assign a string type golang

I'm having a hard time assigned a string to a type string. So I have this type:

type Username string

I then have a function that returns a string

What I'm trying to do is set username to that returned string:

u := &Username{returnedString}

I also tried

var u Username
u = returnedString

But I always get an error.

As others have pointed out, you'll need to do an explicit type conversion:

someString := funcThatReturnsString()
u := Username(someString)

I'd recommend reading this article on implicit type conversion. Specifically, Honnef references Go's specifications on assignability:

> A value x is assignable to a variable of type T ("x is assignable to T") in any of these cases:
>
> * x's type V and T have identical underlying types and at least one of V or T is not a named type.

So in your example, the returnedString is already a "named type": it has type string. If you had instead done something like

var u Username
u = "some string"

you would have been ok, as the "some string" would be implicitly converted into type Username and both string and Username have the underlying type of string.

You've made a user-defined type "Username". You cannot assign a value of one type to another type (in this case a string to Username) without converting. Incidentally you can assign a string literal to it.

var u Username
u = "test"

Will work.

var u Username
	var s string

	s = "test"
	u = s

Will give you the error:
>cannot use s (type string) as type Username in assignment

You can convert from one type to another, so...:

var u Username
	var s string
	s = "test"
	u = Username(s)

...will also work. "u" will be of type Username with the value "test".

For the record these are not meant as idiomatic examples. I am simply trying to clearly illustrate what's going on with the types here.

Type casting in go is explained in the tutorial "A Tour of Go"

See here for an example using your types.