How to create a new function from another function using reflection

Question

Using this code as a template

package main

import "fmt"

type myStruct struct {
    Value int
}

type counter int

func newFuncHandler(fn func(myStruct) error) (interface{}, *counter) {
    count := counter(0)

    newFn := func(e myStruct) error {
        count = count + 1
        return fn(e)
    }

    return newFn, &count
}

func main() {
    fn := func(d myStruct) error {
        // doing some stuff
        return nil
    }

    handle, c := newFuncHandler(fn)
    handleFn := handle.(func(d myStruct) error)

    handleFn(myStruct{Value: 2})
    handleFn(myStruct{Value: 2})
    handleFn(myStruct{Value: 2})
    handleFn(myStruct{Value: 2})
    handleFn(myStruct{Value: 2})

    fmt.Println(*c) // 5
}

how to modify newFuncHandler so that it, given a function with unknown signature, returns a function with the same signature but with additional code for the function body. newFuncHandler should not know of the myStruct type

For example

func newFuncHandler(fn interface{}) (interface{}, *counter) {
    count := counter(0)
    // some reflection magic

    // newFn has the same signature as fn (hardcoded in this case)
    newFn := func(e myStruct) error {
        // add custom code 
        count = count + 1

        // call the original fn
        return fn(e)
    }

    return newFn, &count
}

Answer 1

Use reflect.MakeFunc to make a function. Use Value.Call to call a function.

func newFuncHandler(v interface{}) (interface{}, *counter) {
    count := counter(0)
    fn := reflect.ValueOf(v)
    newFn := reflect.MakeFunc(fn.Type(), func(args []reflect.Value) (results []reflect.Value) {
        count = count + 1
        return fn.Call(args)
    })
    return newFn.Interface(), &count
}

Run it on the playground.