A custom error type in Go

A struct error carries data, and can decide what it matches.

type HTTPError struct {
    Status int
    Cause  error
}

func (e *HTTPError) Error() string {
    return fmt.Sprintf("http %d: %v", e.Status, e.Cause)
}

func (e *HTTPError) Unwrap() error { return e.Cause }

func (e *HTTPError) Is(target error) bool {
    return target == ErrServerSide && e.Status >= 500
}

var ErrServerSide = errors.New("server side")

func classify(status int) bool {
    err := &HTTPError{Status: status, Cause: errors.New("boom")}
    return errors.Is(err, ErrServerSide)
}

How it works

1. Error() on a pointer receiver is the usual choice.
2. Unwrap exposes an inner error to Is and As.
3. Adding an Is method lets one error match a category.

Keywords and builtins used here

• bool
• error
• func
• int
• return
• string
• struct
• type
• var

The run, in numbers

Lines

21

Characters to type

475

Tokens

121

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 69 seconds.

Type this snippet

Step 1 of 2 in Your own errors, step 8 of 15 in Errors.

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