
Similar to the registry is inspecting the name and manually routing to the function, for example:
func callByName(name string) {
switch name {
case "MyFunc1":
MyFunc1()
case "MyFunc2":
MyFunc2()
case "MyFunc3":
MyFunc3()
default:
panic("Unknown function name")
}
}
Using it:
for k := 1; k <= 3; k++ {
callByName(fmt.Sprintf("MyFunc%d", k))
}
Try this on the Go Playground.
Note: It's up to you if you want to call the function identified by its name in the callByName() helper function, or you may choose to return a function value (of type func()) and have it called in the caller's place.
Transforming functions to methods
Also note that if your functions would actually be methods of some type, you could do it without a registry. Using reflection, you can get a method by name: Value.MethodByName(). You can also get / enumerate all methods without knowing their names using Value.NumMethod() and Value.Method() (also see Type.NumMethod() and Type.Method() if you need the name of the method or its parameter types).
This is how it could be done:
type MyType int
func (m MyType) MyFunc1() {
fmt.Println("Hello MyFunc1")
}
func (m MyType) MyFunc2() {
fmt.Println("Hello MyFunc2")
}
func (m MyType) MyFunc3() {
fmt.Println("Hello MyFunc3")
}
func main() {
v := reflect.ValueOf(MyType(0))
for k := 1; k <= 3; k++ {
v.MethodByName(fmt.Sprintf("MyFunc%d", k)).Call(nil)
}
}
Output is the same. Try it on the Go Playground.
Another alternative would be to range over an array of your functions
package main
import (
"fmt"
)
func MyFunc1() {
fmt.Println("I am Masood")
}
func MyFunc2() {
fmt.Println("I am a programmer")
}
func MyFunc3() {
fmt.Println("I want to buy a car")
}
func main() {
for _, fn := range []func(){MyFunc1, MyFunc2, MyFunc3} {
fn()
}
}
Related
Suppose I have the following struct:
package manager
type Manager struct {
strings []string
}
func (m *Manager) AddString(s string) {
m.strings = append(m.strings, s)
}
func (m *Manager) RemoveString(s string) {
for i, str := range m.strings {
if str == s {
m.strings = append(m.strings[:i], m.strings[i+1:]...)
}
}
}
This pattern is not thread safe, so the following test fails due to some race condition (array index out of bounds):
func TestManagerConcurrently(t *testing.T) {
m := &manager.Manager{}
wg := sync.WaitGroup{}
for i:=0; i<100; i++ {
wg.Add(1)
go func () {
m.AddString("a")
m.AddString("b")
m.AddString("c")
m.RemoveString("b")
wg.Done()
} ()
}
wg.Wait()
fmt.Println(m)
}
I'm new to Go, and from googling around I suppose I should use channels (?). So one way to make this concurrent would be like this:
type ManagerA struct {
Manager
addStringChan chan string
removeStringChan chan string
}
func NewManagerA() *ManagerA {
ma := &ManagerA{
addStringChan: make(chan string),
removeStringChan: make(chan string),
}
go func () {
for {
select {
case msg := <-ma.addStringChan:
ma.AddString(msg)
case msg := <-ma.removeStringChan:
ma.RemoveString(msg)
}
}
}()
return ma
}
func (m* ManagerA) AddStringA(s string) {
m.addStringChan <- s
}
func (m* ManagerA) RemoveStringA(s string) {
m.removeStringChan <- s
}
I would like to expose an API similar to the non-concurrent example, hence AddStringA, RemoveStringA.
This seems to work as expected concurrently (although I guess the inner goroutine should also exit at some point). My problem with this is that there is a lot of extra boilerplate:
need to define & initialize channels
define inner goroutine loop with select
map functions to channel calls
It seems a bit much to me. Is there a way to simplify this (refactor / syntax / library)?
I think the best way to implement this would be to use a Mutex instead? But is it still possible to simplify this sort of boilerplate?
Using a mutex would be perfectly idiomatic like this:
type Manager struct {
mu sync.Mutex
strings []string
}
func (m *Manager) AddString(s string) {
m.mu.Lock()
m.strings = append(m.strings, s)
m.mu.Unlock()
}
func (m *Manager) RemoveString(s string) {
m.mu.Lock()
for i, str := range m.strings {
if str == s {
m.strings = append(m.strings[:i], m.strings[i+1:]...)
}
}
m.mu.Unlock()
}
You could do this with channels, but as you noted it is a lot of extra work for not much gain. Just use a mutex is my advice!
