How do I Capture Specific Errors in Go?

Capturing Specific Errors in Go

When interacting with standard Go library functions that return errors, it can be useful to identify and handle specific types of errors, such as "connection lost" or "connection refused." Here's how to achieve this:

Using errors.Is and errors.As

For modern Go versions, the recommended approach is to use the errors.Is and errors.As functions from the standard library. These functions allow you to check if an error matches a specific type or extract a subset of the error information. For example:

if errors.Is(err, syscall.ECONNREFUSED) {
    // err is a connection refused error
}

Matching Error Strings

If a standard library function does not provide specific error types, you can resort to comparing the error string:

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

Checking Specific Error Types

Some libraries may export specific error types that can be checked directly:

if _, ok := err.(net.Error); ok {
    // err is a net.Error instance
}

Determining All Possible Errors

Identifying all possible errors returned by a standard library function requires reading the source code. Alternatively, you can inspect the documentation, such as the godoc for the net package, where errors are often described.

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