Errors

In Go+ it’s idiomatic to communicate errors via an explicit, separate return value. This contrasts with the exceptions used in languages like Java and Ruby and the overloaded single result / error value sometimes used in C. Go+’s approach makes it easy to see which functions return errors and to handle them using the same language constructs employed for any other, non-error tasks.

import ( "errors" )

By convention, errors are the last return value and have type error, a built-in interface.

func f1(arg int) (int, error) { if arg == 42 {

errors.New constructs a basic error value with the given error message.

 return -1, errors.new("can't work with 42") }

A nil value in the error position indicates that there was no error.

 return arg + 3, nil }

It’s possible to use custom types as errors by implementing the Error() method on them. Here’s a variant on the example above that uses a custom type to explicitly represent an argument error.

type argError struct { arg int prob string } func (e *argError) Error() string { return sprintf("%d - %s", e.arg, e.prob) } func f2(arg int) (int, error) { if arg == 42 {

In this case we use &argError syntax to build a new struct, supplying values for the two fields arg and prob.

 return -1, &argError{arg, "can't work with it"} } return arg + 3, nil }

The two loops below test out each of our error-returning functions. Note that the use of an inline error check on the if line is a common idiom in Go+ code.

for _, i := range []int{7, 42} { if r, e := f1(i); e != nil { println "f1 failed:", e } else { println "f1 worked:", r } } for _, i := range []int{7, 42} { if r, e := f2(i); e != nil { println "f2 failed:", e } else { println "f2 worked:", r } }

If you want to programmatically use the data in a custom error, you’ll need to get the error as an instance of the custom error type via type assertion.

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

Next example: Function Values