I am trying to create function pointer to a function that has a method receiver. However, I can't figure out how to get it to work (if it is possible)?
Essentially, I have the following:
type Foo struct {...}
func (T Foo) Bar bool {
...
}
type BarFunc (Foo) func() bool // Does not work.
The last line of the code gives the error
syntax error: unexpected func, expecting semicolon or newline
If you want to create a function pointer to a method, you have two ways. The first is essentially turning a method with one argument into a function with two:
type Summable int
func (s Summable) Add(n int) int {
return s+n
}
var f func(s Summable, n int) int = (Summable).Add
// ...
fmt.Println(f(1, 2))
The second way will "bind" the value of s (at the time of evaluation) to the Summable receiver method Add, and then assign it to the variable f:
s := Summable(1)
var f func(n int) int = s.Add
fmt.Println(f(2))
Playground: http://play.golang.org/p/ctovxsFV2z.
Any changes to s after f is assigned will have no affect on the result: https://play.golang.org/p/UhPdYW5wUOP
And for an example more familiar to those of us used to a typedef in C for function pointers:
package main
import "fmt"
type DyadicMath func (int, int) int // your function pointer type
func doAdd(one int, two int) (ret int) {
ret = one + two;
return
}
func Work(input []int, addthis int, workfunc DyadicMath) {
for _, val := range input {
fmt.Println("--> ",workfunc(val, addthis))
}
}
func main() {
stuff := []int{ 1,2,3,4,5 }
Work(stuff,10,doAdd)
doMult := func (one int, two int) (ret int) {
ret = one * two;
return
}
Work(stuff,10,doMult)
}
https://play.golang.org/p/G5xzJXLexc
I am very likely off-target (just started on Golang) but what if you create a pointer then examine type:
pfun := Bar
fmt.Println("type of pfun is:", reflect.TypeOf(pfun))
then it seems that you can declare the type of pointer correctly:
https://play.golang.org/p/SV8W0J9JDuQ
Related
This question already has answers here:
How can I access a struct field with generics (type T has no field or method)?
(1 answer)
Generic function to work on different structs with common members from external package?
(1 answer)
Closed 3 months ago.
There are two struct types, Foo and Bar, with an int data member val. I am trying to write a generic function that can handle both types. I tried the following and this did not work.
package main
import "fmt"
type Foo struct {
val int
}
type Bar struct {
val int
}
func Add[T any](slice []T) int {
var sum int
for _, elem := range slice {
sum += elem.val
}
return sum
}
func Test() {
f1 := Foo{val: 2}
f2 := Foo{val: 2}
fslice := []Foo{f1, f2}
fsum := Add(fslice)
fmt.Printf("fsum = %d\n", fsum)
b1 := Bar{val: 3}
b2 := Bar{val: 3}
bslice := []Bar{b1, b2}
bsum := Add(bslice)
fmt.Printf("bsum = %d\n", bsum)
}
func main() {
Test()
}
The compiler throws the following error.
$ go run generics1.go
# command-line-arguments
./generics1.go:16:15: elem.val undefined (type T has no field or method val)
Go playground link: https://go.dev/play/p/mdOMH3xuwu7
What could be a possible way to approach this?
Per golang 1.18 release note
The Go compiler does not support accessing a struct field x.f where x is of type parameter type even if all types in the type parameter's type set have a field f. We may remove this restriction in a future release.
You could define one GetVal() interface method to retrieve the val, and use this method as part of type constraint of generic.
Sample codes
type Foo struct {
val int
}
func (f Foo) GetVal() int {
return f.val
}
type Bar struct {
val int
}
func (b Bar) GetVal() int {
return b.val
}
type MyType interface {
Foo | Bar
GetVal() int
}
func Add[T MyType](slice []T) int {
var sum int
for _, elem := range slice {
sum += elem.GetVal()
}
return sum
}
https://go.dev/play/p/0eJZpqy7q8f
I have the following situation.
type A func(x int) int
type B func(x int) A
var testFunc = func() B {
var ret interface{} = func(x int) A {
return func(y int) int {
return x * y
}
}
return ret.(func(x int) A)
//return ret.(B)
}
func main() {
var res = testFunc()(1)(2)
fmt.Println("Here the result", res)
}
Looking at the snippet above, this is what I am trying to do.
First I define a type A which is a function which takes an int and returns an int.
