how to compare funcs in golang

Question

I'm attempting write a test for my package and failing at comparing funcs. Here's essentially what i'm doing.

package main

import (
    "fmt"
    "reflect"
)

type HandlerFunc func(cmd interface{})

type Bus struct {
    handlers map[reflect.Type]HandlerFunc
}

func (bus *Bus) RegisterHandler(cmd interface{}, handler HandlerFunc) {
    bus.handlers[reflect.TypeOf(cmd)] = handler
}

func (bus *Bus) GetHandler(cmd interface{}) HandlerFunc {
    t := reflect.TypeOf(cmd)

    for kind, handler := range bus.handlers {
        if t == kind {
            return handler
        }
    }

    return nil
}

func New() *Bus {
    return &Bus{
        handlers:    make(map[reflect.Type]HandlerFunc),
    }
}

type RegisterUserCommand struct {}

func main() {
    bus := New()
    handler := func (cmd interface{}) {}

    bus.RegisterHandler(&RegisterUserCommand{}, handler)

    retrieved := bus.GetHandler(&RegisterUserCommand{})

    if retrieved != handler {
        fmt.Println("Not the same!")
        return
    }

    fmt.Println("Same!")
}

Comparing retrieved with handler causes the following error

invalid operation: (func(interface {}))(retrieved) != handler (func can only be compared to nil)

How can i properly test the function i'm retrieving is the same one added previously?

This is not an answer to your actual question, but why do you iterate through the map of types to handlers in `GetHandler`? You can return `bus.handlers[t]` and that will return `nil` for undefined handlers. — Matt K, Apr 29 '15 at 03:11
possibly related to http://stackoverflow.com/questions/9643205/how-do-i-compare-two-functions-for-pointer-equality-in-the-latest-go-weekly — Dave C, Apr 29 '15 at 03:19

score 4 · Answer 1 · answered Apr 29 '15 at 03:08

Given that you can't compare functions, you can write your test in a different way. You can make handler set a boolean value in your test and check that you've got the right function by calling it and seeing if the boolean changes.

Here's an example:

func main() {
    bus := New()
    called := false
    handler := func (cmd interface{}) { called = true }

    bus.RegisterHandler(&RegisterUserCommand{}, handler)
    bus.GetHandler(&RegisterUserCommand{})(nil)

    if called {
        fmt.Println("We got the right handler!")
        return
    }

    fmt.Println("We didn't get the right handler")
}

how to compare funcs in golang

1 Answers1