Runtime access to symbols in Go - go

Question

In writing a Web server in Go, I'd like to be able to dereference symbols at runtime, to allow me to figure out which functions to call from a configuration file, something like the call to the fictional "eval" function in the example below. That would allow me to select handlers from a library of handlers, and to deploy a new server with just a config file. Is there any way to accomplish this in Go?
config.json
{ "url": "/api/apple", "handler": "Apple", "method": "get" }
{ "url": "/api/banana", "handler": "Banana", "method": "get" }
play.go
package main
import (
"github.com/gorilla/mux"
"net/http"
"encoding/json"
"log"
)
type ConfigEntry struct {
URL string `json:"url"`
Method string `json:"method"`
Handler string `json:"handler"`
}
func main() {
ifp, err := os.Open("config.json")
if err != nil {
log.Fatal(err)
}
dec := json.NewDecoder(ifp)
r := mux.NewRouter()
for {
var config ConfigEntry
if err = dec.Decode(&m); err == io.EOF {
break
} else if err != nil {
log.Fatal(err)
}
r.HandleFunc(config.URL, eval(config.Handler + "Handler")).Methods(config.Method)
}
http.Handle("/", r)
http.ListenAndServe(8080, nil)
}
func AppleHandler(w http.ResponseWriter, r *http.Request) (status int, err error) {
w.Write("Apples!\n")
return http.StatusOK, nil
}
func BananaHandler(w http.ResponseWriter, r *http.Request) (status int, err error) {
w.Write("Bananas!\n")
return http.StatusOK, nil
}

Answer 1

There's nothing like eval in Go, which is a good thing since things like that are very dangerous.
What you can do is have a map mapping the handler strings in your config file to the handler functions in your code:
var handlers = map[string]func(http.ResponseWriter, *http.Request) (int, error){
"Apple": AppleHandler,
"Banana": BananaHandler,
}
Then you can register those handlers by simply doing:
handler, ok := handlers[config.Handler]
if !ok {
log.Fatal(fmt.Errorf("Handler not found"))
}
r.HandleFunc(config.URL, handler).Methods(config.Method)

Answer 2

There's some limited way to access things during runtime with the reflect package. However it doesn't allow you to search for all suitable standalone functions in a package. It would be possible if they are all methods on a known struct type/value.
As an alternative your given example you could simply use a map[string]func(...) to store all handlers, initialize it at startup (during init()) and fetch the handlers from there. But that also more or less what the existing http muxes are doing.