publish errors

examples.txt

@@ -19,7 +19,7 @@ Structs
 Methods
 Interfaces
 # Embedding
-# Errors
+Errors
 Goroutines
 Channels
 Channel Buffering

examples/errors.sh

@@ -0,0 +1,10 @@
+$ go run errors.go
+f1 worked: 10
+f1 failed: can't work with 42
+f2 worked: 10
+f2 failed: 42 - can't work with it
+42
+can't work with it
+
+# See this [great post](http://blog.golang.org/2011/07/error-handling-and-go.html)
+# on the Go blog for more on error handling in Go.

examples/errors/errors.go

@@ -1,26 +1,85 @@
-// Errors by return value...
+// In Go it's idiomatic to communicate errors via an
+// explicit, separate return value. This contrasts with
+// the exceptions used in languages like Java and Ruby and
+// the overloaded single result / error value sometimes
+// used in C. Go's approach makes it easy to see which
+// functions return errors and to handle them using the
+// same language constructs employed for any other,
+// non-error tasks.
 
 package main
 
 import "errors"
 import "fmt"
 
-func myFun(arg int) (int, error) {
+// By convention, errors are the last return value and
+// have type `error`, a built-in interface.
+func f1(arg int) (int, error) {
     if arg == 42 {
+
+        // `errors.New` constructs a basic `error` value
+        // with the given error message.
         return -1, errors.New("can't work with 42")
 
     }
+
+    // A nil value in the error position indicates that
+    // there was no error.
+    return arg + 3, nil
+}
+
+// It's possible to use custom types as `error`s by
+// implementing the `Error()` method on them. Here's a
+// variant on the example above that uses a custom type
+// to explicitly represent an argument error.
+type argError struct {
+    arg  int
+    prob string
+}
+
+func (e *argError) Error() string {
+    return fmt.Sprintf("%d - %s", e.arg, e.prob)
+}
+
+func f2(arg int) (int, error) {
+    if arg == 42 {
+
+        // In this case we use `&argError` syntax to build
+        // a new struct, supplying values for the two
+        // fields `arg` and `prob`.
+        return -1, &argError{arg, "can't work with it"}
+    }
     return arg + 3, nil
 }
 
 func main() {
-    r, _ := myFun(7)
-    fmt.Println(r)
 
-    _, e := myFun(42)
-    fmt.Println(e)
+    // The two loops below test out each of our
+    // error-returning functions. Note that the use of an
+    // inline error check on the `if` line is a common
+    // idiom in Go code.
+    for _, i := range []int{7, 42} {
+        if r, e := f1(i); e != nil {
+            fmt.Println("f1 failed:", e)
+        } else {
+            fmt.Println("f1 worked:", r)
+        }
+    }
+    for _, i := range []int{7, 42} {
+        if r, e := f2(i); e != nil {
+            fmt.Println("f2 failed:", e)
+        } else {
+            fmt.Println("f2 worked:", r)
+        }
+    }
+
+    // If you want to programmatically use the data in
+    // a custom error, you'll need to get the error  as an
+    // instance of the custom error type via type
+    // assertion.
+    _, e := f2(42)
+    if ae, ok := e.(*argError); ok {
+        fmt.Println(ae.arg)
+        fmt.Println(ae.prob)
+    }
 }
-
-// todo: custom errors
-
-// todo: data conveying errors