go语言中log包的使用

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package main   import (     "github.com/robertkrimen/otto"     "log" )   func main() {     log.Printf("Creating JS interpreter")     js := otto.New()       var function otto.Value       log.Printf("Defining setEnrichFunction")     js.Set("setEnrichFunction", func(call otto.FunctionCall) otto.Value {         function = call.Argument(0)         if class := function.Class(); class != "Function" {             log.Fatalf("setEnrichFunction: expected Function, got %s instead.", class)         }         return otto.UndefinedValue()     })       log.Printf("Registering enrich function")     js.Run(`         setEnrichFunction(function(data) {             data.timestamp = new Date().toUTCString();         });     `)       data := map[string]string{         "foo": "bar",         "theAnswer": "42",     }       log.Printf("raw data: %#v", data)       arg, err := js.ToValue(data)     if err != nil {         log.Fatalf("couldn't convert message to JS value")     }       log.Printf("Calling enrich function")     _, err = function.Call(otto.NullValue(), arg)     if err != nil {         log.Fatalf("calling enrich function failed: %v", err)     }       log.Printf("enriched data: %#v", data) }