Improved error reporting

moo.js

@@ -53,6 +53,38 @@
     }
   }
 
+  function pad(s, length) {
+    if (s.length > length) {
+      return s
+    }
+    return Array(length - s.length + 1).join(" ") + s
+  }
+
+  function lastNLines(string, numLines) {
+    var position = string.length
+    var lineBreaks = 0;
+    while (true) {
+      var idx = string.lastIndexOf("\n", position - 1)
+      if (idx === -1) {
+        break;
+      } else {
+        lineBreaks++
+      }
+      position = idx
+      if (lineBreaks === numLines) {
+        break;
+      }
+      if (position === 0) {
+        break;
+      }
+    }
+    var startPosition = 
+      lineBreaks < numLines ?
+      0 : 
+      position + 1
+    return string.substring(startPosition).split("\n")
+  }
+
   function objectToRules(object) {
     var keys = Object.getOwnPropertyNames(object)
     var result = []
@@ -530,7 +562,8 @@
 
     // throw, if no rule with {error: true}
     if (group.shouldThrow) {
-      throw new Error(this.formatError(token, "invalid syntax"))
+      var err = new Error(this.formatError(token, "invalid syntax"))
+      throw err;
     }
 
     if (group.pop) this.popState()
@@ -571,13 +604,28 @@
         col: this.col,
       }
     }
-    var start = Math.max(0, token.offset - token.col + 1)
-    var eol = token.lineBreaks ? token.text.indexOf('\n') : token.text.length
-    var firstLine = this.buffer.substring(start, token.offset + eol)
-    message += " at line " + token.line + " col " + token.col + ":\n\n"
-    message += "  " + firstLine + "\n"
-    message += "  " + Array(token.col).join(" ") + "^"
-    return message
+
+    var numLinesAround = 2
+    var firstDisplayedLine = Math.max(token.line - numLinesAround, 1)
+    var lastDisplayedLine = token.line + numLinesAround
+    var lastLineDigits = String(lastDisplayedLine).length
+    var displayedLines = lastNLines(
+        this.buffer, 
+        (this.line - token.line) + numLinesAround + 1
+      )
+      .slice(0, 5)
+    var errorLines = []
+    errorLines.push(message + " at line " + token.line + " col " + token.col + ":")
+    errorLines.push("")
+    for (var i = 0; i < displayedLines.length; i++) {
+      var line = displayedLines[i]
+      var lineNo = firstDisplayedLine + i
+      errorLines.push(pad(String(lineNo), lastLineDigits) + "  " + line);
+      if (lineNo === token.line) {
+        errorLines.push(pad("", lastLineDigits + token.col + 1) + "^")
+      }
+    }
+    return errorLines.join("\n")
   }
 
   Lexer.prototype.clone = function() {

test/test.js

@@ -905,8 +905,9 @@ describe('errors', () => {
     expect(lexer.next()).toMatchObject({value: '456'})
     expect(() => lexer.next()).toThrow(
       "invalid syntax at line 2 col 4:\n\n" +
-      "  456baa\n" +
-      "     ^"
+      "1  123\n" +
+      "2  456baa\n" +
+      "      ^"
     )
   })
 
@@ -921,8 +922,12 @@ describe('errors', () => {
     expect(tok).toMatchObject({type: 'error', value: ' 12\n345\n6', lineBreaks: 2})
     expect(lexer.formatError(tok, "numbers!")).toBe(
       "numbers! at line 3 col 2:\n\n" +
-      "  g 12\n" +
-      "   ^"
+      "1  abc\n" +
+      "2  def\n" +
+      "3  g 12\n" +
+      "    ^\n" +
+      "4  345\n" +
+      "5  6"
     )
   })
 
@@ -937,8 +942,9 @@ describe('errors', () => {
     expect(lexer.col).toBe(9)
     expect(lexer.formatError(undefined, "EOF!")).toBe(
       "EOF! at line 2 col 9:\n\n" +
-      "  def quxx\n" +
-      "          ^"
+      "1  abc\n" +
+      "2  def quxx\n" +
+      "           ^"
     )
   })
 
@@ -954,8 +960,10 @@ describe('errors', () => {
     expect(lexer.col).toBe(1)
     expect(lexer.formatError(undefined, "oh no!")).toBe(
       "oh no! at line 2 col 1:\n\n" +
-      "  def quxx\n" +
-      "  ^"
+      "1  abc\n" +
+      "2  def quxx\n" +
+      "   ^\n" +
+      "3  bar"
     )
   })