Hands-On System Programming with Go

Defining a reader

Any Go application can define a custom implementation of the io.Reader interface. A good general rule when implementing interfaces is to accept interfaces and return concrete types, avoiding unnecessary abstraction.

Let's look at a practical example. We want to implement a custom reader that takes the content from another reader and transforms it into uppercase; we could call this AngryReader, for instance:

func NewAngryReader(r io.Reader) *AngryReader {    return &AngryReader{r: r}}type AngryReader struct {    r io.Reader}func (a *AngryReader) Read(b []byte) (int, error) {    n, err := a.r.Read(b)    for r, i, w := rune(0), 0, 0; i < n; i += w {        // read a rune        r, w = utf8.DecodeRune(b[i:])        // skip if not a letter if !unicode.IsLetter(r) ...