How to create and use error constants in Go to define common error types?

To create and use error constants in Go, you can follow these steps:

Define a new package to contain your error constants. Let's call it errors.

package errors

Define a new file within the errors package, for example, constants.go.

package errors

import "errors"

var (
  ErrNotFound  = errors.New("not found")
  ErrInvalid   = errors.New("invalid data")
  ErrForbidden = errors.New("forbidden")
)

In this example, we define three error constants: ErrNotFound, ErrInvalid, and ErrForbidden. Each constant is set to a new instance of the error type with an appropriate error message.

Use the error constants in your code:

package main

import (
  "fmt"
  "github.com/your-username/your-package/errors"
)

func main() {
  // Example usage 1
  if err != nil {
    fmt.Println(errors.ErrNotFound)
  }

  // Example usage 2
  if err != nil {
    if errors.Is(err, errors.ErrForbidden) {
      fmt.Println("This is a forbidden error.")
    }
  }
}

In the first example usage, we're printing the error constant errors.ErrNotFound directly when the error occurs.

In the second example usage, we're checking if the error is equivalent to errors.ErrForbidden using the errors.Is() function from the errors package. If it is, we print a specific message.

Note that the errors.Is(err, target) function checks if the error err is equal to or implements target. This is especially useful when dealing with error wrapping.

That's it! These steps allow you to create and use error constants in Go to define common error types.