Why can I type alias functions and use them without casting?

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Turns out, this is a misunderstanding that I had about how Go dealt with types, which can be resolved by reading the relevant part of the spec:

http://golang.org/ref/spec#Type_identity

The relevant distinction that I was unaware of was that of named and unnamed types.

Named types are types with a name, such as int, int64, float, string, bool. In addition, any type you create using ‘type’ is a named type.

Unnamed types are those such as []string, map[string]string, [4]int. They have no name, simply a description corresponding to how they are to be structured.

If you compare two named types, the names must match in order for them to be interchangeable. If you compare a named and an unnamed type, then as long as the underlying representation matches, you’re good to go!

e.g. given the following types:

type MyInt int
type MyMap map[int]int
type MySlice []int
type MyFunc func(int)

the following is invalid:

var i int = 2
var i2 MyInt = 4
i = i2 //both named (int and MyInt) and names don't match, so invalid

the following is fine:

is := make([]int)
m := make(map[int]int)
f := func(i int){}

//OK: comparing named and unnamed type, and underlying representation
//is the same:
func doSlice(input MySlice){...}
doSlice(is)

func doMap(input MyMap){...}
doMap(m)

func doFunc(input MyFunc){...}
doFunc(f)

I’m a bit gutted I didn’t know that sooner, so I hope that clarifies the type lark a little for someone else! And means much less casting than I at first thought 🙂

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