Generic type in a switch statement

user_4463429 0
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Just started learning generics. I'm making a command processor and I honestly don't know how to word this so I'm just going to show an example problem:

var ErrInvalidCommand = errors.New("invalid command")

type TransactionalFn[T any] func(ctx context.Context, db T) error

func NewTransactionalCommand[T any](fn TransactionalFn[T]) *TransactionalCommand[T] {
    return &TransactionalCommand[T]{
        fn: fn,
    }
}

type TransactionalCommand[T any] struct {
    fn TransactionalFn[T]
}

func (cmd *TransactionalCommand[T]) StartTransaction() error {
    return nil
}

func (cmd *TransactionalCommand[T]) Commit() error {
    return nil
}

func (cmd *TransactionalCommand[T]) Rollback() error {
    return nil
}

type CMD interface{}

type CommandManager struct{}

func (m *CommandManager) Handle(ctx context.Context, cmd CMD) error {
    switch t := cmd.(type) {
    case *TransactionalCommand[any]:
        return m.handleTransactionalCommand(ctx, t)
    default:
        fmt.Printf("%T\n", cmd)
        return ErrInvalidCommand
    }
}

func (m *CommandManager) handleTransactionalCommand(ctx context.Context, cmd *TransactionalCommand[any]) error {
    if err := cmd.StartTransaction(); err != nil {
        return err
    }

    if err := cmd.fn(ctx, nil); err != nil {
        if err := cmd.Rollback(); err != nil {
            return err
        }
    }

    if err := cmd.Commit(); err != nil {
        return err
    }

    return nil
}

// tests
type db struct{}

func (*db) Do() {
    fmt.Println("doing stuff")
}

func TestCMD(t *testing.T) {
    ctx := context.Background()
    fn := func(ctx context.Context, db *db) error {
        fmt.Println("test cmd")
        db.Do()
        return nil
    }
    tFn := bus.NewTransactionalCommand(fn)

    mng := &bus.CommandManager{}
    err := mng.Handle(ctx, tFn)
    if err != nil {
        t.Fatal(err)
    }
}

mng.handle returns ErrInvalidCommand so the test fails because cmd is *TransactionalCommand[*db] and not *TransactionalCommand[any]

Let me give another, more abstract example:

type A[T any] struct{}

func (*A[T]) DoA() { fmt.Println("do A") }

type B[T any] struct{}

func (*B[T]) DoB() { fmt.Println("do B") }

func Handle(s interface{}) {
    switch x := s.(type) {
    case *A[any]:
        x.DoA()
    case *B[any]:
        x.DoB()
    default:
        fmt.Printf("%T\n", s)
    }
}



func TestFuncSwitch(t *testing.T) {
    i := &A[int]{}

    Handle(i) // expected to print "do A"
}

Why doesn't this switch statement case *A[any] match *A[int]? How to make CommandManager.Handle(...) accept generic Commands?

go generics
2 Answers

*A[any] does not match *A[int] because any is a static type, not a wildcard. Therefore instantiating a generic struct with different types yields different types.

In order to correctly match a generic struct in a type switch, you must instantiate it with a type parameter:

func Handle[T any](s interface{}) {
    switch x := any(s).(type) {
    case *A[T]:
        x.DoA()
    case *B[T]:
        x.DoB()
    default:
        panic("no match")
    }
}

Though in absence of other function arguments to infer T, you will have to call Handle with explicit instantiation. T won't be inferred from the struct alone.

func main() {
    i := &A[int]{}
    Handle[int](i) // expected to print "do A"
}

Playground: https://go.dev/play/p/2e5E9LSWPmk

However when Handle is actually a method, as in your database code, this has the drawback of choosing the type parameter when instantiating the receiver.

In order to improve the code here you can make Handle a top-level function:

func Handle[T any](ctx context.Context, cmd CMD) error {
    switch t := cmd.(type) {
    case *TransactionalCommand[T]:
        return handleTransactionalCommand(ctx, t)
    default:
        fmt.Printf("%T\n", cmd)
        return ErrInvalidCommand
    }
}

Then you have the problem of how to supply the argument db T to the command function. For this, you might:

  • simply pass an additional *db argument to Handle and handleTransactionalCommand, which also helps with type parameter inference. Call as Handle(ctx, &db{}, tFn). Playground: https://go.dev/play/p/6WESb86KN5D

  • pass an instance of CommandManager (like solution above but *db is wrapped). Much more verbose, as it requires explicit instantiation everywhere. Playground: https://go.dev/play/p/SpXczsUM5aW

  • use a parametrized interface instead (like below). So you don't even have to type-switch. Playground: https://go.dev/play/p/EgULEIL6AV5

type CMD[T any] interface {
    Exec(ctx context.Context, db T) error
}
user_41088030

Why does the generic type switch fail to compile?

  • This is in fact the result of an intentional decision of the Go team. It turned out that allowing type switches on parametrized types can cause confusion

  • In an earlier version of this design, we permitted using type assertions and type switches on variables whose type was a type parameter, or whose type was based on a type parameter. We removed this facility because it is always possible to convert a value of any type to the empty interface type, and then use a type assertion or type switch on that. Also, it was sometimes confusing that in a constraint with a type set that uses approximation elements, a type assertion or type switch would use the actual type argument, not the underlying type of the type argument (the difference is explained in the section on identifying the matched predeclared type)

    From the Type Parameters Proposal

Let me turn the emphasized statement into code. If the type constraint uses type approximation (note the tildes)...

func PrintStringOrInt[T ~string | ~int](v T)

...and if there also was a custom type with int as the underlying type...

type Seconds int

...and if PrintOrString() is called with a Seconds parameter...

PrintStringOrInt(Seconds(42))

...then the switch block would not enter the int case but go right into the default case, because Seconds is not an int. Developers might expect that case int: matches the type Seconds as well.

To allow a case statement to match both Seconds and int would require a new syntax, like, for example,

case ~int:

As of this writing, the discussion is still open, and maybe it will result in an entirely new option for switching on a type parameter (such as, switch type T).

More details, please refer to proposal: spec: generics: type switch on parametric types

Trick: convert the type into 'any'

Luckily, we do not need to wait for this proposal to get implemented in a future release. There is a super simple workaround available right now.

Instead of switching on v.(type), switch on any(v).(type).

switch any(v).(type) {
    ...

This trick converts v into an empty interface{} (a.k.a. any), for which the switch happily does the type matching.

Source: A tip and a trick when working with generics

user_30113800
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