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Error handling

Error handling

Go does not have try/catch or try/except. Instead almost every function returns (or should return) an error value. It's good practice to return an error value in every function and also check it after reading values from functions/channels/etc. As a result it's very common to see if err != nil code blocks.

Go's error handling is very controversial. Some call it genius and others not so much. For more information read the Error handling and Go blog.

Errors

error type is similar to Stringer().

type error interface {
    Error() string
}

Create a method for the struct type named Error() to return error codes/messages.

func (e MyType) Error() string {
    return fmt.Sprintf("error message")
}

According to Go docs, errors strings should not be capitalized. Most built-in and package methods return an error value if an error occurs, otherwise they will return nil for error which means no error.

Avoiding if err != nil fatigue

Checking for errors after every function call will result in a lot of if err != nil blocks. One good way is to create a function to check the error and perform actions based on it. For example:

func checkError(err) {
    if err != nil {
        // Do something
    }
}

Solution to the Errors exercise

The errors exercise is part of tour of go.

// 02.7-01-errors1.go
package main

import (
    "fmt"
    "math"
)

type ErrNegativeSqrt float64

func (e ErrNegativeSqrt) Error() string {
    return fmt.Sprintf("cannot Sqrt negative number: %v", float64(e))
}

func Sqrt(x float64) (float64, error) {

    if x < 0 {
        return 0, ErrNegativeSqrt(x)
    }

    // Don't need else here - why?
    return math.Sqrt(x), nil
}

func main() {
    fmt.Println(Sqrt(2))
    fmt.Println(Sqrt(-2))
}

Instead of creating an Error() method, we could create a new error type and return that using fmt.Errorf:

// 02.7-02-errors2.go
package main

import (
    "fmt"
    "math"
)

func Sqrt(x float64) (float64, error) {

    if x < 0 {
        return 0, fmt.Errorf("cannot Sqrt negative number: %v", float64(x))
    }

    return math.Sqrt(x), nil
}

func main() {
    fmt.Println(Sqrt(2))
    fmt.Println(Sqrt(-2))
}

Continue reading ⇒ 03 - Useful Go packages

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