title: What "number of bytes written" should my io.Writer return when it intentionally does not write all the bytes it is passed
body: <p>I want to create a struct that implements <code>io.Writer</code>, ie <code>Write(p []byte) (n int, err error)</code>. The struct will omit certain bytes that is passed to it and thus not actually write all the bytes that is passed in the <code>p</code> parameter. (The actual purpose isn't important, but it could for example be to ignore the first 10 bytes written, ignore every second byte or something else that alters the number of bytes written from what the caller passed to the method).</p>
  
  <p><a href="http://golang.org/pkg/io/#Writer" rel="nofollow">The documentation</a> for <code>io.Writer</code>'s <code>Write</code> method says the following (emphasis mine):</p>
  
  <blockquote>
    <p>Write writes len(p) bytes from p to the underlying data stream. It
    returns the number of bytes written from p (0 &lt;= n &lt;= len(p)) and any
    error encountered that caused the write to stop early. <strong>Write must
    return a non-nil error if it returns n &lt; len(p)</strong>. Write must not modify
    the slice data, even temporarily.</p>
  </blockquote>
  
  <p>What should I return as the <code>n</code> value? To comply with the documentation, on success I should return <code>len(p)</code>. However, the method might actually not have written that many bytes, and the return value will be "wrong".</p>