Optional Fetcher

[gitperson1980]

In some circumstances with certain structs and as needed, would it be possible to create provisions to use fetcher?

[antonmedv]

Can you please explain a little bit more?

As vm.Fetcher was removed in latest Expr versions. It’s still possible to do the same logic using the ast.Patch functionality.

I will write a doc on how to do it.

[gitperson1980]

type Bar struct{
   D int
}

type Foo struct {
	A int
	B bool
	Mp map[string]*Bar   // has elements  MP1 MP2 MP3 MP4 MP5 MP6   MP1-3 is call heavily and would like to cache it as pointers as below
	mp1 *Bar   //lower case to avoid export in serialization but cannot be accessed by expr
	mp2 *Bar   //lower case to avoid export in serialization
	mp3 *Bar   //lower case to avoid export in serialization
}

FooExample = Foo{A: 1, B: true, Mp: make(map[string]*Bar)}
FooExample.Mp["MP1"]= Bar{}
FooExample.Mp["MP2"]= Bar{}
FooExample.Mp["MP3"]= Bar{}
FooExample.Mp["MP4"]= Bar{}
FooExample.Mp["MP5"]= Bar{}
FooExample.mp1=FooExample.Mp["MP1"]    //cache the most used
FooExample.mp2=FooExample.Mp["MP2"]    //cache the most used
FooExample.mp3=FooExample.Mp["MP3"]    //cache the most used

FooExample.mp1.D   // direct and avoids the overhead of a map lookup

[antonmedv]

call heavily and would like to cache it as pointers as below

Expose MP1 and no map lookup will be done.

[gitperson1980]

I cannot expose it as I don't want MP1-3 added in a gob serialization routine in another process and increase the size of the serialized output with duplicate information. This is why it is lowercase. However it can be accessed by a fetcher.

What might be nice is to be able to fetch if fetcher exists/defined else fall though to standard fetch.

func (v *Foo) Fetch(i interface{}) interface{} {
	var string_i string
	if s, ok := i.(string); ok {
		string_i = s
	} else {
		string_i = fmt.Sprint(i)
	}
	switch string_i {
	case "mp1":
		return v.mp1
	case "mp2":
		return v.mp2
	case "mp3":
		return v.mp3
	}
	return nil
}

[antonmedv]

I see. What about losing a separate structure for Env?