Merge pull request #58 from tsingbx/error

Add doc for error interface

Author: xushiwei

217-Interface-Values/interface-values-3.gop

@@ -10,12 +10,12 @@ import (
 
 var w io.Writer
 
-fmt.printf "%T\n", w // "<nil>"
+fmt.printf("%T\n", w) // "<nil>"
 
 w = os.Stdout
-fmt.printf "%T\n", w // "*os.File"
+fmt.printf("%T\n", w) // "*os.File"
 
 w = new(bytes.Buffer)
-fmt.printf "%T\n", w // "*bytes.Buffer"
+fmt.printf("%T\n", w) // "*bytes.Buffer"
 
 // Internally, fmt uses reflection to obtain the name of the int erface’s dynamic type.

218-The-error-Interface/error-1.gop

@@ -0,0 +1,5 @@
+// error interface has a single method that returns an error message:
+
+type error interface {
+	Error() string
+}

218-The-error-Interface/error-2.gop

@@ -0,0 +1,20 @@
+// The simplest way to create an error is by calling errors.New, which returns a new error for
+// a given error message. The entire errors package is only four lines long:
+
+package errors
+
+func New(text string) error {
+	return &errorString{text}
+}
+
+type errorString struct {
+	text string
+}
+
+func (e *errorString) Error() string {
+	return e.text
+}
+
+// The underlying type of errorString is a struct, not a string , to protect its representation from
+// inadvertent (or premeditated) updates. And the reason that the pointer type *errorString,
+// not errorString alone, satisfies the error interface is so that every call to New allocates a distinct error instance that is equal to no other.

218-The-error-Interface/error-3.gop

@@ -0,0 +1,8 @@
+// We would not want a distinguished error such as io.EOF to compare equal to one that merely happened to have the same message.
+
+import (
+	"errors"
+	"fmt"
+)
+
+fmt.Println(errors.New("EOF") == errors.New("EOF")) // "false"

218-The-error-Interface/error-4.gop

@@ -0,0 +1,14 @@
+// Calls to errors.New are relatively infrequent because there’s a convenient wrapper function,
+// fmt.Errorf, that does string formatting too.
+
+package fmt
+
+import "errors"
+
+func Sprintf(format string, args ...interface{}) (s string) {
+	//...
+}
+
+func Errorf(format string, args ...interface{}) error {
+	return errors.New(Sprintf(format, args...))
+}