将函数分配给返回接口值的函数类型，

Question

Im trying to define a couple of interface to refactor some code and I'm getting a problem where Go is not letting me assign functions to variables. This is the set up

main.go

    type Gettable interface {
        Get() int64
    }

    type MyFunction func(int64) (Gettable, error)

    func main() {

        var f MyFunction
        f = sub.TestFn2
        a, _ := f(1)
        fmt.Println(a)
    }

main/sub

package sub

type MyStruct struct {
    Val int64
}

func (v MyStruct) Get() int64 {
    return v.Val
}

func TestFn2(a int64) (MyStruct, error) {
    return MyStruct{a}, nil
}

I'm trying to define a generic function type, and in the sub package create the concrete functions

and ideally i want to store the functions in a map and call them with something like

FnMap["fnName"]()

I'm not there yet,

im getting an error saying

/topics.go:27:4: cannot use sub.TestFn2 (type func(int64) (sub.MyStruct, error)) as type MyFunction in assignment

but MyStruct clearly implements the interface Gettable

Answer 1

This error occurs because of signature don't match. You code shared is:

//shared code
//---------------------------------------------
type Gettable interface {
    Get() int64
}
type MyFunction func(int64) (Gettable, error)

So you need replace MyStruct by Gettable.

//main/sub
//---------------------------------------------
type MyStruct struct {
    Val int64
}

func (v MyStruct) Get() int64 {
    return v.Val
}

//this signature of TestFn2 is different of MyStruct
//-[TestFn2] func (a int64) (MyStruct, error)
//-[MyFunction] func(int64) (Gettable, error)
func TestFn2(a int64) (Gettable, error) {//<-- replace by Gettable here
    return MyStruct{a}, nil
}

Running your code:

//main.go
//---------------------------------------------

func main() {
        var f MyFunction
        f = TestFn2
        a, _ := f(1)
        fmt.Println(a)
}   

Result is:

{1}

See in playground: https://play.golang.org/p/sRsXix8E_83