| 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. | ||
package main |
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import ( |
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"errors" |
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"fmt" |
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) |
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| By convention, errors are the last return value and have type `error`, a built-in interface. | func f1(arg int) (int, error) { |
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if arg == 42 { |
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| `errors.New` constructs a basic `error` value with the given error message. | return -1, errors.New("can't work with 42") |
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} |
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| A `nil` value in the error position indicates that there was no error. | return arg + 3, nil |
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} |
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| It's possible to use custom types as `error`s 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 { |
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arg int |
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prob string |
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} |
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func (e *argError) Error() string { |
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return fmt.Sprintf("%d - %s", e.arg, e.prob) |
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} |
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func f2(arg int) (int, error) { |
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if arg == 42 { |
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| 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"} |
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} |
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return arg + 3, nil |
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} |
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func main() { |
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| 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} { |
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if r, e := f1(i); e != nil { |
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fmt.Println("f1 failed:", e) |
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} else { |
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fmt.Println("f1 worked:", r) |
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} |
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} |
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for _, i := range []int{7, 42} { |
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if r, e := f2(i); e != nil { |
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fmt.Println("f2 failed:", e) |
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} else { |
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fmt.Println("f2 worked:", r) |
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} |
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} |
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| 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) |
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if ae, ok := e.(*argError); ok { |
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fmt.Println(ae.arg) |
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fmt.Println(ae.prob) |
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} |
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} |
f1 worked: 10 f1 failed: can't work with 42 f2 worked: 10 f2 failed: 42 - can't work with it 42 can't work with it |