If you simply need to make the access to the struct thread-safe, use mutex:
type Manager struct {
sync.Mutex
data []string
}
func (m *Manager) AddString(s string) {
m.Lock()
m.strings = append(m.strings, s)
m.Unlock()
}
I have a package named "seeder":
package seeder
import "fmt"
func MyFunc1() {
fmt.Println("I am Masood")
}
func MyFunc2() {
fmt.Println("I am a programmer")
}
func MyFunc3() {
fmt.Println("I want to buy a car")
}
Now I want to call all functions with MyFunc prefix
package main
import "./seeder"
func main() {
for k := 1; k <= 3; k++ {
seeder.MyFunc1() // This calls MyFunc1 three times
}
}
I want something like this:
for k := 1; k <= 3; k++ {
seeder.MyFunc + k ()
}
and this output:
I am Masood
I am a programmer
I want to buy a car
EDIT1:
In this example, parentKey is a string variable which changed in a loop
for parentKey, _ := range uRLSjson{
pppp := seeder + "." + strings.ToUpper(parentKey)
gorilla.HandleFunc("/", pppp).Name(parentKey)
}
But GC said:
use of package seeder without selector
You can't get a function by its name, and that is what you're trying to do. The reason is that if the Go tool can detect that a function is not referred to explicitly (and thus unreachable), it may not even get compiled into the executable binary. For details see Splitting client/server code.
With a function registry
One way to do what you want is to build a "function registry" prior to calling them:
registry := map[string]func(){
"MyFunc1": MyFunc1,
"MyFunc2": MyFunc2,
"MyFunc3": MyFunc3,
}
for k := 1; k <= 3; k++ {
registry[fmt.Sprintf("MyFunc%d", k)]()
}
Output (try it on the Go Playground):
Hello MyFunc1
Hello MyFunc2
Hello MyFunc3
Manual "routing"
Similar to the registry is inspecting the name and manually routing to the function, for example:
func callByName(name string) {
switch name {
case "MyFunc1":
MyFunc1()
case "MyFunc2":
MyFunc2()
case "MyFunc3":
MyFunc3()
default:
panic("Unknown function name")
}
}
Using it:
for k := 1; k <= 3; k++ {
callByName(fmt.Sprintf("MyFunc%d", k))
}
Try this on the Go Playground.
Note: It's up to you if you want to call the function identified by its name in the callByName() helper function, or you may choose to return a function value (of type func()) and have it called in the caller's place.
Transforming functions to methods
Also note that if your functions would actually be methods of some type, you could do it without a registry. Using reflection, you can get a method by name: Value.MethodByName(). You can also get / enumerate all methods without knowing their names using Value.NumMethod() and Value.Method() (also see Type.NumMethod() and Type.Method() if you need the name of the method or its parameter types).
This is how it could be done:
type MyType int
func (m MyType) MyFunc1() {
fmt.Println("Hello MyFunc1")
}
func (m MyType) MyFunc2() {
fmt.Println("Hello MyFunc2")
}
func (m MyType) MyFunc3() {
fmt.Println("Hello MyFunc3")
}
func main() {
v := reflect.ValueOf(MyType(0))
for k := 1; k <= 3; k++ {
v.MethodByName(fmt.Sprintf("MyFunc%d", k)).Call(nil)
}
}
Output is the same. Try it on the Go Playground.
Another alternative would be to range over an array of your functions
package main
import (
"fmt"
)
func MyFunc1() {
fmt.Println("I am Masood")
}
func MyFunc2() {
fmt.Println("I am a programmer")
}
func MyFunc3() {
fmt.Println("I want to buy a car")
}
func main() {
for _, fn := range []func(){MyFunc1, MyFunc2, MyFunc3} {
fn()
}
}
Suppose I have a dependency that looks like this:
type Dependency interface {
Retrieve(transform func(row *Row) string) []string
}
And I'm using it in code that I'd like to unit test
// ...
result := dep.Retrieve(func(row *Row) string {
// ... do stuff
})
// ...
This is a contrived example, but consider it for something like gcloud pubsub, which has a Receive method that calls a user-defined function for each message it pulls.
If I use mockgen to get a mock of Dependency, how do I tell the mock how to behave? I'd want it to call its input function some number of times with some sets of input.
Here is some code while trying to understand what you wanted to achieve.
I don't use mockgen, but manually, you can do what you want. I totally changed the behavior between the original and the mocked call. Does it answer your questions?
package main
import (
"fmt"
)
var str = [...]string{
"world",
"night",
}
type Dependency interface {
Get(val string) string
}
type dependency struct {
i int
}
func (d *dependency) Get(fn func(int) string) string {
d.i++
d.i = d.i % len(str)
return "hello " + fn(d.i)
}
func main() {
d := dependency{}
myfn := func(idx int) string { return str[idx] }
fmt.Println(d.Get(myfn))
fmt.Println(d.Get(myfn))
fmt.Println(d.Get(myfn))
m := mock{}
fmt.Println(m.Get(myfn))
}
type mock struct {
}
func (m *mock) Get(fn func(int) string) string {
i := 0
j := (i + 1) % len(str)
k := (j + 1) % len(str)
return "mocked " + fn(i) + fn(j) + fn(k)
}
The play link:
https://play.golang.org/p/bb7WrmlIEN
Cycle loops, when I increment i by the function, but no via i++.
package main
import "fmt"
func increment(i int) (int) {
i++
return i
}
func condition_true(i int) (bool) {
if i < 10 {
return true
} else {
return false
}
}
func main() {
for i := 1; condition_true(i); increment(i) {
fmt.Println(i)
}
}
compiler to play around
You should do i = increment(i).
