From 7e9fd3da5a19205f632ad0d07cde8df339810178 Mon Sep 17 00:00:00 2001
From: Ted van Riel <80752652+tvanriel@users.noreply.github.com>
Date: Sat, 4 Jun 2022 11:04:23 +0200
Subject: [PATCH] feat: better error display

Errors generated by l.Error() will now be wrapped with
"lexer(pos=line,pos): " to improve the developer experience.

Fix: bbuck/go-lexer#6

Signed-off-by: Ted van Riel <80752652+tvanriel@users.noreply.github.com>
---
 lexer.go                 | 15 +++++++++++++--
 lexer_test/lexer_test.go |  2 +-
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/lexer.go b/lexer.go
index 286dde6..966acdc 100644
--- a/lexer.go
+++ b/lexer.go
@@ -28,7 +28,7 @@
 package lexer
 
 import (
-	"errors"
+	"fmt"
 	"strings"
 	"unicode/utf8"
 )
@@ -188,11 +188,22 @@ func (l *L) NextToken() (*Token, bool) {
 	}
 }
 
+// Get the line number and position in that line the lexer position is currently on.
+func getPos(l *L) (int, int) {
+	untilNow := l.source[:l.position]
+	linenum := strings.Count(untilNow, "\n") + 1
+	lastNewLineIndex := strings.LastIndex(untilNow, "\n")
+	posInLine := l.position - lastNewLineIndex
+	return linenum, posInLine
+}
+
 // Partial yyLexer implementation
 
 func (l *L) Error(e string) {
 	if l.ErrorHandler != nil {
-		l.Err = errors.New(e)
+
+		linenum, pos := getPos(l)
+		l.Err = fmt.Errorf("lexer (pos=%d,%d): %v", linenum, pos, e)
 		l.ErrorHandler(e)
 	} else {
 		panic(e)
diff --git a/lexer_test/lexer_test.go b/lexer_test/lexer_test.go
index e56bdc8..7e67e12 100644
--- a/lexer_test/lexer_test.go
+++ b/lexer_test/lexer_test.go
@@ -197,7 +197,7 @@ func Test_LexerError(t *testing.T) {
 		return
 	}
 
-	if l.Err.Error() != "unexpected token '1'" {
+	if l.Err.Error() != "lexer (pos=1,2): unexpected token '1'" {
 		t.Errorf("Expected specific message from error, but got %q", l.Err.Error())
 		return
 	}