packer: Clean up some of the JSON syntax stuff

CHANGELOG.md

@@ -1,6 +1,8 @@
 ## 0.1.4 (unreleased)
 
+IMPROVEMENTS:
 
+* core: Template syntax errors now show line and character number. [GH-56]
 
 ## 0.1.3 (July 1, 2013)
 

packer/template.go

@@ -57,29 +57,6 @@ type rawProvisionerConfig struct {
 	rawConfig interface{}
 }
 
-// displaySyntaxError returns a location for the json syntax error
-// Adapted from:
-// https://groups.google.com/forum/#!topic/golang-nuts/fizimmXtVfc
-func displaySyntaxError(js []byte, syntaxError error) (err error) {
-	syntax, ok := syntaxError.(*json.SyntaxError)
-	if !ok {
-		err = syntaxError
-		return
-	}
-	newline := []byte{'\x0a'}
-	space := []byte{' '}
-
-	start, end := bytes.LastIndex(js[:syntax.Offset], newline)+1, len(js)
-	if idx := bytes.Index(js[start:], newline); idx >= 0 {
-		end = start + idx
-	}
-	
-	line, pos := bytes.Count(js[:start], newline)+1, int(syntax.Offset) - start - 1
-	
-	err = fmt.Errorf("\nError in line %d: %s \n%s\n%s^", line, syntaxError, js[start:end], bytes.Repeat(space, pos))
-	return
-}
-
 // ParseTemplate takes a byte slice and parses a Template from it, returning
 // the template and possibly errors while loading the template. The error
 // could potentially be a MultiError, representing multiple errors. Knowing
@@ -89,7 +66,27 @@ func ParseTemplate(data []byte) (t *Template, err error) {
 	var rawTpl rawTemplate
 	err = json.Unmarshal(data, &rawTpl)
 	if err != nil {
-		err = displaySyntaxError(data, err)
+		syntaxErr, ok := err.(*json.SyntaxError)
+		if !ok {
+			return
+		}
+
+		// We have a syntax error. Extract out the line number and friends.
+		// https://groups.google.com/forum/#!topic/golang-nuts/fizimmXtVfc
+		newline := []byte{'\x0a'}
+
+		start := bytes.LastIndex(data[:syntaxErr.Offset], newline)+1
+		end := len(data)
+		if idx := bytes.Index(data[start:], newline); idx >= 0 {
+			end = start + idx
+		}
+
+		line := bytes.Count(data[:start], newline)+1
+		pos := int(syntaxErr.Offset) - start - 1
+
+		err = fmt.Errorf("Error in line %d, char %d: %s\n%s",
+			line, pos, syntaxErr, data[start:end])
+
 		return
 	}