Directly assign map[string]int to map[string]interface{}?

[Glen Huang]

Hi,

I have a function that accepts an argument of type map[string]interface{}, and I also have a value of  type map[string]int.

Currently it seems I can't directly pass the value to the function. Is there anyway I can directly coerce it or a new value of the exact matching type must be created?

I find it quite surprising that you can directly assign any variables to interface{} but not when they are both "scoped inside" a map or a slice. Is the asymmetry by design? 

Regards

[Glen Huang]

Another slightly related topic:

switch v := i.(type) {
case map[string]interface{}, map[string]int:
    fmt.Print(v)

}

Ideally v is of type map[string]interface{}, but it's interface{} instead.

[Axel Wagner]

Hi,

I've wrote about the theoretical side of that here:

https://blog.merovius.de/2018/06/03/why-doesnt-go-have-variance-in.html

The post uses slices, but the same arguments apply to maps as well. Specifically, while it *may* seem like you should be able to use a map[string]int as a map[string]interface{}, if you *could*, you would also have to explain what happens when you write something to it that is not an int. i.e. how would this code behave: https://play.golang.org/p/StJrbhmjCHh

There really is no good answer to this. So as long as the type allows both reading and writing, making map variant really isn't possible.

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[Dan Kortschak]

Have a read of https://research.swtch.com/interfaces. There you'll see
that the memory layout of int and interface{} are not the same. This
means you can't just treat one as the other, which essentially is what
you are asking for.

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[Dan Kortschak]

Go is a statically typed language, but the time you get into the case,
you must know the concrete type of the v. You allows it to be either
map[string]interface{} or map[string]int, this is not a single known
type, so the original input type (interface{}) is used.

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[Glen Huang]

This is a great write-up! Going from explaining the terms to applying the terms to the actual problems.

I learned a lot. Thanks.

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[Volker Dobler]

It boils down to the meaning of interface{}.

interface{} means interface{} and _not_ "any type".

While you can assign anything to a variable of type interface{}

this does not mean that a variable of type interface{} _is_

"any type". There is no "any type" type in Go.

V.