proposal: spec: reduce error handling boilerplate using catch statement

[philippta]

Proposal Details

Proposal

Reduce boilerplate of handling errors by introducing a new catch statement, which works similar to switch and select, given the implicit error value is non-nil.

// return bare error 
f, err := os.Open(src) catch err 


// return annotated error
f, err := os.Open(src) catch fmt.Errorf("open file: %w", err)


// switch on error type using a boolean expression
f, err := os.Open(src) catch {
    case os.IsNotExist(err): errors.New("file not found")
    case os.Permission(err): errors.New("permission denied")
    default: fmt.Errorf("open file: %w", err)
}

Multiple return values

Given a function has multiple return values, the catch statement would look like this:

// return bare error 
f, err := os.Open(src) catch nil, err 


// return annotated error
f, err := os.Open(src) catch nil, fmt.Errorf("open file: %w", err)


// switch on error type using a boolean expression
f, err := os.Open(src) catch {
    case os.IsNotExist(err): nil, errors.New("file not found")
    case os.Permission(err): nil, errors.New("permission denied")
    default: nil, fmt.Errorf("open file: %w", err)
}

The default case

The default case is effectively syntatic sugar for:

    case err != nil: ...

No default provided in catch block

Omitting the default case in the catch block, should result in a compilation error for being non-exhaustive.

Rationale

Adding a new symbol like ? or !, although inspired by other languages, feels out of place in Go code, which is why I believe adding a new keyword instead is a more reasonable approach.

Many of the proposals from this meta issue only handle the err != nil case, leaving it up to the developer to find a solution when trying to assert on the type of the error.

The new catch statement feels more Go-like by borrowing well-established syntax from switch and select using the case keyword to assert on the error type using a boolean expression.

Possible considerations for go fmt

Consider to align error handlers of case statements similar to how map key-value pairs are formatted:

f, err := os.Open(src) catch {
    case os.IsNotExist(err): errors.New("file not found")
    case os.Permission(err): errors.New("permission denied")
    default:                 fmt.Errorf("open file: %w", err)
}

[seankhliao]

Please fill out https://github.com/golang/proposal/blob/master/go2-language-changes.md when proposing language changes

[seankhliao]

I believe this is sufficiently similar to #67955