// Created on savesnippets.com · https://savesnippets.com/e3yJ7WvnG58ZAI
package main

import (
    "errors"
    "fmt"
)

type ValidationError struct {
    Field   string
    Message string
}

func (e *ValidationError) Error() string {
    return fmt.Sprintf("invalid %s: %s", e.Field, e.Message)
}

func validate(name string) error {
    if name == "" {
        return &ValidationError{Field: "name", Message: "cannot be empty"}
    }
    if len(name) > 50 {
        return &ValidationError{Field: "name", Message: "too long"}
    }
    return nil
}

func main() {
    err := validate("")
    fmt.Println(err)                                     // invalid name: cannot be empty

    var ve *ValidationError
    if errors.As(err, &ve) {
        fmt.Printf("field=%s reason=%s\n", ve.Field, ve.Message)
    }
}