Feature request: Add setters

[ekle]

generated message-types have only getters but no setters at the moment.
This makes it impossible to create an interface which can change values of them.

This would be especially useful for the return messages of rpc calls.
For example a generic function which adds an request-id to every response.

[dsymonds]

Nope. The code bloat does not warrant the minor utility, which you can achieve with your own wrapper types, or with reflection.

[daemonl]

Are you sure we can't discuss this? I don't see this as minor, I have the exact same issue, want to set ResponseID from RequestID.

Alternatively, and this may already exist, could there be / is there already a way to add a hook for 'extra' parts like setters?

[jbowens]

+1

[jspiro]

@dsymonds With due respect, that's an obnoxiously terse response to a legitimate request. If it was so easy @ekle would not have asked. I expect you'll close the discussion next? This is typical golang community "you're a bad programmer for asking" behavior; we can be better than that.

Let's discuss this like engineers. In response to your assertions:

• Many of the other protobuf generators (e.g. C++ and Java) generate setters.
• I've spent the last hour trying to modify a fork to add setters and it's a nightmare if you're not familiar with the code.
• Setters are not minor. If you do NEED setters, you cannot add them to the original structs outside of the package, and generating them requires a non-trivial amount of work.
• Reflection for production code: Seriously?
• Setters for protos are particularly necessary in Go. Go is not templated, and you cannot create an interface for struct fields; there's no easy way to write a function that sets a particular field name that many proto messages share. You'd need one for each proto type or wrappers and interfaces. A setter is an elegant and trivial solution since you a single small interface satisfies it. That's the Go way.
• Code bloat? It's generated code. Protobufs are supposed to provide lots of varied functionality out of the box. Are you suggesting that the setters are going to get compiled in even if you don't use them, and that they could possibly take up much space?
• Code bloat part 2: Hello world in Go is 5mb.

Manually writing wrappers for all messages is a terrible solution; generating them is marginally better but both are a large amount of non-trivial work for a problem easily and naturally solved in this library: Adding setters.

[jspiro]

In the spirit of showing my work, this patch was sufficient to get my project moving again, and I was able to eliminate a lot redundant functions with a two line interface as a result.

[ekle]

@jspiro thats a cool patch. at the moment i'm using a big Makefile with some bash/grep magic to transform the existing getters to setters in a new file.

[dsnet]

@jspiro, I don't think it's helpful to summarize @dsymonds as simply saying "you're a bad programmer for asking" since he says nothing of the sort. Even if someone closes the issue, you can always respond back with more discussion points as you have done here.

Points to consider:

1. Ability to set some generic field on differing proto types. While setters seem like the right solution since you can create an interface for the field you are interested in. However, I believe this is wrong abstraction to perform that work. This approach makes the assumption that all proto.Message types are generated by protoc-gen-go, however there are some custom defined types that satisfy the proto.Message interface; it is not reasonable to expect them to setter methods (or even setter methods with the same signature). One of the core issues with the proto.Message interface is that it is marginally better than interface{}. It is trivial for any user-defined type to implement proto.Message, but the problem is that the proto package (and others) have no idea how to handle these custom types. The proper approach is to extend (this is a breaking change) the proto.Message interface to include reflection information about the protocol buffer itself. We're not talking about Go reflection, but a reflection interface designed specifically for protobuf. I will file a separate issue about this.
   1a. Reflection for production code. (irrelevant to setters) While reflection is not necessarily the most performant, please don't scoff at the use of it. The proto library itself is heavily reliant on reflection to operate. That being said, Go reflection is not the correct approach to solve point 1. Again, it assumes that all protocol buffers are structs with some field of the name your interested in.
2. Code bloat. Increased code size of protocol buffers is a real concern for mobile and applications with low memory. While the cost is not terrible (since each setter function will be the equivalent of a single conditional check and a store instruction), it is still a non-zero cost. It is not fair to compare Go programs to the "hello world" program since Go programs have a heavy initial code-size cost for the runtime and core standard library (while C programs can dynamically link with libc).
3. Setters permit other optimizations. One use case that the current proto library does not handle well is where a service unmarshals a protobuf and only mutate a small number of fields before marshaling it again. Using settings and getters alone can allow an optimization where the unmarshaller does a form of lazy decoding and retrieves fields on demands. However, in order to make this optimization, it pre-supposes that it is not possible to access message fields as if there were just struct fields.

/cc @alandonovan, who is interested in protobuf reflection.
/cc @cherrymui, who is interested in the performance side of things.

[dsnet]

See #364.

[dsnet]

I've been working on #364 and I am convinced that is the right solution for the root issues going on here. I understand the temptation want to use Go interfaces to set some generic field in a set of messages, but it makes the very subtle assumption that all of them will have the same method signature, which is not a world we want to back ourselves into.

[codyaray]

I ran into this issue today and wrote a little protoc generator for setters. Thought I'd share:
https://github.com/codyaray/proto-go-setter

Still a WIP. Right now it supports primitive types. Working on adding more complex types.

[RaymondReddington]

why dont you support setter.
how can i Abstract interface from many messages?
shall we write setter one by one and then Abstract the setter's interface?