Then I define a type B which is a function that takes an int and returns an A, i.e. B is an higher level function returning an A.
Then I define a testFunc, which is a function which returns a B, as defined in its signature.
Now, let's look at the body of testFunc. In here I define a ret variable as interface{} and then I set its value to a function that takes an int and returns an A, which is what B is. So, within testFunc, I end up doing the following operations:
I type-seert ret, which is a variable of type interface{}, against the type func(x int) A
Then, while returning ret, what I sense is that, given the signature of testFunc which specifies B as return type, there is an implicit type-conversion from func(x int) A to B
My problem is the following. If, in testFunc, I type-assert the ret variable against func(x int) A, everything works as charm.
But if I type-assert the ret variable against B, which is what you see in the commented line at the end of testFunc, I get the error
panic: interface conversion: interface {} is func(int) main.A, not main.B
Why the somehow implicit conversion from func(int) main.A to B works while the explicit type-assertion does not?
Here is a "playground" reproducing the case.
RESPONSE AFTER HAVING READ THE CONTRIBUTION GIVEN IN THE ANSWERS AND COMMENTS
B is based on func(int) main.A as its source type but it is a different type. Therefore the type assertion of ret against type func(int) main.A actually works, but the type-assertion against B generates an error.
When returning, the function testFunc performs a type-convertion, i.e. copies ret to a new value of type B implicitly performing the B(ret) operation.
If I had used type aliases, i.e. if I had defined B as an alias of func(int) main.A like this
type B = func(int) main.A
type assertion ret.(B) would have worked since in this case B is just another name, i.e. an alias, of func(int) main.A.
You are not type-casting, you using type-assertion. Type assertion tests if the underlying type of an interface is the requested type.
var ret interface{} = func(x int) A {...}
Above, ret is an interface containing a func(x int) A, and it is not B. You can copy the function to a B, and then the type-assertion works:
var ret interface{} = B(func(x int) A {...})
// Here, ret is of type B
return ret.(B)
This question already has an answer here:
golang how can I use struct name as map key
(1 answer)
Closed 9 months ago.
We have a following function:
func (h *Handler) Handle(message interface{}) error {
//here there is a switch for different messages
switch m := message.(type) {
}
}
This signature is given and can't be changed. There are around 20 different message types the handler processes.
Now, there are some of these messages (around 4) which need special post-processing. In a different package.
Thus, I am thinking to do this like this:
func (h *Handler) Handle(message interface{}) error {
//here there is a switch for different messages
switch m := message.(type) {
}
//only post-process if original message processing succeeds
postProcessorPkg.Process(message)
}
Now, in the Process function, I want to quickly lookup if the message type is indeed of the ones we need postprocessing for. I don't want to do a switch again here. There are many handlers, in different packages, with varying amount of message types, and it should be generic.
So I was thinking of registering the message type in the postprocessor and then just do a lookup:
func (p *Postprocessor) Register(msgtype interface{}) {
registeredTypes[msgtype] = msgtype
}
and then
func (p *Postprocessor) Process(msgtype interface{}) error {
if ok := registeredTypes[msgtype]; !ok {
return errors.New("Unsupported message type")
}
prop := GetProp(registeredTypes[msgtype])
doSmthWithProp(prop)
}
This will all not work now because I can only "register" instances of the message, not the message type itself, as far as I know. Thus the map would only match a specific instance of a message, not its type, which is what I need.
So I guess this needs redesign. I can completely ditch the registering and the map lookup, but
I can't change the Handle function to a specific type (signature will need to remain message interface{}
I would like to avoid to have to use reflect, just because I will have a hard time defending such a solution with some colleagues.
As there is no possibility to set a type as the map key, I finally decided to implement the following solution, which is based on #Chrono Kitsune 's solution:
type Postprocess interface {
NeedsPostprocess() bool
}
type MsgWithPostProcess struct {}
func (p *MsgWithPostProcess) NeedsPostprocess() bool {
return true
}
type Msg1 struct {
MsgWithPostProcess
//other stuff
}
type Msg2 struct {
MsgWithPostProcess
//other stuff
}
type Msg3 struct {
//no postprocessing needed
}
func (p *Postprocessor) Process(msgtype interface{}) error {
if _, ok := msgtype.(Postprocess); ok {
//do postprocessing
}
}
As of my simple test I did, only Msg1 and Msg2 will be postprocessed, but not Msg3, which is what I wanted.
