In Go, if you define a new type e.g.:
type MyInt int
You can't then pass a MyInt to a function expecting an int, or vice versa:
func test(i MyInt) {
//do something with i
}
func main() {
anInt := 0
test(anInt) //doesn't work, int is not of type MyInt
}
Fine. But why is it then that the same does not apply to functions? e.g.:
type MyFunc func(i int)
func (m MyFunc) Run(i int) {
m(i)
}
func run(f MyFunc, i int) {
f.Run(i)
}
func main() {
var newfunc func(int) //explicit declaration
newfunc = func(i int) {
fmt.Println(i)
}
run(newfunc, 10) //works just fine, even though types seem to differ
}
Now, I'm not complaining because it saves me having to explicitly cast newfunc to type MyFunc, as I would have to do in the first example; it just seems inconsistent. I'm sure there is a good reason for it; can anyone enlighten me?
The reason I ask is mainly because I would like to shorten some of my rather long function types in this way, but I want to make sure it's expected and acceptable to do this :)
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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