Use map[string]SpecificType with method of map[string]SomeInterface into

I get cannot use map[string]MyType literal (type map[string]MyType) as type map[string]IterableWithID in argument to MapToList with the code below, how do I pass in a concrete map type to method that expects a interface type?

https://play.golang.org/p/G7VzMwrRRw

Go's interface convention doesn't quite work the same way as in, say, Java (and the designers apparently didn't like the idea of getters and setters very much :-/ ). So you've got two core problems:

1. A map[string]Foo is not the same as a map[string]Bar, even if Bar implements Foo, so you have to break it out a bit (use make() beforehand, then assign in a single assignment).
2. Interface methods are called by value with no pointers, so you really need to do foo = foo.Method(bar) in your callers or get really pointer-happy to implement something like this.

What you can do to more-or-less simulate what you want:

type IterableWithID interface {
    SetID(id string) IterableWithID     // use as foo = foo.SetID(bar)
}

func (t MyType) SetID(id string) IterableWithID {
    t.ID = id
    return t
}

...and to deal with the typing problem

t := make(map[string]IterableWithID)
t["foo"] = MyType{}
MapToList(t)              // This is a map[string]IterableWithID, so compiler's happy. 

...and finally...

value = value.SetID(key)  // We set back the copy of the value we mutated

The final value= deals with the fact that the method gets a fresh copy of the value object, so the original would be untouched by your method (the change would simply vanish).

Updated code on the Go Playground

...but it's not particularly idiomatic Go--they really want you to just reference struct members rather than use Java-style mutators in interfaces (though TBH I'm not so keen on that little detail--mutators are supes handy to do validation).

You can't do what you want to do because the two map types are different. It doesn't matter that the element type of one is a type that implements the interface which is the element type of the other. The map type that you pass into the function has to be map[string]IterableWithID. You could create a map of that type, assign values of type MyType to the map, and pass that to the function.

See https://play.golang.org/p/NfsTlunHkW

Also, you probably don't want to be returning a pointer to a slice in MapToList. Just return the slice itself. A slice contains a reference to the underlying array.