Difference between http.Handle and http.HandleFunc?

No, it's different. Let's examine:

func Handle(pattern string, handler Handler) {
    DefaultServeMux.Handle(pattern, handler) 
}

Handle expects us to pass a Handler. Handler is an interface

type Handler interface {
    ServeHTTP(ResponseWriter, *Request)
}

if any type implements ServeHTTP(ResponseWriter, *Request) for example: myCustomHandler then we can pass it like Handle(pattern string, myCustomHandler).

In the second scenario:

HandleFunc(pattern string, func(w ResponseWriter, r *Request) {
    // do some stuff
}

HandleFunc expects a function where Handle expects a Handler interface.

So, if you just want to pass a function then you can use http.HandleFunc(..). Like @David showed that behind the scenes it implements Handler interface by calling ServeHTTP.

type HandlerFunc func(ResponseWriter, *Request)
    
// ServeHTTP calls f(w, r).
func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
    f(w, r)
}

Basically, the HTTP server's "mux" has a map of path -> handler interface

Interfaces are used here, I assume, to allow you to implement complex path handlers that have state.

For example the file server from the standard package is a struct that contains the root dir for file service and implements the handler interface.

That said, for simple stuff, a func is easier and more clear. So they added a special generator so you can easily pass in a func.

Take a look at: server.go

from line: 1216 (as of today)

  1216  type HandlerFunc func(ResponseWriter, *Request)
  1217  
  1218  // ServeHTTP calls f(w, r).
  1219  func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
  1220      f(w, r)
  1221  }

What they are doing is implementing the interface on a custom type (which happens to match the api of the interface) that just calls itself.

In simple terms:

Problem: I want to create an object (type) that responds to HTTP requests.

Solution: Use http.Handle for that. It accepts an http.Handler as the second argument. http.Handler is an interface and should implement ServeHTTP from the http package.

---

Problem: I want a function to respond to my HTTP request.

Solution: Use http.HandleFunc for that. It accepts an http.HandlerFunc as the second argument.

http.HandlerFunc is a function type and should implement ServeHTTP from the http package.