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package main
import (
"io"
"log"
"os"
"strings"
)
type WrappedReader struct {
reader io.Reader
}
func (wr WrappedReader) Read(p []byte) (n int, err error) {
return wr.reader.Read(p)
}
// Of course, this implementation is completely stupid because it does not write
// to the intended writer, as any honest WriteTo implementation should. its
// implemtion is just to make obvious the divergence of behaviour with yaegi.
func (wr WrappedReader) WriteTo(w io.Writer) (n int64, err error) {
// Ignore w, send to Stdout to prove whether this WriteTo is used.
data, err := io.ReadAll(wr)
if err != nil {
return 0, err
}
nn, err := os.Stdout.Write(data)
return int64(nn), err
}
func main() {
f := strings.NewReader("hello world")
wr := WrappedReader{reader: f}
// behind the scenes, io.Copy is supposed to use wr.WriteTo if the implementation exists.
// With Go, it works as expected, i.e. the output is sent to os.Stdout.
// With Yaegi, it doesn't, i.e. the output is sent to io.Discard.
if _, err := io.Copy(io.Discard, wr); err != nil {
log.Fatal(err)
}
}
// Output:
// hello world
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