This question was the first hit I found on Google but the title is somewhat misleading. So I'll leave this here to add some food for thought with the title of the question in mind.
First, the issue with maps is that its key must be a comparable value. This is why for example a slice cannot be used is a map key. A slice is not comparable and is therefore not allowed. You can use an array (fixed sized slice) but not a slice for the same reason.
Second, you have in the reflect.TypeOf(...).String()a way to get a canonical string representation for types. Though it is not unambiguous unless you include the package path, as you can see here.
package main
import (
"fmt"
s2 "go/scanner"
"reflect"
s1 "text/scanner"
)
type X struct{}
func main() {
fmt.Println(reflect.TypeOf(1).String())
fmt.Println(reflect.TypeOf(X{}).String())
fmt.Println(reflect.TypeOf(&X{}).String())
fmt.Println(reflect.TypeOf(s1.Scanner{}).String())
fmt.Println(reflect.TypeOf(s2.Scanner{}).String())
fmt.Println(reflect.TypeOf(s1.Scanner{}).PkgPath(), reflect.TypeOf(s1.Scanner{}).String())
fmt.Println(reflect.TypeOf(s2.Scanner{}).PkgPath(), reflect.TypeOf(s2.Scanner{}).String())
}
int
main.X
*main.X
scanner.Scanner
scanner.Scanner
text/scanner scanner.Scanner
go/scanner scanner.Scanner
https://play.golang.org/p/NLODZNdik6r
With this information, you can (if you feel so inclined) create a map which let's go from a reflect.Type to a key and back again, like this.
package main
import (
"fmt"
s2 "go/scanner"
"reflect"
s1 "text/scanner"
)
type TypeMap struct {
m []reflect.Type
}
func (m *TypeMap) Get(t reflect.Type) int {
for i, x := range m.m {
if x == t {
return i
}
}
m.m = append(m.m, t)
return len(m.m) - 1
}
func (m *TypeMap) Reverse(t int) reflect.Type {
return m.m[t]
}
type X struct{}
func main() {
var m TypeMap
fmt.Println(m.Get(reflect.TypeOf(1)))
fmt.Println(m.Reverse(0))
fmt.Println(m.Get(reflect.TypeOf(1)))
fmt.Println(m.Reverse(0))
fmt.Println(m.Get(reflect.TypeOf(1)))
fmt.Println(m.Reverse(0))
fmt.Println(m.Get(reflect.TypeOf(X{})))
fmt.Println(m.Reverse(1))
fmt.Println(m.Get(reflect.TypeOf(&X{})))
fmt.Println(m.Reverse(2))
fmt.Println(m.Get(reflect.TypeOf(s1.Scanner{})))
fmt.Println(m.Reverse(3).PkgPath(), m.Reverse(3))
fmt.Println(m.Get(reflect.TypeOf(s2.Scanner{})))
fmt.Println(m.Reverse(4).PkgPath(), m.Reverse(4))
}
0
int
0
int
0
int
1
main.X
2
*main.X
3
text/scanner scanner.Scanner
4
go/scanner scanner.Scanner
In the above case I'm assuming that N is small. Also note the use of the identity of reflect.TypeOf, it will return the same pointer for the same type on subsequent calls.
If N is not small, you may want to do something a bit more complex.
package main
import (
"fmt"
s2 "go/scanner"
"reflect"
s1 "text/scanner"
)
type PkgPathNum struct {
PkgPath string
Num int
}
type TypeMap struct {
m map[string][]PkgPathNum
r []reflect.Type
}
func (m *TypeMap) Get(t reflect.Type) int {
k := t.String()
xs := m.m[k]
pkgPath := t.PkgPath()
for _, x := range xs {
if x.PkgPath == pkgPath {
return x.Num
}
}
n := len(m.r)
m.r = append(m.r, t)
xs = append(xs, PkgPathNum{pkgPath, n})
if m.m == nil {
m.m = make(map[string][]PkgPathNum)
}
m.m[k] = xs
return n
}
func (m *TypeMap) Reverse(t int) reflect.Type {
return m.r[t]
}
type X struct{}
func main() {
var m TypeMap
fmt.Println(m.Get(reflect.TypeOf(1)))
fmt.Println(m.Reverse(0))
fmt.Println(m.Get(reflect.TypeOf(X{})))
fmt.Println(m.Reverse(1))
fmt.Println(m.Get(reflect.TypeOf(&X{})))
fmt.Println(m.Reverse(2))
fmt.Println(m.Get(reflect.TypeOf(s1.Scanner{})))
fmt.Println(m.Reverse(3).PkgPath(), m.Reverse(3))
fmt.Println(m.Get(reflect.TypeOf(s2.Scanner{})))
fmt.Println(m.Reverse(4).PkgPath(), m.Reverse(4))
}
0
int
1
main.X
2
*main.X
3
text/scanner scanner.Scanner
4
go/scanner scanner.Scanner
https://play.golang.org/p/2fiMZ8qCQtY
Note the subtitles of pointer to type, that, X and *X actually are different types.
