feat: JSON output for the `list` sub-command

cmd/list.go

@@ -1,16 +1,44 @@
 package cmd
 
 import (
+	"encoding/json"
 	"fmt"
+	"gopkg.in/yaml.v2"
+	"io"
+	"os"
 
 	"github.com/boson-project/faas"
 	"github.com/boson-project/faas/knative"
-	"github.com/ory/viper"
 	"github.com/spf13/cobra"
 )
 
+var formats = map[string]fmtFn{
+	"plain": fmtPlain,
+	"json":  fmtJSON,
+	"yaml":  fmtYAML,
+}
+
+var validFormats []string
+
+func completeFormats(cmd *cobra.Command, args []string, toComplete string) (formats []string, directive cobra.ShellCompDirective) {
+	formats = validFormats
+	directive = cobra.ShellCompDirectiveDefault
+	return
+}
+
 func init() {
 	root.AddCommand(listCmd)
+
+	validFormats = make([]string, 0, len(formats))
+	for name := range formats {
+		validFormats = append(validFormats, name)
+	}
+
+	listCmd.Flags().StringP("output", "o", "plain", "optionally specify output format (plain,json,yaml)")
+	err := listCmd.RegisterFlagCompletionFunc("output", completeFormats)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+	}
 }
 
 var listCmd = &cobra.Command{
@@ -21,8 +49,39 @@ var listCmd = &cobra.Command{
 	RunE:       list,
 }
 
+type fmtFn func(writer io.Writer, names []string) error
+
+func fmtPlain(writer io.Writer, names []string) error {
+	for _, name := range names {
+		_, err := fmt.Fprintf(writer, "%s\n", name)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func fmtJSON(writer io.Writer, names []string) error {
+	encoder := json.NewEncoder(writer)
+	return encoder.Encode(names)
+}
+
+func fmtYAML(writer io.Writer, names []string) error {
+	encoder := yaml.NewEncoder(writer)
+	return encoder.Encode(names)
+}
+
 func list(cmd *cobra.Command, args []string) (err error) {
-	verbose := viper.GetBool("verbose")
+
+	verbose, err := cmd.Flags().GetBool("verbose")
+	if err != nil {
+		return
+	}
+
+	format, err := cmd.Flags().GetString("output")
+	if err != nil {
+		return
+	}
 
 	lister, err := knative.NewLister(faas.DefaultNamespace)
 	if err != nil {
@@ -42,8 +101,11 @@ func list(cmd *cobra.Command, args []string) (err error) {
 	if err != nil {
 		return
 	}
-	for _, name := range names {
-		fmt.Printf("%s\n", name)
+
+	fmtFn, ok := formats[format]
+	if !ok {
+		return fmt.Errorf("invalid format name: %s", format)
 	}
-	return
+
+	return fmtFn(os.Stdout, names)
 }