If I understand this correctly, you want the ability to add a function that handles any method invocation on some value. This approach means that there is no static type checking at all.

You can already do part of this using the reflect package. Given a reflect.Value, the reflect package can look up a method by name, and call it.

What you can't do is define a method dynamically and call it with static code. As Go is a statically typed language, it seems very unlikely that that would ever be permitted.

One way to do this is to embed a Go interpreter like this one: https://github.com/traefik/yaegi. This allows you to define Go functions at runtime although indirectly, through the interpreter.

I feel this may be an XY question though. Why do you actually want to do this? If we know your ultimate goal, we might be able to better advise you on how such a thing is done in Go, as opposed to trying to copy the Java approach.

Based on the discussion above, and the emoji voting, this is a likely decline. Leaving open for four weeks for final comments.

No further comments.

I thought it was pretty clear, that this is the desired behavior. There is no XY. This is what I want and why I want to be able to do it that way.

Lets say I have the following Interface

type Person interface {
    FirstName() string
    LastName() string
    Age() string
}

and I had a map[string]interface{} with that data, with the Dynamic Proxy Invocation Handler I can just tell it to map the method names to the keys in an the map like so.

being able to intercept the call to FirstName() and get the function name and be able to return the value in the map by return map[functionName]interface{} is pretty straightfoward and clear.

This is an EXTREMELY powerful ability. It allows you to put type safety onto unsafe things like map[string]interface{} with a single implemenation.

Lots of things like Google Datastore and other key value stores, graph stores and document stores; use a generic map[string]interface{} representation and you either have to code lots of boilerplate mappings to get a type safe interface for the Entity or you have to generate a bunch of boilerplate code like it is 1995 and you are using Visual Basic 4 to get data from an RDBMS and map it to objects.

I thought it was pretty clear, that this is the desired behavior. There is no XY. This is what I want and why I want to be able to do it that way.

Lets say I have the following Interface

type Person interface {
    FirstName() string
    LastName() string
    Age() string
}

and I had a map[string]interface{} with that data, with the Dynamic Proxy Invocation Handler I can just tell it to map the method names to the keys in an the map like so.

being able to intercept the call to FirstName() and get the function name and be able to return the value in the map by return map[functionName]interface{} is pretty straightfoward and clear.

This is an EXTREMELY powerful ability. It allows you to put type safety onto unsafe things like map[string]interface{} with a single implemenation.

Lots of things like Google Datastore and other key value stores, graph stores and document stores; use a generic map[string]interface{} representation and you either have to code lots of boilerplate mappings to get a type safe interface for the Entity or you have to generate a bunch of boilerplate code like it is 1995 and you are using Visual Basic 4 to get data from an RDBMS and map it to objects.

Have a look at this https://github.com/cocotyty/dpig