How to use switch type on value of reflect.TypeOf(value)

Question

How is it possible to use a switch type on the value of reflect.TypeOf(value)

import (
    "fmt"
    "reflect"
)

func main() {
    var value int = 123
    t := reflect.TypeOf(value)
    switch tv := t.(type) {
    case int:
        fmt.Println("type: INT")
    }
    fmt.Println(t)
}

Answer 1

t's type is the reflect.Type interface type, and the concrete type stored in it is always the unexported *reflect.rtype pointer type descriptor, so there's no sense type-switching on that.

Use a switch statement and compare to other reflect.Type values like this:

for _, v := range []any{
    int(123), "foo", float64(1.0),
} {
    t := reflect.TypeOf(v)

    switch t {
    case reflect.TypeOf(int(0)):
        fmt.Println("type: INT")
    case reflect.TypeOf(""):
        fmt.Println("type: STRING")
    default:
        fmt.Println("type: other:", t)
    }
}

This will output (try it on the Go Playground):

type: INT
type: STRING
type: other: float64

If you have to run this many times, you can cache the type descriptors you're interested in:

var (
    typeInt    = reflect.TypeOf(int(0))
    typeString = reflect.TypeOf("")
)

Then the switch:

switch t {
case typeInt:
    fmt.Println("type: INT")
case typeString:
    fmt.Println("type: STRING")
default:
    fmt.Println("type: other:", t)
}

This outputs the same. Try this one on the Go Playground.