Try and Catch in Golang

Golang (as opposed to Java) does not have exceptions such as try/catch/finally blocks. It has strict error handling with functions called panic and recover and a statement named defer. It is a totally different approach. Is this a better approach than Java takes? (Sorry that I keep comparing it to Java. I am coming from the Java world.)

When we handle exceptions in Java we enclose the commands into a ‘try’ block denoting that something may happen that we want to handle later in a ‘catch’ block. Then we have the ‘finally’ block that contains all the things that are to be executed no matter what. The problem with this approach is that it separates the commands that belong to the same concern.

We want to deal with some file. So we open a file and later, no matter what, we want to close it. When the programmer writes the finally block the file opening is far away somewhere at the start of the method. To remember all the things that we have to do to clean up the actions at the start of the method you have to scroll up to the start of the method where the ‘try’ block starts.

Okay, I know that your method is too long if you have to scroll back. Your methods follow clean code principles and are not longer than ten lines each (including JavaDoc). Even though the issue is still there, it is formulated according to the order of execution expected and not according to the order the logic dictates. The logic says the following: if I open a file, I will want to close it. If I allocate some resource I will want to release it. It is better keeping these concerns together. We are not programming in Assembly, where you write the mnemonics in the strict order of execution. We define the algorithmic solution in a high-level language and the compiler will generate the Assembly. Real work has to be done by the brain, mechanical work is for the CPU. These days we have CPUs.

Golang has the command ‘defer’ for this purpose. You open a file and you mention on the next line that you will want it to be closed some time calling the function you provide. This is the much better approach, which the developers of the Java language also know hence the introduction of the interface ‘closeable’ and try-with-resources statement.

Still, programmers coming from the Java world being introduced to Go are longing for exception handling. If you really want you can mimic it in Go. It will not be the same and I do not really think one should take something that is good and instead use something old and mediocre, but you can write

Block{
        Try: func() {
            fmt.Println("I tried")
            Throw("Oh,...sh...")
        },
        Catch: func(e Exception) {
            fmt.Printf("Caught %v\n", e)
        },
        Finally: func() {
            fmt.Println("Finally...")
        },
    }.Do()

Homework: find out the sample code that is before these lines (Go constructs) that make this possible. Solution is here: https://play.golang.org/p/LXroobH8SM.

package main
 
import (
    "fmt"
)
 
type Block struct {
    Try     func()
    Catch   func(Exception)
    Finally func()
}
 
type Exception interface{}
 
func Throw(up Exception) {
    panic(up)
}
 
func (tcf Block) Do() {
    if tcf.Finally != nil {
 
        defer tcf.Finally()
    }
    if tcf.Catch != nil {
        defer func() {
            if r := recover(); r != nil {
                tcf.Catch(r)
            }
        }()
    }
    tcf.Try()
}
 
func main() {
    fmt.Println("We started")
    Block{
        Try: func() {
            fmt.Println("I tried")
            Throw("Oh,...sh...")
        },
        Catch: func(e Exception) {
            fmt.Printf("Caught %v\n", e)
        },
        Finally: func() {
            fmt.Println("Finally...")
        },
    }.Do()
    fmt.Println("We went on")
}

See also a recent similar solution at http://hackthology.com/exceptions-for-go-as-a-library.html from Tim Henderson.