How to do it...

The following steps cover how to write and run your application:

1. From your Terminal or console application, create a new directory called ~/projects/go-programming-cookbook/chapter1/interfaces.
2. Navigate to this directory.
3. Run the following command:

$ go mod init github.com/PacktPublishing/Go-Programming-Cookbook-Second-Edition/chapter1/interfaces

You should see a file called go.mod that contains the following:

module github.com/PacktPublishing/Go-Programming-Cookbook-Second-Edition/chapter1/interfaces    

4. Copy tests from ~/projects/go-programming-cookbook-original/chapter1/interfaces or use this as an exercise to write some of your own code!

5. Create a file called interfaces.go with the following contents:

        package interfaces

        import (
                "fmt"
                "io"
                "os"
        )

        // Copy copies data from in to out first directly,
        // then using a buffer. It also writes to stdout
        func Copy(in io.ReadSeeker, out io.Writer) error {
                // we write to out, but also Stdout
                w := io.MultiWriter(out, os.Stdout)
        
                // a standard copy, this can be dangerous if there's a 
                // lot of data in in
                if _, err := io.Copy(w, in); err != nil {
                    return err
                }
        
                in.Seek(0, 0)

                // buffered write using 64 byte chunks
                buf := make([]byte, 64)
                if _, err := io.CopyBuffer(w, in, buf); err != nil {
                    return err
                }

                // lets print a new line
                fmt.Println()
        
                return nil
        }

6. Create a file called pipes.go with the following contents:

        package interfaces

        import (
                "io"
                "os"
        )
        
        // PipeExample helps give some more examples of using io  
        //interfaces
        func PipeExample() error {
                // the pipe reader and pipe writer implement
                // io.Reader and io.Writer
                r, w := io.Pipe()

                // this needs to be run in a separate go routine
                // as it will block waiting for the reader
                // close at the end for cleanup
                go func() {
                    // for now we'll write something basic,
                    // this could also be used to encode json
                    // base64 encode, etc.
                    w.Write([]byte("test\n"))
                    w.Close()
                }()
        
                if _, err := io.Copy(os.Stdout, r); err != nil {
                    return err
                }
                return nil
        }

7. Create a new directory named example and navigate to it.
8. Create a main.go file with the following contents:

        package main

        import (
             "bytes"
             "fmt"
        
             "github.com/PacktPublishing/
              Go-Programming-Cookbook-Second-Edition/
              chapter1/bytestrings"
        )
        
        func main() {
                in := bytes.NewReader([]byte("example"))
                out := &bytes.Buffer{}
                fmt.Print("stdout on Copy = ")
                if err := interfaces.Copy(in, out); err != nil {
                        panic(err)
                }
        
                fmt.Println("out bytes buffer =", out.String())
        
                fmt.Print("stdout on PipeExample = ")
                if err := interfaces.PipeExample(); err != nil {
                        panic(err)
                }
        }

9. Run go run ..

10. You may also run the following:

$ go build 
$ ./example

You should see the following output:

$ go run .
stdout on Copy = exampleexample
out bytes buffer = exampleexample
stdout on PipeExample = test

11. If you copied or wrote your own tests, go up one directory and run go test, and ensure that all tests pass.