# Errors

Go programs express error state with `error` values. The `error` type is a built-in interface similar to `fmt.Stringer`:

```go
type error interface {
    Error() string
}
```

 Functions often return an `error` value, and calling code should handle errors by testing whether the error equals `nil`. A nil `error` denotes success; a non-nil `error` denotes failure.

### Example

```go
package main

import (
	"fmt"
	"time"
)

type MyError struct {
	When time.Time
	What string
}

func (e *MyError) Error() string {
	return fmt.Sprintf("at %v, %s",
		e.When, e.What)
}

func run() error {
	return &MyError{
		time.Now(),
		"it didn't work",
	}
}

func main() {
	if err := run(); err != nil {
		fmt.Println(err)
	}
}

```

```bash
at 2009-11-10 23:00:00 +0000 UTC m=+0.000000001, it didn't work
```