Additional arguments to http function Golang - go

Question

I am trying to pass string to handler in given example.
package main
import (
"fmt"
"net/http"
)
func handler(w http.ResponseWriter, r *http.Request) {
fmt.Fprintf(w, "Hi there, I love %s!", r.URL.Path[1:])
}
func main() {
http.HandleFunc("/", handler)
http.ListenAndServe(":8080", nil)
}
Here is what i tried but it throws an error as it expects regular number of arguments:
package main
import (
"fmt"
"net/http"
)
func handler(w http.ResponseWriter, r *http.Request, s *string) {
fmt.Fprintf(w, "Hi there, I love %s!", *s)
}
func main() {
files := "bar"
http.HandleFunc("/", handler(&files))
http.ListenAndServe(":8080", nil)
}

Answer 1

I'm a little unclear on what you're trying to do, but based off what you said, why not try to encapsulate the data you want to pass in like this:
package main
import (
"fmt"
"net/http"
)
type FilesHandler struct {
Files string
}
func (fh *FilesHandler) handler(w http.ResponseWriter, r *http.Request) {
fmt.Fprintf(w, "Hi there, I love %s!", fh.Files)
}
func main() {
myFilesHandler := &FilesHandler{Files: "bar"}
http.HandleFunc("/", myFilesHandler.handler)
http.ListenAndServe(":8080", nil)
}
This provides a little more granular control of what you make available to your Handler.

Answer 2

There are lots of options here, you could:
Use a closure to set state for the enclosed handler
Use a method on a struct for the handler, and set global state there
Use the request context to store a value, then get it out
Use a package global to store the value
Write your own router with a new signature (not as complex as it sounds, but probably not a good idea)
Write a helper function to do things like extract params from the url
It depends what s is really - is it a constant, is it based on some state, does it belong in a separate package?

Answer 3

One of ways is to store data in global variable:
package main
import (
"fmt"
"net/http"
)
var replies map[string]string
func handler1(w http.ResponseWriter, r *http.Request) {
r.ParseForm()
question := r.FormValue("question")
var answer string
var ok bool
if answer, ok = replies[question]; !ok {
answer = "I have no answer for this"
}
fmt.Fprintf(w, "Hi there, I love %s! My answer is: %s", question, answer)
}
func main() {
//files := "bar"
replies = map[string]string{
"UK": "London",
"FR": "Paris",
}
http.HandleFunc("/", handler1)
http.ListenAndServe(":8080", nil)
}
Here for brevity I've commented out files and put data as is into the map. You may read the file and put them there.
Usage with CURL:
$ curl -X POST 127.0.0.1:8080/ -d "question=FR"
Hi there, I love FR! My answer is: Paris
$ curl -X POST 127.0.0.1:8080/ -d "question=US"
Hi there, I love US! My answer is: I have no answer for this