The io Reader interface

The io.Reader interface is a standard Go interface that defines a single method, Read, used to read data from a source sequentially. It is implemented by types like *tar.Reader to allow streaming access to tar archive contents without loading the entire file into memory.

import "io"

type Reader interface {
    Read(p []byte) (n int, err error)
}

The Read method copies data from the source into the provided byte slice p, returning the number of bytes read and any error encountered, such as io.EOF when the stream ends.

The io.Reader interface is a universal contract in Go that allows any data source to be read byte-by-byte. It matters because it lets you write generic code that can handle files, network connections, or archives like tar files in the same way. Think of it like a universal hose nozzle that fits any water source, letting you drain the water without needing to know where it came from.