Custom errors

Context

You can define custom error types by implementing the error interface (Error() string). Custom errors can carry additional context (e.g., status codes). Use errors.Is and errors.As to inspect error chains.

Example

Define a custom error with an error code.

Code example

package main

import (
    "errors"
    "fmt"
)

type ValidationError struct {
    Field string
    Value interface{}
}

func (e ValidationError) Error() string {
    return fmt.Sprintf("validation failed for field '%s' with value %v", e.Field, e.Value)
}

func validateAge(age int) error {
    if age < 0 {
        return ValidationError{Field: "age", Value: age}
    }
    return nil
}

func main() {
    err := validateAge(-5)
    if err != nil {
        // Type assertion to get extra fields
        if ve, ok := err.(ValidationError); ok {
            fmt.Printf("Custom error: %s (field=%s, value=%v)\n", err, ve.Field, ve.Value)
        } else {
            fmt.Println("Error:", err)
        }
    }
}

Output

Custom error: validation failed for field 'age' with value -5 (field=age, value=-5)

Useful links

• Go by Example: Custom Errors
• errors package