jsontoml: port to v2 (#726)

Fixes #719

cmd/tomljson/main.go

@@ -1,67 +1,29 @@
-// Tomljson reads TOML and converts to JSON.
+// Package tomljson is a program that converts TOML to JSON.
 //
 // Usage:
 //   cat file.toml | tomljson > file.json
-//   tomljson file1.toml > file.json
+//   tomljson file.toml > file.json
 package main
 
 import (
 	"encoding/json"
 	"errors"
-	"flag"
 	"fmt"
 	"io"
-	"os"
 
 	"github.com/pelletier/go-toml/v2"
+	"github.com/pelletier/go-toml/v2/internal/cli"
 )
 
-func usage() {
-	fmt.Fprint(os.Stderr, `tomljson can be used in two ways:
+func main() {
+	usage := `tomljson can be used in two ways:
 Reading from stdin:
   cat file.toml | tomljson > file.json
 
 Reading from a file:
   tomljson file.toml > file.json
-`)
-}
-
-func init() {
-	flag.Usage = usage
-}
-
-func main() {
-	flag.Parse()
-	os.Exit(processMain(flag.Args(), os.Stdin, os.Stdout, os.Stderr))
-}
-
-func processMain(files []string, input io.Reader, output, error io.Writer) int {
-	err := run(files, input, output)
-	if err != nil {
-		var derr *toml.DecodeError
-		if errors.As(err, &derr) {
-			fmt.Fprintln(error, derr.String())
-			row, col := derr.Position()
-			fmt.Fprintln(error, "error occurred at row", row, "column", col)
-		} else {
-			fmt.Fprintln(error, err.Error())
-		}
-		return -1
-	}
-	return 0
-}
-
-func run(files []string, input io.Reader, output io.Writer) error {
-	if len(files) > 0 {
-		f, err := os.Open(files[0])
-		if err != nil {
-			return err
-		}
-		defer f.Close()
-		input = f
-	}
-
-	return convert(input, output)
+`
+	cli.Execute(usage, convert)
 }
 
 func convert(r io.Reader, w io.Writer) error {
@@ -70,6 +32,11 @@ func convert(r io.Reader, w io.Writer) error {
 	d := toml.NewDecoder(r)
 	err := d.Decode(&v)
 	if err != nil {
+		var derr *toml.DecodeError
+		if errors.As(err, &derr) {
+			row, col := derr.Position()
+			return fmt.Errorf("%s\nerror occurred at row %d column %d", derr.String(), row, col)
+		}
 		return err
 	}