When you write:
f, err := os.OpenFile(filename, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
the types of 'f' and 'err' are defined statically and magically as whatever
the declared return types of os.OpenFile are. It's like you wrote:
var f type1
var err type2
f, err = os.OpenFile(...)
where type1 and type2 were replaced by looking at the function definition
of os.OpenFile.
"return f, err" will then assign those values to the return types *your*
function declared - which are *os.File and error respectively. So if "f"
or "err" are of the wrong type, you'll get a compile-time error, meaning
you can't even build your program, let alone run it. Example:
https://play.golang.org/p/IOLMmMUGjCL
Therefore, there's no point checking the type of "f", since the compiler
has already done this.
The interesting one is "err". You are returning type "error", which is an
interface type. This means that "err" can have any concrete type which
satisfies that interface, and it will work. Example:
https://play.golang.org/p/Hdb9smP9hYh
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