Converting a custom type to string in Go

In this bizarre example, someone has created a new type which is really just a string:

type CustomType string
const (
        Foobar CustomType = "somestring"
)
func SomeFunction() string {
        return Foobar
}

However, this code fails to compile:

> cannot use Foobar (type CustomType) as type string in return argument

How would you fix SomeFunction so that it is able to return the string value of Foobar ("somestring") ?

Convert the value to a string:

func SomeFunction() string {
        return string(Foobar)
}

Better to define a String function for the Customtype - it can make your life easier over time - you have better control over things as and if the structure evolves. If you really need SomeFunction then let it return Foobar.String()

   package main
    
    import (
    	"fmt"
    )
    
    type CustomType string
    
    const (
    	Foobar CustomType = "somestring"
    )
    
    func main() {
    	fmt.Println("Hello, playground", Foobar)
    	fmt.Printf("%s", Foobar)
    	fmt.Println("\n\n")
    	fmt.Println(SomeFunction())
    }
    
    func (c CustomType) String() string {
    	fmt.Println("Executing String() for CustomType!")
    	return string(c)
    }
    
    func SomeFunction() string {
    	return Foobar.String()
    }

https://play.golang.org/p/jMKMcQjQj3

> For every type T, there is a corresponding conversion operation T(x)
> that converts the value x to type T. A conversion from one type to
> another is allowed if both have the same underlying type, or if both
> are unnamed pointer types that point to variables of the same
> underlying type; these conversions change the type but not the
> representation of the value. If x is assignable to T, a conversion
> is permitted but is usually redundant. - Taken from The Go
> Programming Language - by Alan A. A. Donovan

As per your example here are some of the different examples which will return the value.

package main
import "fmt"
type CustomType string
const (
	Foobar CustomType = "somestring"
)
func SomeFunction() CustomType {
	return Foobar
}
func SomeOtherFunction() string {
	return string(Foobar)
}
func SomeOtherFunction2() CustomType {
	return CustomType("somestring") // Here value is a static string.
}
func main() {
	fmt.Println(SomeFunction())
	fmt.Println(SomeOtherFunction())
	fmt.Println(SomeOtherFunction2())
}

It will output:

somestring
somestring
somestring

The Go Playground link

You can convert like this:

var i int = 42
var f float64 = float64(i)

check here

you can return like this:

return string(Foobar)