Error Handling

        
          package main

import (
    "errors"
    "fmt"
    "strconv"
)

// Custom error types
type ValidationError struct {
    Field   string
    Message string
}

func (e ValidationError) Error() string {
    return fmt.Sprintf("validation error on field '%s': %s", e.Field, e.Message)
}

type NetworkError struct {
    Code    int
    Message string
}

func (e NetworkError) Error() string {
    return fmt.Sprintf("network error %d: %s", e.Code, e.Message)
}

// Function that returns an error
func divide(a, b float64) (float64, error) {
    if b == 0 {
        return 0, errors.New("division by zero")
    }
    return a / b, nil
}

// Function with custom error
func validateAge(age int) error {
    if age < 0 {
        return ValidationError{Field: "age", Message: "cannot be negative"}
    }
    if age > 150 {
        return ValidationError{Field: "age", Message: "seems unrealistic"}
    }
    return nil
}

// Function that can return different error types
func processUserData(data map[string]interface{}) error {
    // Simulate network call
    if data["network_error"] != nil {
        return NetworkError{Code: 500, Message: "server error"}
    }

    if name, ok := data["name"].(string); !ok {
        return ValidationError{Field: "name", Message: "must be string"}
    } else if name == "" {
        return ValidationError{Field: "name", Message: "cannot be empty"}
    }

    if age, ok := data["age"].(float64); !ok {
        return ValidationError{Field: "age", Message: "must be number"}
    } else {
        return validateAge(int(age))
    }
}

// Panic and recover
func riskyOperation() {
    defer func() {
        if r := recover(); r != nil {
            fmt.Println("Recovered from panic:", r)
        }
    }()

    // Simulate a panic
    panic("something went wrong!")
}

// Error wrapping (Go 1.13+)
func processFile(filename string) error {
    data, err := readFile(filename)
    if err != nil {
        return fmt.Errorf("failed to process file %s: %w", filename, err)
    }
    return processData(data)
}

// Mock functions for demonstration
func readFile(filename string) ([]byte, error) {
    if filename == "" {
        return nil, errors.New("filename cannot be empty")
    }
    return []byte("file content"), nil
}

func processData(data []byte) error {
    if len(data) == 0 {
        return errors.New("data cannot be empty")
    }
    return nil
}

func main() {
    // Basic error handling
    result, err := divide(10, 0)
    if err != nil {
        fmt.Println("Error:", err)
    } else {
        fmt.Printf("Result: %.2f\n", result)
    }

    // Custom errors
    if err := validateAge(-5); err != nil {
        fmt.Println("Validation error:", err)
        // Type assertion to access custom error fields
        if valErr, ok := err.(ValidationError); ok {
            fmt.Printf("Field: %s, Message: %s\n", valErr.Field, valErr.Message)
        }
    }

    // Process user data
    userData := map[string]interface{}{
        "name": "John",
        "age":  25,
    }

    if err := processUserData(userData); err != nil {
        fmt.Println("User data error:", err)
    }

    // Panic and recover
    fmt.Println("About to run risky operation...")
    riskyOperation()
    fmt.Println("Program continues after recovery")

    // Error wrapping
    if err := processFile(""); err != nil {
        fmt.Println("Wrapped error:", err)
        // Check if error wraps another error
        if wrappedErr := errors.Unwrap(err); wrappedErr != nil {
            fmt.Println("Original error:", wrappedErr)
        }
    }
}