Go programming language is easy to learn, but there are some tricky traps. This article series is trying to show these booby traps so that you avoid them.

Type assertions might be a bit tricky sometimes:

package main

import "fmt"

func main() {
	var data interface{} = "string"
	if cast, ok := data.(int); ok {
		fmt.Printf("%v is an int\n", cast)
	} else {
		fmt.Printf("%v is not an int\n", cast)
	}
}

On the Playground

When we run this, we get:

$ go run broken.go
0 is not an int

Why is that? How could we fix it?

Explanation

Expression cast, ok := data.(int) performs a type assertion. Which means:

• We try to cast data into an int
• If it works, cast is set with the int value and ok is true
• If it doesn’t work, cast is set with int zero value and ok is false

Thus, as data is not an integer, ok is false and cast is set with 0 and it is not set with string value of data.

We can fix performing another cast, as follows:

package main

import "fmt"

func main() {
    var data interface{} = "string"
    if cast, ok := data.(int); ok {
        fmt.Printf("%v is an int\n", cast)
    } else {
		str := data.(string)
        fmt.Printf("%v is not an int\n", str)
    }
}

On the Playground

Enjoy!