Wandering Thoughts archives

Go: using type assertions to safely reach through interface types

To start with, suppose that you have a Go net.Conn value, call it conn, that you want to shutdown() (for writing) on if possible. Some but not all specific concrete net connection types make this available as a .CloseWrite() method (eg it's available for TCP sockets but not for UDP ones), but net.Conn is an interface type and it doesn't include a .CloseWrite() method so you can't directly call conn.CloseWrite().

(In Go's software engineering view of the world this is a sensible choice. net.Conn is the set of interfaces that all connections can support. If you included .CloseWrite() in the interface anyways you would force some connections, eg UDP sockets, to implement a do-nothing or always-error version of the method and then people would write Go code that blindly called .CloseWrite() and expected it to always work.)

So sometimes conn will be of a concrete type that supports this (and sometimes it won't be). You want to somehow call .CloseWrite() if it's supported by your particular value (well, the particular concrete type of your particular value). In Python we would do this either with a hasattr() check or just by calling obj.CloseWrite() and catching AttributeError, but we're in Go and Go does things differently.

If you're a certain sort of beginning Go programmer coming from Python, you grind your teeth in irritation, look up just what concrete types support .CloseWrite(), and write the following brute force code using a type switch:

func shutdownWrite(conn net.Conn) {
    switch i := conn.(type) {
    case *net.TCPConn:
        i.CloseWrite()
    case *net.UnixConn:
        i.CloseWrite()
    }
}

(Then this code doesn't compile under Go 1.0 because net.UnixConn doesn't implement .CloseWrite() in Go 1.0.)

What this code is doing in its brute force way is changing the type of conn into something where we know that we can call .CloseWrite() and where the Go compiler will let us do so. The compiler won't let us directly call conn.CloseWrite() because .CloseWrite() is not part of the net.Conn interface, but it will let us call, say, net.TCPConn.CloseWrite(), because it is part of net.TCPConn's public methods. So if conn is actually a net.TCPConn value (well, a pointer to it) we can convert its type through this type switch and then make the call. Unfortunately this code has the great drawback that it has to specifically know which concrete types that sit behind net.Conn do and don't implement .CloseWrite(). This is bad for various reasons.

(I am mangling some Go details here in the interests of nominal clarity.)

The experienced Go programmers in the audience are shaking their heads sadly right now, because there is a more general and typesafe way to do this. We just need to say what we actually mean. First we need a type that will let us call .CloseWrite(); this has to be an interface type because we need to convert conn to it (somehow).

type Closer interface {
    CloseWrite() error
}

(It's important to get the argument and return types exactly right even if you're going to ignore the return value.)

Now we need to coerce conn to having that type if and only if this is possible; if we blindly coerce conn to this type (in one of a number of ways) we will get a runtime error when we're handed a net.Conn with a concrete type that lacks a .CloseWriter() method. In Go, this safe coercion is done with the two-result form of a type assertion:

func shutdownWrite(conn net.Conn) {
    v, ok := conn.(Closer)
    if ok {
        v.CloseWrite()
    }
}

(We can't just call conn.CloseWrite() after the coercion because we haven't changed the type of conn itself, we've just manufactured another variable, v, that has the right type.)

This is both typesafe and general. Any conn value of a concrete type that implements .CloseWrite() will work and it will work transparently, while if conn is of a concrete type that doesn't implement .CloseWrite() there are no runtime panics; all of this is exactly what we want. The same technique can be used in exactly the same way to reach through any interface type to get access to any (public) methods on the underlying concrete types; set up an interface type with the methods you want, try coercing, and then call things appropriately.

(I actually like this typesafe conversion and method access better than the Python equivalent because it feels less hacky and more a direct expression of what I want.)

I think that it follows that any type switch code of the first form, one where you just call the same routine (or a few routines) on the new types, is a danger sign of doing things the wrong way. You probably want to use interface type conversion instead.

(Had I read the right bit of Effective Go carefully I might have seen this right away, but Effective Go doesn't quite address this directly. All of this is probably obvious to experienced Go programmers.)

Update: there are several good ideas and improvements (and things I didn't know or realize) in the the golang reddit comments on this entry.

Some notes on my first experience with Go

I've finally wound up writing my first Go program. The program is a Go version of what seems to have turned into my standard language test program, namely a netcat-like program that takes standard input, sends it off to somewhere over the network, and writes to standard out what it gets back from the network. Partly because Go made it easy and partly due to an excess of new thing enthusiasm the program grew far beyond my initial basic specifications.

(I'm somewhat bemused but a netcat-like program really has become a standard program I write in new languages and to try out things like new buffering libraries. It's actually not a bad test.)

On the whole the experience was quite pleasant. The specific need I had is something I normally would have handled with a Python program and writing my Go program was not particularly much more work and bookkeeping than the Python equivalent would have been (it took much longer to write because I was semi-learning Go as I went and I already know Python). The code has reasonably few variable declarations and most of them are non-annoying; Go's := idiom really helps with this since it means that in many circumstances you don't have to declare a variable or specifically name its type.

One important thing I wish I'd know at the start is that you should ignore most everything the Go documentation overview pages tells you about what to read. Effective Go is in practice the quick guide to Go for C programmers, or at least for C programmers who have some general idea about Go to start with, and is the closest thing Go has to Python's excellent tutorial. The language reference is overly detailed and too hard to read for learning and the interactivity of the beginning tutorial makes it completely unsuitable for quick starts.

One of the reasons that I got as far as I did as fast as I did is that Go's networking library has a relatively high-level view of the world. There is no Python equivalent of Go's net.Dial() or net.Listen() APIs, at least not in the standard library; the existence of both of them made handling an absurdly wide variety of network protocols basically trivial (along with a bunch of complexity of hostname and port number lookups). On the flipside this API is not complete (especially in Go 1.0) and has a number of really annoying omissions. This is especially frustrating since I have the (Go) source for the net package and can see perfectly well that what I want access to already exists in the package; it's just not exported and (unlike Python) you can't fish into a package to grab stuff yourself.

My code wound up using goroutines and channels, although in a relatively basic way. Designing program flow in terms of channels definitely took several attempts before I had everything sorted out cleanly; earlier versions of the code had all sorts of oddities before I sorted out exactly what I wanted and how to express that in channel data flows. My broad takeaway from this experience is that it's very important to think carefully about what you want to do before you start eagerly designing a complex network of channels and goroutines. It was easy for me to get distracted by the latter and miss an obvious, relatively simple solution that was under my nose.

My feelings about channels and goroutines are mixed. On the one hand I think that using them simplified the logic of my code (and made it much easier to support TLS), even if it took a while to sort out that logic. On the other hand having to use goroutines is responsible for a serious wart in one aspect of the program, a wart I see no way around; the wart arises because there's no way for outside code to force a goroutine blocked in IO to gracefully abort that IO (this is a fundamental issue with channels).

This is rambling long enough as it is, so I think that I will save my language disagreements for another day. Well, except to say that I think that the standard Go package for parsing arguments and argument flags handles command line options utterly the wrong way and I need to get a real argument parsing package before I write another Go command.

(Go's standard flag package apparently follows some argument parsing standard that Google likes. It is pretty non-Unixy while looking just enough like normal Unix argument handling to fool you.)