Name Your Interface Arguments

Consider the following interface:

type Copier interface {
  Copy(string, string) int
}

Based on the code alone, can you deduce what kinds of strings you should pass into the Copy function?

We know the function signature expects 2 string types, but what are they? Filenames? URLs? Raw string data? For that matter, what the heck is that int that's being returned?

Let's add some named arguments and return data to make it more clear.

type Copier interface {
  Copy(sourceFile string, destinationFile string) (bytesCopied int)
}

Much better. We can see what the expectations are now. The first argument is the sourceFile, the second argument is the destinationFile, and bytesCopied, an integer, is returned.