类型开关未检测到float32

Question

package main

import "fmt"

func main() {
    printType(4.0)
}

func printType(i interface{}) {
    switch i := i.(type) {
    case float32:
        fmt.Println("This is a float type", i)
    }
}

For some reason this code does not detect 4.0 value as float32 but it detects it as float64 - why is that? I run it on win x64 machine.

Answer 1

For some reason this code does not detect 4.0 value as float32 but it detects it as float64 - why is that?

Because float64 is the default type for an untyped floating-point constant. Relevant part emphasized:

An untyped constant has a default type which is the type to which the constant is implicitly converted in contexts where a typed value is required, for instance, in a short variable declaration such as i := 0 where there is no explicit type. The default type of an untyped constant is bool, rune, int, float64, complex128 or string respectively, depending on whether it is a boolean, rune, integer, floating-point, complex, or string constant.