Perform an action on a collection of items

In my actual code, I'm parsing an XML document using encoding/xml, and I basically have a bunch of nested structures of the following form — all of which may occur multiple times, except the top-level statements element:

statements
  statement
    opcode
      args
        pre
        post

I'm fairly new to Go, and I'm clearly misunderstanding how interface{} (the empty interface) works:

.\stmtgen.go:58: cannot use print_name (type func(Statement)) as type func(interface {}) in argument to performAction
.\stmtgen.go:58: cannot use slist (type []Statement) as type []interface {} in argument to performAction

Relevant example code:

<!-- language: lang-golang -->

package main
import "fmt"

// Actually a structure in my code, but this suffices for demonstration.
type Opcode int

// A Statement has a Name and multiple Opcodes may use this Name.
type Statement struct {
    Name    string
    Opcodes []Opcode
}

// Print the statement name.
func print_name(stmt Statement) {
    fmt.Println(stmt.Name)
}

// Perform an action on each item of a collection.
func performAction(action func(interface{}), v []interface{}) {
    for i := range v {
        action(v[i])
    }
}

func main() {
    slist := make([]Statement, 3)
    slist[0] = Statement{"Statement 1"}
    slist[1] = Statement{"Statement 2"}
    slist[2] = Statement{"Statement 3"}

    //ERROR HERE
    performAction(print_name, slist)
}

Must I create functions to print the values for every single type?

An empty interface{} can contain any value and passed around as the type interface{}. when you need the value from it, you can perform a type assertion like this:

var anyValue interface{}
anyValue = "hello"

strValue := anyValue.(string) 

If anyValue is not of the type being asserted then it will cause a panic

the type assertion can also be used to return a bool if the interface is of that type with a multiple return

strValue, ok := anyValue.(string)
if ok {
    //anyValue contains a string!
}

If you dont know the type of the interface, you can use a switch to determine it's type like this:

switch val := anyValue.(type) {
case string:
    // anyValue contains a string
    // and val is a string
    break
case int:
    // anyValue contains an int
    // and val is and int
    break
default:
    //unhandled interface type
}

Hopefully this makes the empty interface{} type clearer.

interfaces{...} which have methods declared in them are different, they can not have members (like structs can), only methods, and their underlying type must implement all the methods declared in the interface. You could have an interface actionPerformer (interface names should have the suffix "er" as they are doing something)

type actionPerformer interface {
    action(interface{})
}

A type that implements all the methods in an interface can be cast to that interface type, then if you call one of those methods on the interface, it will run the method on the underlying type.
For example, if the Statement struct implements the action(interface{}) method, the Statement struct can be cast to an actionPerformer type and if the action(interface{}) function is called on the actionPerformer, the action function on the Statement struct is run. So you could have multiple types that all have the action(interface{}) method and they can all be cast to an actionPerformer which you can call the action function on.

func (code Opcode) action(arg interface{}) {
    fmt.Println(arg.(int) + int(code))
}

func (stmt Statement) action(arg interface{}) {
    fmt.Println(arg.(string), stmt.Name)
}

stmt := Statement{"Statement 1", nil}
stmtActionPerformer := actionPerformer(stmt)

opcode := Opcode(5)
opcodeActionPerformer := actionPerformer(opcode)

stmtActionPerformer.action("hello") // will print "hello "+whatever the statements name is
opcodeActionPerformer.action(2) //will print be 7

Type assertions can still be used on these types of interface e.g.

stmt := stmtActionPerformer.(Statement)
fmt.Println(stmt.Name)

This is a contrived example, but with this in mind, you might want to write your code using interfaces like this.
Remember casting between interfaces is costly, so should be done sparingly, however they are a powerful tool when used correctly.

For your example, a simple printNames function would be much more efficient than all that interface casting (note that in golang, names should be in the CamelCase format, not using underscores)

func printNames(stmts []Statement) {
    for _, stmt := range stmts {
	    fmt.Println(stmt.Name)
    }
}

It might also be useful to have a type StatementList and add methods to it:

type StatementList []Statement

func (list StatementList) printNames() {
    for _, stmt := range list {
        fmt.Println(stmt.Name)
    }
}

Getting the hang of this stuff make golang a lot more fun, hope this helps

You have to declare the parameters of performAction exactly same like the arguments type.

func performAction(action func(Statement), v []Statement) {
    for i := range v {
        action(v[i])
    }
}

Or you could use interface{} on all parameters instead. Then cast it according to the needs.

func performAction(action interface{}, v interface{}) {
    for _, each := range v.([]Statement) {
        action.(func(Statement))(each)
    }
}

• data with type []Statement cannot be assigned to []interface{}
• also for type func(Statement) cannot be assigned to func(interface{})

Use interface{}, then cast it to the original type.

this works for me:

package main

import (
	"fmt"
)

// Actually a structure in my code, but this suffices for demonstration.
type Opcode int

// A Statement has a Name and multiple Opcodes may use this Name.
type Statement struct {
	Name    string
	Opcodes []Opcode
}

// Print the statement name.
func print_name(stmt interface{}) {
	if s, ok := stmt.(Statement); !ok {
		panic("typ err")
	} else {
		fmt.Println(s.Name)
	}
}

// Perform an action on each item of a collection.
func performAction(action func(interface{}), v []interface{}) {
	for i := range v {
		action(v[i])
	}
}

func main() {
	slist := make([]interface{}, 3)
	slist[0] = Statement{"Statement 1", nil}
	slist[1] = Statement{"Statement 2", nil}
	slist[2] = Statement{"Statement 3", nil}

	performAction(print_name, slist)
	/*output:
	Statement 1
	Statement 2
	Statement 3
	*/
}