Function in Function
Need to pass more parameters to a function type parameter, you will know everything at the end.
When we do API configuration we often find ourselves having to pass functions in parameters. For example http.HandleFunc need a function in second parameter:
func http.HandleFunc(pattern string, handler func(http.ResponseWriter, *http.Request))
But you would like to be able to pass an additional parameter or you would like to extend this function.
Here is an example:
package main
import (
"log"
"net/http"
)
func main() {
http.HandleFunc("/hello", middlewareOne("hello", http.HandlerFunc(hello)))
http.HandleFunc("/headers", middlewareOne("headers", http.HandlerFunc(headers)))
http.ListenAndServe(":8080", nil)
}
func middlewareOne(value string, next http.Handler) func(http.ResponseWriter, *http.Request) {
return func(w http.ResponseWriter, r *http.Request) {
log.Printf("Executing middlewareOne: %s", value)
next.ServeHTTP(w, r)
}
}
func hello(w http.ResponseWriter, r *http.Request) {
log.Print("Executing hello")
w.Write([]byte("OK"))
}
func headers(w http.ResponseWriter, r *http.Request) {
log.Print("Executing headers")
w.Write([]byte("OK"))
}
Go playground: https://go.dev/play/p/NsXH7pH085X.
middlewareOne allows us to log a message at each HTTP request and takes as input a new parameter that was not accepted by http.HandleFunc.
$ go run .
2022/08/10 11:40:45 Executing middlewareOne: hello
2022/08/10 11:40:45 Executing hello
2022/08/10 11:41:05 Executing middlewareOne: headers
2022/08/10 11:41:05 Executing headers
For most of the HTTP libraries, when you will have to develop middleware you will have to do it this way, thanks to this trick you will be able to pass additional configuration parameters like the configuration of your logger or your opentelemetry lib etc.
Check https://github.com/justinas/alice, that can help you to chain your HTTP middlewares.
We can take another case that can happen more often. Let’s say you want to loop over an interfaced function slice, but the interface prevents you from passing parameters. You can do it like this:
package main
import "fmt"
type Task func()
func main() {
defaultTask := func(id string) Task {
return func() {
fmt.Println(id)
}
}
tasks := []Task{
func() {
fmt.Println("1")
},
defaultTask("2"),
defaultTask("3"),
}
for _, t := range tasks {
t()
}
}
Go playground: https://go.dev/play/p/RHYBqB9qeZ3.
$ go run .
1
2
3