Errors in Go

Sentinel errors, custom types, and wrapping.

var ErrNotFound = errors.New("not found")

type ValidationError struct{ Field string }

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

func find(id int) error {
    if id < 0 {
        return fmt.Errorf("lookup %d: %w", id, ErrNotFound)
    }
    return nil
}

How it works

1. errors.New makes a comparable sentinel.
2. A type with an Error() string method is an error.
3. %w in Errorf wraps for errors.Is.

Keywords and builtins used here

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

The run, in numbers

Lines

14

Characters to type

287

Tokens

67

Three-star pace

95 tpm

At the three-star pace of 95 tokens a minute, this run takes about 42 seconds.

Type this snippet

Step 2 of 3 in The error value, step 2 of 15 in Errors.

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