README: interface demo

_demo/errors/errors.go

@@ -0,0 +1,22 @@
+package main
+
+// New returns an error that formats as the given text.
+// Each call to New returns a distinct error value even if the text is identical.
+func New(text string) error {
+	return &errorString{text}
+}
+
+// errorString is a trivial implementation of error.
+type errorString struct {
+	s string
+}
+
+func (e *errorString) Error() string {
+	return e.s
+}
+
+func main() {
+	err := New("an error")
+	println(err)
+	println(err.Error())
+}