Otherwise, the i used in the loop is not modified.
for i := 1; condition_true(i); i = increment(i) {
fmt.Println(i)
}
That one works as you'd expect.
https://play.golang.org/p/dwHbV1iY0_
Alternatively, allow increment to modify i by receiving a pointer to it:
func increment(i *int) {
*i++
}
And then use it like this in the loop:
for i := 1; condition_true(i); increment(&i) {
fmt.Println(i)
}
This is happening because the increment function isn't actually changing the i value because the i is passed by value into the function.
Simply remove the increment in the for loop and replace it with i++
I try to write a function which takes any other function and wraps a new function around it. This is what I have tried so far:
package main
import (
"fmt"
)
func protect (unprotected func (...interface{})) (func (...interface{})) {
return func (args ...interface{}) {
fmt.Println ("protected");
unprotected (args...);
};
}
func main () {
a := func () {
fmt.Println ("unprotected");
};
b := protect (a);
b ();
}
When I compile this I get the error:
cannot use a (type func()) as type func(...interface { }) in function argument
Why is a function without arguments not compatible to a function with a variable number of arguments? What can I do to make them compatible?
Update:
The protected function should be compatible with the original:
func take_func_int_int (f func (x int) (y int)) (int) {
return f (1)
}
func main () {
a := func (x int) (y int) {
return 2 * x
}
b := protect (a)
take_func_int_int (a)
take_func_int_int (b)
}
Types are pretty concrete in Go. You could try
a := func(_ ...interface{}) {
fmt.Println("unprotected")
}
func (...interface{}) does not mean "any function that takes any number of any kind of arguments", it means "only a function which takes a variable number of interface{} arguments"
Alternatively rather than func(...interface{}) you can just use interface{} and the reflect package. See http://github.com/hoisie/web.go for an example.
EDIT: Specifically, this:
package main
import (
"fmt"
"reflect"
)
func protect(oldfunc interface{}) (func (...interface{})) {
if reflect.TypeOf(oldfunc).Kind() != reflect.Func {
panic("protected item is not a function")
}
return func (args ...interface{}) {
fmt.Println("Protected")
vargs := make([]reflect.Value, len(args))
for n, v := range args {
vargs[n] = reflect.ValueOf(v)
}
reflect.ValueOf(oldfunc).Call(vargs)
}
}
func main() {
a := func() {
fmt.Println("unprotected")
}
b := func(s string) {
fmt.Println(s)
}
c := protect(a)
d := protect(b)
c()
d("hello")
}
Ouput is
Protected
unprotected
Protected
hello
EDIT: To answer the update
Like I said above, types are pretty concrete in Go. The protect function returns a type func(...interface{}) which will never be assignable to func(int)int. I think you're probably either over-engineering your problem or misunderstanding it. However, here's a highly discouraged code snippet that would make it work.
First change protect to also return values:
func protect(oldfunc interface{}) (func (...interface{}) []interface{}) {
if reflect.TypeOf(oldfunc).Kind() != reflect.Func {
panic("protected item is not a function")
}
return func (args ...interface{}) []interface{} {
fmt.Println("Protected")
vargs := make([]reflect.Value, len(args))
for n, v := range args {
vargs[n] = reflect.ValueOf(v)
}
ret_vals := reflect.ValueOf(oldfunc).Call(vargs)
to_return := make([]interface{}, len(ret_vals))
for n, v := range ret_vals {
to_return[n] = v.Interface()
}
return to_return
}
}
Then make a convert function:
func convert(f func(...interface{}) (func(int) int) {
return func(x int) int {
r := f(x)
return r[0].(int)
}
}
Then your call would look like
take_func_int_int(convert(b))
But I promise this isn't what you actually want to do.
Step back and try to rework the problem. I've completely killed type-safety in these examples. What are you trying to accomplish?
package main
import "fmt"
// Here's a function that will take an arbitrary number
// of `int`s as arguments.
func sum(nums ...int) {
fmt.Print(nums, " ")
total := 0
for _, num := range nums {
total += num
}
fmt.Println(total)
}
func main() {
// Variadic functions can be called in the usual way
// with individual arguments.
sum(1, 2)
sum(1, 2, 3)
// If you already have multiple args in a slice,
// apply them to a variadic function using
// `func(slice...)` like this.
nums := []int{1, 2, 3, 4}
sum(nums...)
}