What cast / assertion need I do in Go in order to pass to a function expecting a generic function like func(interface{}) interface{}, a more specific function like func(int) int instead?
For example, in code like this, fooA can be passed to MakeExclamer, but not fooB:
func MakeExclamer(foo func (interface{}) interface{}, n int) func () {
return func() {
fmt.Printf("%v!!!", foo(n))
}
}
func fooA(x interface{}) interface{} {
return x.(int)*2
}
func fooB(x int) int {
return x * 10
}
func main() {
exclamerA := MakeExclamer(fooA, 12)
exclamerA()
exclamerB := MakeExclamer(fooB, 66)
// >> cannot use fooB (type func(int) int) as type func(interface {}) interface {} in argument to MakeExclamer
exclamerB()
}
(Go Playground link: https://play.golang.org/p/xGzfco0IAG)
I'm not interested much in alternative code structure patterns, since this is how I want it to work: a specific function should be passed to a general function transformer (accepting function of type Any -> Any) that will return another general function (Any -> Any). This may not be idiomatic in Go, but it is the pattern that I want my code to follow.
To use type assertions, every possible type must be enumerated in MakeExclamer:
func MakeExclamer(fn interface{}, arg interface{}) func() {
switch fn := fn.(type) {
case func(int) int:
return func() {
fmt.Printf("%v!!!\n", fn(arg.(int)))
}
case func(interface{}) interface{}:
return func() {
fmt.Printf("%v!!!\n", fn(arg))
}
default:
panic("not supported")
}
}
To accept a function of any type, the fn argument is declared as type interface{}. The code uses a type switch to handle the different function types.
playground example
Reflection can be used to write a more general function.
func MakeExclamer(fn interface{}, arg interface{}) func() {
fnr := reflect.ValueOf(fn)
argr := reflect.ValueOf(arg)
return func() {
resultr := fnr.Call([]reflect.Value{argr})
fmt.Printf("%v!!!\n", resultr[0].Interface())
}
}
playground example
First things first : When it comes to typing in Go, everything is theoretically possible. That's because even though the compiler does a lot of checks at compile-time, it is possible to change the runtime... at runtime. So-called runtime hacks, where you dynamically manipulate runtime structs that you're NOT supposed to handle.
Now, you have an interesting question, whose answer doesn't include the need to use the 'unsafe' package. However, the way I found of generalizing a function involves heavy reflection.
How to call a function (via reflection) ?
The documentation for the reflect package can be found here.
So, like all elements in Golang, functions have a Type. Without going through all fields, functions do take an array of arguments and produce an array of results. It is possible to investigate the Type of arguments and results through the In(int) and Out(int) method.
func investigate(fn interface{}) {
fnType := reflect.TypeOf(fn)
for idx := 0; idx < fnType.NumIn(); idx ++ {
fmt.Printf("Input arg %d has type %v\n", idx, fnType.In(idx))
}
for idx := 0; idx < fnType.NumOut(); idx ++ {
fmt.Printf("Output arg %d has type %v\n", idx, fnType.Out(idx))
}
}
We won't use this code. However, two important things are to be noted at this point :
The generic type under which a function can be passed around without caring about its type is interface{}. Something like "func(interface{}) interface{}" is not a generalization of a function, it is already a concrete type. Hence, "func(interface{}) interface{}" is not a generalization of "func(int) int", those are two different function types entirely. This is why you can't use type assertions/cast to convert from one function type to another.
A function can be represented as something that takes an input array and produces and output array.
