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error in Go isn't a keyword: it's a built-in interface with a single method.

type error interface {
    Error() string
}

Anything that has Error() string is an error. Combined with the convention of returning (T, error) from functions, it's the foundation of error handling in Go.

Sentinel errors

import "errors"

var ErrNotFound = errors.New("non trovato")

func find(key string) (string, error) {
    // ...
    return "", ErrNotFound
}

A sentinel is an exported error value that the caller can compare against to recognize a specific error case.

Custom errors (with data)

To carry context (e.g. the missing key) you define a type:

type ErrNotFound struct {
    Key string
}

func (e *ErrNotFound) Error() string {
    return "non trovato: " + e.Key
}

func find(key string) (string, error) {
    return "", &ErrNotFound{Key: key}
}

errors.Is and errors.As (Go 1.13+)

Comparing with == works only for "pure" sentinels. To be robust against wrapped errors, use the errors package:

import "errors"

if errors.Is(err, ErrNotFound) {
    // err IS ErrNotFound (even if wrapped)
}

var nfe *ErrNotFound
if errors.As(err, &nfe) {
    // err IS (or wraps) an *ErrNotFound; nfe points to the concrete value
    fmt.Println("chiave mancante:", nfe.Key)
}

• errors.Is(err, target): true if target is somewhere in the wrapping chain.
• errors.As(err, &target): assigns to target the first error in the chain of its type.

Wrapping with %w

if err := find("k"); err != nil {
    return fmt.Errorf("operazione X: %w", err)
}

%w (NOT %s or %v) creates a new error that wraps the original, keeping the chain navigable by errors.Is/As.

Handling pattern

v, err := op()
if err != nil {
    return fmt.Errorf("op fallita per %q: %w", id, err)
}

Idioms:

• Check err != nil immediately, no "early happy path".
• Add context at each level with fmt.Errorf("... : %w", err).
• "Upstream" (near main / handler), discriminate with errors.Is/As.

Try it

func (e *MyErr) ??? {}

Recap

• error = interface { Error() string }, built-in.
• Sentinels with errors.New("...") for known cases without context.
• Custom error types with fields to carry context.
• errors.Is(err, target): comparison robust to wrapping.
• errors.As(err, &target): extraction of the concrete type.
• fmt.Errorf("...: %w", err): wrapping that preserves the chain.
• Never %v / %s to wrap: ALWAYS use %w.