errors.Is in Go

Comparing against a sentinel through any amount of wrapping.

var (
    ErrLocked  = errors.New("step locked")
    ErrMissing = errors.New("step missing")
)

func play(idx int) error {
    if idx > 10 {
        return fmt.Errorf("play %d: %w", idx, ErrMissing)
    }
    if idx > 3 {
        return fmt.Errorf("play %d: %w", idx, ErrLocked)
    }
    return nil
}

func describe(idx int) string {
    err := play(idx)
    switch {
    case err == nil:
        return "playing"
    case errors.Is(err, ErrLocked):
        return "earn a star first"
    case errors.Is(err, ErrMissing):
        return "no such step"
    }
    return "unknown failure"
}

How it works

1. == fails once the error is wrapped; errors.Is does not.
2. Sentinels are exported vars so callers can name them.
3. errors.Is also matches an Is method you define.

Keywords and builtins used here

• case
• error
• func
• if
• int
• return
• string
• switch
• var

The run, in numbers

Lines

27

Characters to type

493

Tokens

115

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 69 seconds.

Type this snippet

Step 2 of 4 in Wrapping, step 5 of 15 in Errors.

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