Now, in order to call a function, you have to get not its Type, but its Value. Once you get its value, you can call it using an array of arguments, which must all be Values.
The prototype is:
func (v Value) Call(in []Value) []Value
Using this method, it is possible to call any function.
The code
So, the only thing you need is to convert whichever arguments array you have to an array of Values, then you will be able to call your function.
Here is your code:
package main
import (
"fmt"
"reflect"
)
func MakeExclamer(foo interface{}, n int) func() {
exclamer := generalize(foo, n)
return func() {
fmt.Printf("%v!!!\n", exclamer())
}
}
func fooA(x interface{}) interface{} {
return x.(int) * 2
}
func fooB(x int) int {
return x * 10
}
func generalize(implem interface{}, args ...interface{}) func() interface{} {
valIn := make([]reflect.Value, len(args), len(args))
fnVal := reflect.ValueOf(implem)
for idx, elt := range args {
valIn[idx] = reflect.ValueOf(elt)
}
ret := func() interface{} {
res := fnVal.Call(valIn)
// We assume the function produces exactly one result
return res[0].Interface()
}
return ret
}
func main() {
exclamerA := MakeExclamer(fooA, 12)
exclamerA()
exclamerB := MakeExclamer(fooB, 66)
exclamerB()
}
Playground
The important bit is the generalize function which makes the translation between your arguments and an array of Values, then returns a new function whith all parameters already filled.
Do not hesitate if you need any precision !
I have several methods that I'm calling for some cases (like Add, Delete, etc..). However over time the number of cases is increasing and my switch-case is getting longer. So I thought I'd create a map of methods, like Go map of functions; here the mapping of functions is trivial. However, is it possible to create a map of methods in Go?
When we have a method:
func (f *Foo) Add(a string, b int) { }
The syntax below create compile-time error:
actions := map[string]func(a, b){
"add": f.Add(a,b),
}
Is it possible to create a map of methods in Go?
Yes. Currently:
actions := map[string]func(a string, b int){
"add": func(a string, b int) { f.Add(a, b) },
}
Later: see the go11func document guelfi mentioned.
There is currently no way to store both receiver and method in a single value (unless you store it in a struct). This is currently worked on and it may change with Go 1.1 (see http://golang.org/s/go11func).
You may, however, assign a method to a function value (without a receiver) and pass the receiver to the value later:
package main
import "fmt"
type Foo struct {
n int
}
func (f *Foo) Bar(m int) int {
return f.n + m
}
func main() {
foo := &Foo{2}
f := (*Foo).Bar
fmt.Printf("%T\n", f)
fmt.Println(f(foo, 42))
}
This value can be stored in a map like anything else.
I met with a similar question.
How can this be done today, 9 years later:
the thing is that the receiver must be passed to the method map as the first argument. Which is pretty unusual.
package main
import (
"fmt"
"log"
)
type mType struct {
str string
}
func (m *mType) getStr(s string) {
fmt.Println(s)
fmt.Println(m.str)
}
var (
testmap = make(map[string]func(m *mType, s string))
)
func main() {
test := &mType{
str: "Internal string",
}
testmap["GetSTR"] = (*mType).getStr
method, ok := testmap["GetSTR"]
if !ok {
log.Fatal("something goes wrong")
}
method(test, "External string")
}
https://go.dev/play/p/yy3aR_kMzHP
You can do this using Method Expressions:
https://golang.org/ref/spec#Method_expressions
However, this makes the function take the receiver as a first argument:
actions := map[string]func(Foo, string, int){
"add": Foo.Add
}
Similarly, you can get a function with the signature func(*Foo, string, int) using (*Foo).Add
If you want to use pointer to type Foo as receiver, like in:
func (f *Foo) Add(a string, b int) { }
then you can map string to function of (*Foo, string, int), like this:
var operations = map[string]func(*Foo, string, int){
"add": (*Foo).Add,
"delete": (*Foo).Delete,
}
Then you would use it as:
var foo Foo = ...
var op string = GetOp() // "add", "delete", ...
operations[op](&foo, a, b)
where GetOp() returns an operation as string, for example from a user input.
a and b are your string and int arguments to methods.
This assumes that all methods have the same signatures. They can also have return value(s), again of the same type(s).
It is also possible to do this with Foo as receiver instead of *Foo. In that case we don't have to de-reference it in the map, and we pass foo instead of &foo.