In Go, why can we use *os.File as parameter in bufio.NewScanner when the definition says it should only accept io.Reader?

In the Go language, when we define a function or method to receive a parameter type of io.Reader, it can actually accept any type that implements the io.Reader interface. Therefore, when we use os.File as a parameter in bufio.NewScanner, it is because the os.File type implements the io.Reader interface. This means that the os.File type can be passed as a parameter to a function or method that accepts the io.Reader type, and the io.Reader method can be used inside the function or method to operate on os.File. This flexibility is a reflection of the interface characteristics of the Go language, making the code more concise and reusable. So, even if the parameter type is declared as io.Reader, we can still use os.File as parameter passed to bufio.NewScanner.

Question content

Try to learn go and keep using bufio.newscanner to read the file content. I use the following code to do this:

input_file, err := os.Open("input.txt")

if err != nil {
    panic(err)
}

scanner := bufio.NewScanner(input_file)
//do stuff

Thought I would look at the definition and see something weird (at least to me), the os.open("input.txt") above actually returns an * os.file and bufio.newscanner expect a io. reader as parameter. reader is an interface, while file is a struct, which doesn't implement that interface or anything like that (if that's possible).

But it looks like this is totally fine. Am I missing something about how go works? I have a c# background and it seems to me that the parameters are of different types so the compiler shouldn't allow this, right?

Just curious, don't know where else to ask this question.

Solution

os.file is actually implementing the io.reader interface.

This means that it implements all methods provided by the io.reader interface with the same signature.

In this particular case, this method:

func (f *File) Read(b []byte) (n int, err error)

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