Try It Yourself - SyntaxStudy Editor

HTML / CSS / JS

package main

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

// Custom error type
type ParseError struct {
    Input string
    Err   error
}

func (e *ParseError) Error() string {
    return fmt.Sprintf("ParseError: cannot parse %q: %v", e.Input, e.Err)
}

func (e *ParseError) Unwrap() error { return e.Err }

func parsePositive(s string) (int, error) {
    n, err := strconv.Atoi(s)
    if err != nil {
        return 0, &ParseError{Input: s, Err: err}
    }
    if n <= 0 {
        return 0, fmt.Errorf("parsePositive: %d is not positive", n)
    }
    return n, nil
}

func main() {
    inputs := []string{"42", "-5", "abc", "100"}

for _, s := range inputs {
        n, err := parsePositive(s)
        if err != nil {
            // Type assertion
            var pe *ParseError
            if errors.As(err, &pe) {
                fmt.Printf("type error for %q: %v
", pe.Input, pe.Err)
            } else {
                fmt.Printf("value error: %v
", err)
            }
            continue
        }
        fmt.Printf("parsed: %d
", n)
    }
}