How to `catch` specific errors

Question

For example, I am using one Go standard library function as:

func Dial(network, address string) (*Client, error)

This function may return e

Answer 1

There's no standard way to do this.

The most obvious way, which should only be used if no other method is available, is to compare the error string against what you expect:

if err.Error() == "connection lost" { ... }

Or perhaps more robust in some situations:

if strings.HasSuffix(err.Error(), ": connection lost") { ... }

But many libraries will return specific error types, which makes this much easier.

In your case, what's relevant are the various error types exported by the net package: AddrError, DNSConfigError, DNSError, Error, etc.

You probably care most about net.Error, which is used for network errors. So you could check thusly:

if _, ok := err.(net.Error); ok {
    // You know it's a net.Error instance
    if err.Error() == "connection lost" { ... }
}

What's more, how to get all the errors a specific standard library function may return?

The only fool-proof way to do this is to read the source for the library. Before going to that extreme, a first step is simply to read the godoc, as in the case of the net package, the errors are pretty well documented.