Tour of Go exercise #22: Reader, what does the question mean?

Exercise: Readers

Implement a Reader type that emits an infinite stream of the ASCII character 'A'.

I don't understand the question, how to emit character 'A'? into which variable should I set that character?

Here's what I tried:

package main

import "golang.org/x/tour/reader"

type MyReader struct{}

// TODO: Add a Read([]byte) (int, error) method to MyReader.

func main() {
    reader.Validate(MyReader{}) // what did this function expect?
}

func (m MyReader) Read(b []byte) (i int, e error) {
	b = append(b, 'A') // this is wrong..
	return 1, nil      // this is also wrong..
}

Ah I understand XD

I think it would be better to say: "rewrite all values in []byte into 'A's"

package main

import "golang.org/x/tour/reader"

type MyReader struct{}

// TODO: Add a Read([]byte) (int, error) method to MyReader.
func (m MyReader) Read(b []byte) (i int, e error) {
	for x := range b {
		b[x] = 'A'
	}
	return len(b), nil
}

func main() {
	reader.Validate(MyReader{})
}

An io.Reader.Read role is to write a given memory location with data read from its source.

To implement a stream of 'A', the function must write given memory location with 'A' values.

It is not required to fill in the entire slice provided in input, it can decide how many bytes of the input slice is written (Read reads up to len(p) bytes into p), it must return that number to indicate to the consumer the length of data to process.

By convention an io.Reader indicates its end by returning an io.EOF error. If the reader does not return an error, it behaves as an infinite source of data to its consumer which can never detect an exit condition.

Note that a call to Read that returns 0 bytes read can happen and does not indicate anything particular, Callers should treat a return of 0 and nil as indicating that nothing happened; Which makes this non-solution https://play.golang.org/p/aiUyc4UDYi2 fails with a timeout.

In regard to that, the solution provided here https://stackoverflow.com/a/68077578/4466350 return copy(b, "A"), nil is really just right. It writes the minimum required, with an elegant use of built-ins and syntax facilities, and it never returns an error.

Simplest one:

func (s MyReader) Read(b []byte) (int, error) {
	b[0] = byte('A')
	return 1, nil
}

You can generalize the idea to create an eternal reader, alwaysReader, from which you always read the same byte value over and over (it never results in EOF):

package readers

type alwaysReader struct {
	value byte
}

func (r alwaysReader) Read(p []byte) (n int, err error) {
	for i := range p {
		p[i] = r.value
	}
	return len(p), nil
}

func NewAlwaysReader(value byte) alwaysReader {
	return alwaysReader { value }
}

NewAlwaysReader() is the constructor for alwaysReader (which isn't exported). The result of NewAlwaysReader('A') is a reader from whom you will always read 'A'.

A clarifying unit test for alwaysReader:

package readers_test

import (
	"bytes"
	"io"
	"readers"
	"testing"
)

func TestEmptyReader(t *testing.T) {
	const numBytes = 128
	const value = 'A'

	buf := bytes.NewBuffer(make([]byte, 0, numBytes))
	reader := io.LimitReader(readers.NewAlwaysReader(value), numBytes)

	n, err := io.Copy(buf, reader)
	if err != nil {
		t.Fatal("copy failed: %w")
	}
	if n != numBytes {
		t.Errorf("%d bytes read but %d expected", n, numBytes)
	}
	for i, elem := range buf.Bytes() {
		if elem != value {
			t.Errorf("byte at position %d has not the value %v but %v", i, value, elem)
		}
	}
}

Since we can read from the alwaysReader forever, we need to decorate it with a io.LimitReader so that we end up reading at most numBytes from it. Otherwise, the bytes.Buffer will eventually run out of memory for reallocating its internal buffer because of io.Copy().

Note that the following implementation of Read() for alwaysReader is also valid:

func (r alwaysReader) Read(p []byte) (n int, err error) {
	if len(p) > 0 {
		p[0] = r.value
		return 1, nil
	}
	return 0, nil
}

The former Read() implementation fills the whole byte slice with the byte value, whereas the latter writes a single byte.

The alleged answer is didn't work for me, even without the typos.
Try as I did, that string would not go into b.

func (r MyReader) Read(b []byte) (int,  error) {
	return copy(b, "A"), nil
}

My solution: just add one byte at a time, store the index i using closure.

package main

import (
	"golang.org/x/tour/reader"
)

type MyReader struct{}

func (mr MyReader) Read(b []byte) (int, error) {
	i := 0
	p := func () int {
		b[i] = 'A'
		i += 1
		
		return i
	}
	
	
	return p(), nil
}

func main() {
	reader.Validate(MyReader{})
}