proposal: errors: As with type parameters

[slnt]

Currently in 1.18 and before, when using the errors.As method, an error type you would like to write into must be predeclared before calling the function. For example:

var myErr *MyCustomError
if errors.As(err, &myErr) {
  // handle myErr
}

This can makes control flow around handling errors "unergonomic".

I'd propose that a new, type parameterized method be added to the errors package in 1.19:

func IsA[T error](err error) (T, bool) {
	var isErr T
	if errors.As(err, &isErr) {
		return isErr, true
	}

	var zero T
	return zero, false
}

This enables more "ergonomic" usage as follows:

err := foo()
if err != nil {
	if myErr, ok := errors.IsA[*MyCustomError](err); ok {
		// handle myErr
	} else if otherErr, ok := errors.IsA[*OtherError](err); ok {
		// handle otherErr
	}

	// handle everything else
}

instead of

err := foo()
if err != nil {
	var myErr *MyCustomError
	if errors.As(err, &myErr) {
		// handle customer error
	}

	var otherErr *OtherError
	if errors.As(err, &otherErr) {
		// handle other error
	}

	// handle everything else
}

This change would reduce the overall LOC needed for handling custom errors, imo improves readability of the function, as well as scopes the errors to the if blocks they are needed in.

Naming is hard so IsA might be better replaced with something else.

[seankhliao]

Interesting, but changing the signature of errors.As is not a backwards compatible change.
It will have to be a new function.

[slnt]

Interesting, but changing the signature of errors.As is not a backwards compatible change. It will have to be a new function.

Yeah, unfortunate. I think if customerErr, ok := errors.IsA[*CustomError](err); ok reads quite nicely. It would be implemented pretty simply in terms of errors.As:

func IsA[T error](err error) (T, bool) {
	var isErr T
	if errors.As(err, &isErr) {
		return isErr, true
	}

	var zero T
	return zero, false
}

[ianlancetaylor]

CC @jba @neild

(My vague recollection is that this was considered and rejected when errors.As was introduced, even beyond the fact that at the time type parameters did not yet exist.)

[neild]

@rsc made the case in the original proposal issue that a type-parameterized version of errors.As would not be an improvement:
#29934 (comment)

@dsnet also pointed out that the non-type-parameterized version of As can be used in a switch, while the type-parameterized one cannot:
#29934 (comment)

I don't recall if there were any other arguments against a type-parameterized As (aside from, obviously, the fact that type parameters didn't exist at the time).

I personally think that a type-parameterized As seems fairly reasonable, although the practical benefit over the current As seems small. It would need a good name. I don't know what that name would be. IsA does not seem right. Under the current design, "Is" is an enhanced form of equality and "As" is an enhanced form of type assertion; blurring that distinction would add confusion.

An argument against adding a type-parameterized As is that the benefit does not justify the cost in API churn and user confusion. I don't have a strong opinion on whether the benefits do outweigh the costs.

[DeedleFake]

I personally think that a type-parameterized As seems fairly reasonable, although the practical benefit over the current As seems small. It would need a good name. I don't know what that name would be. IsA does not seem right. Under the current design, "Is" is an enhanced form of equality and "As" is an enhanced form of type assertion; blurring that distinction would add confusion.

AsA() could work, though it's a little oddly close to the existing one. if pe, ok := errors.AsA[*os.PathError](err); ok { ... } reads pretty well to me, though it should technically be 'an *os.PathError', not 'a'.

New proposal: Automatically alias all functions with names that match ([a-z0-9])A$ to ${1}An.

[zigo101]

I like this proposal, but I fell the call errors.IsA[*MyCustomError](err) is not very natural.
If would be good if we could pass type arguments to unnamed value parameters (_) of type parameter types.
For example,

func IsA[T error](err error, _ T) (T, bool) {
	var isErr T
	if errors.As(err, &isErr) {
		return isErr, true
	}

	var zero T
	return zero, false
}

err := foo()
if err != nil {
	if myErr, ok := errors.IsA(err, MyCustomError); ok {
		// handle myErr
	} else if otherErr, ok := errors.IsA(err, OtherError); ok {
		// handle otherErr
	}

	// handle everything else
}

[zigo101]

Or more generally, it would be good to pass type arguments to any value parameters of type parameter types.
It is equivalent to pass zero values of the type arguments.

[Edit] This could unify built-in generic functions and custom ones to some extent.

func new[T any](T) *T

is much better than the illogical fake declaration:

func new(Type) *Type

[earthboundkid]

I don't see much benefit to duplicating the existing API. This also implies a similar change to eg json.Marshal etc. We would end up with duplicate functions throughout the standard library. And as noted this doesn't add any type safety; it is just more convenient, although even that is debatable. I think generics should be reserved for areas where they either add real type safety or big convenience, and cases like this of minor convenience can stay as they are.

[neild]

And as noted this doesn't add any type safety

That's not quite true: This does add type safety at the language level, rather than leaving it to a go vet check. Under this proposal, this code would not be valid:

type MyError struct{}
func (*MyError) Error() string { return "MyError" }

func main() {
	var err error
	m, ok := errors.AsA[MyError](err) // MyError does not implement error (Error method has pointer receiver)
	fmt.Println(m, ok)
}

The equivalent errors.As call is a run-time panic or go vet failure:

second argument to errors.As must be a non-nil pointer to either a type that implements error, or to any interface type

[slnt]

A sidenote is the original error inspection draft design includes this blurb:

Here we are assuming the use of the contracts draft design to make errors.As explicitly polymorphic:
func As(type E)(err error) (e E, ok bool)

so its not I guess a new idea