On Thu, 08 Feb 2018 16:17:08 +1100 Rob Pike <r...@golang.org> wrote:
Rob Pike writes:
>
> Isn't the visitor pattern just a way to implement type switch in languages
> that don't implement type switch?
>
> That's certainly how I saw it when using C++. Go has a type switch, and so
> has no need for the visitor pattern, a perfect example of the principle
> that a design pattern is just a way to work around shortcomings in the
> language.
In Go you can implement the visitor pattern without using a
type switch. This allows you to add new type of objects
without changing any other code -- just add the functions you
need. See below.
Having had to debug someone else's C++ code which made heavy
of the visitor pattern, I can tell you it was quite difficult
to understand, particularly as it had global side-effects and
as it was beaten into shape over time, the code had gotten
quite messy[1].
My advice to C++/Java refugees: don't use the visitor pattern.
And if you do, don't have side-effects or complicated logic!
Actually don't use any GoF patterns! Go has its own idioms.
Example: // Just sketching the idea...
type Node struct {
kids []*Node
val Value
}
type Visitor interface {
func Visit(val interface{})
}
func (t *Tree) Walk(Visitor v) {
v.Visit(t.val)
for _, k := range t.kids {
k.walk(v)
}
}
type V1 struct {
}
type Stringer interface {
String() string
}
type Sizer interface {
Size() int
}
func (v1 V1)Visit(val interface{}) {
v, ok := val.(Stringer)
if !ok { panic("...") }
fmt.Println(v.String())
}
type V2 struct{
}
func (v2 V2)Visit(val interface{}) {
v, ok := val.(Sizer)
if !ok { panic("...") }
fmt.Println(v.Size())
}
var v1 V1
var v2 V2
...
t *Tree
...
t.walk(v1)
t.walk(v2)
[1] I ended up rewriting it from scratch by simply observing
its behavior. There were many other problems as well such as
linear search and what not. The rewrite took fraction of a
second and cut down everyone's build time by 3 minutes!
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