Add customizable file extensions (#5)

* Always allow empty args in `IF` directive

ext.go

@@ -3,6 +3,7 @@ package difflint
 import (
 	"encoding/json"
 	"log"
+	"os"
 )
 
 var (
@@ -66,7 +67,12 @@ func NewExtMap(path string) *ExtMap {
 	if path != "" {
 		// Unmarshal the JSON file.
 		var extFile ExtFileJSON
-		if err := json.Unmarshal([]byte(path), &extFile); err != nil {
+		bytes, err := os.ReadFile(path)
+		if err != nil {
+			log.Fatalf("error reading JSON file %q: %v", path, err)
+		}
+
+		if err := json.Unmarshal(bytes, &extFile); err != nil {
 			log.Fatalf("error unmarshaling JSON file %q: %v", path, err)
 		}
 

lex.go

@@ -118,18 +118,17 @@ func parseRules(file string, tokens []token, ranges []Range) ([]Rule, error) {
 				return nil, errors.New("unexpected IF directive at " + file + ":" + string(rune(token.line)))
 			}
 
-			r.Hunk.File = file
-			r.Hunk.Range = Range{Start: token.line}
-
 			targets, err := parseTargets(parseTargetsOptions{
-				args:       token.args,
-				allowEmpty: r.ID != nil,
+				args:           token.args,
+				allowEmptyArgs: true,
 			})
 			if err != nil {
 				return nil, err
 			}
 
 			r.Targets = targets
+			r.Hunk.File = file
+			r.Hunk.Range = Range{Start: token.line}
 
 		case directiveEnd:
 			if r.Hunk.File == "" {
@@ -168,13 +167,13 @@ func parseRules(file string, tokens []token, ranges []Range) ([]Rule, error) {
 
 // parseTargets parses the given list of targets and returns the list of targets.
 type parseTargetsOptions struct {
-	args       []string
-	allowEmpty bool
+	args           []string
+	allowEmptyArgs bool
 }
 
 // parseTargets parses the given list of targets and returns the list of targets.
 func parseTargets(o parseTargetsOptions) ([]Target, error) {
-	if !o.allowEmpty && len(o.args) == 0 {
+	if !o.allowEmptyArgs && len(o.args) == 0 {
 		return nil, errors.New("missing target")
 	}
 

rules.go

@@ -56,7 +56,7 @@ func RulesMapFromHunks(hunks []Hunk, options LintOptions) (map[string][]Rule, ma
 	for pathname, ranges := range rangesMap {
 		rules, err := RulesFromFile(pathname, ranges, visited, &wg, options)
 		if err != nil {
-			return nil, nil, errors.Wrapf(err, "failed to parse rules for file %s", pathname)
+			return nil, nil, err
 		}
 
 		for _, rule := range rules {
@@ -71,6 +71,7 @@ func RulesMapFromHunks(hunks []Hunk, options LintOptions) (map[string][]Rule, ma
 	return rulesMap, targetsMap, nil
 }
 
+// RulesFromFile parses rules from the given file and returns the list of rules.
 func RulesFromFile(pathname string, ranges []Range, visited map[string]struct{}, wg *sync.WaitGroup, options LintOptions) ([]Rule, error) {
 	visited[pathname] = struct{}{}