Type switch on generic parameter

[Matt Rosencrantz]

I'd really love to be able to do:

type allowedTypes{

  int64|float64|string

}

func Foo[T allowedTypes](arg T) {

    switch t := arg {

       case int64:

           // something

       case string:

          // something completely different

   }

}

As a way of having the type checker disallow other types from being passed so I don't have to return an error for incorrect types.  I guess this would be the same as asking for typesets to be allowed in normal interfaces like:

func Foo(arg allowedTypes) {...}

Is there another way to achieve this? Is there a reason it is not possible?

Thanks!

Matt

[David Finkel]

The type-parameters proposal discusses this a little.

You can cast your arg to an interface-type before type-asserting.

https://go.googlesource.com/proposal/+/HEAD/design/43651-type-parameters.md#identifying-the-matched-predeclared-type

Note that the design uses the interface{} syntax, while you can now use "any" when converting to an interface.

Worth noting: you'll still have a runtime cost to the type-switch.

 

Thanks!

Matt

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[roger peppe]

See https://github.com/golang/go/issues/45380 for a proposal for a language feature that would allow this. In the meantime, you can convert to any, and type switch on that:

func Foo[T allowedTypes](arg T) {

    switch t := any(arg).(type) {

    case int64:

...