Improve syntax error messages

- Add a source file name to error messages.
- Add a line that an error occurred at to error messages.

1 files changed, 46 insertions, 6 deletions

diff --git a/error/error.go b/error/error.go
index 1745c52..cc4fed5 100644
--- a/error/error.go
+++ b/error/error.go
@@ -1,15 +1,55 @@
 package error
 
-import "fmt"
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
 
 type SpecError struct {
-	Cause error
-	Row   int
+	Cause      error
+	FilePath   string
+	SourceName string
+	Row        int
 }
 
 func (e *SpecError) Error() string {
-	if e.Row == 0 {
-		return fmt.Sprintf("error: %v", e.Cause)
+	var b strings.Builder
+	if e.SourceName != "" {
+		fmt.Fprintf(&b, "%v: ", e.SourceName)
 	}
-	return fmt.Sprintf("%v: error: %v", e.Row, e.Cause)
+	if e.Row != 0 {
+		fmt.Fprintf(&b, "%v: ", e.Row)
+	}
+	fmt.Fprintf(&b, "error: %v", e.Cause)
+
+	line := readLine(e.FilePath, e.Row)
+	if line != "" {
+		fmt.Fprintf(&b, "\n    %v", line)
+	}
+
+	return b.String()
+}
+
+func readLine(filePath string, row int) string {
+	if filePath == "" || row <= 0 {
+		return ""
+	}
+
+	f, err := os.Open(filePath)
+	if err != nil {
+		return ""
+	}
+
+	i := 1
+	s := bufio.NewScanner(f)
+	for s.Scan() {
+		if i == row {
+			return s.Text()
+		}
+		i++
+	}
+
+	return ""
 }