Errors in Golang - assessing errors

Question

I am trying to understand the following example

I understand most of it except for this part:

_, e := f2(42)
if ae, ok := e.(*argError); ok {
    fmt.Println(ae.arg)
    fmt.Println(ae.prob)
}

I'm not sure what this line does :

if ae, ok := e.(*argError); ok {

BadZen BadZen · Accepted Answer · 2015-05-06T00:01:13

e.(*argError)

is a type assertion that casts the value e to *argError type. This is the type f2() returns on error - it is a pointer to an argError struct, which implements the error interface. This type assertion evaluates multi-valued to (ae,ok), where ae is the *argError typed value, if successful, and ok is a boolean letting you know if it was successful.

if statements in go can be separated into an initial assignment part, then a semicolon, then a boolean condition to evaluate to determine the branch.

Altogether, then,

if ae, ok := e.(*argError); ok {

means: try to cast e to *argError, if that is successful (do if block).

Why do this? Because argError has fields that are not in a plain error (arg, prob), and you might want to use those. In real code where you do this, you'd likely also need to handle the case where e was not an argError, but some other error type, in an "else" branch.

Errors in Golang - assessing errors

1 Answers