Implement Read like Reader interface, you can get inspiration from buffer.go

ddata.go

package main

import (
    "io"
    "fmt"

)

type Data struct {
    stream  string
    idx     int
}

func NewData(stream string) *Data {
    return &Data{stream, 0}
}

func (d *Data) Len() int {
    return len(d.stream)
}

// Implement Read interface
// Lowercase to uppercase
// The return value n is the number of bytes read.
// If no data to return, err is io.EOF (unless len(p) is zero);
// otherwise it is nil.)
func (d *Data) Read(s []byte) (n int, err error) {
    for ; d.idx < d.Len() && n < len(s); d.idx ++ {
        chr := d.stream[d.idx]
        if 'a' <= chr && chr <= 'z' {
            s[n] = chr + 'A' - 'a'
        } else {
            s[n] = chr
        }
        n++
    }
    if n == 0 {
        return n, io.EOF
    }
    return n, nil
}

func main() {
    d := NewData("hello world")
    buf := make([]byte, d.Len())

    n, err := io.ReadFull(d, buf)
    fmt.Printf("%s\n", buf)
    fmt.Println(n, err)
}