proper parser error messages

lib/less/parser.js

@@ -39,10 +39,10 @@ var less = exports || {};
 //
 //
 var input,       // LeSS input string
-    i = 0,       // current index in `input`
-    j = 0,       // current chunk
+    i,           // current index in `input`
+    j,           // current chunk
     chunks = [], // chunkified input
-    current = 0, // index of current chunk, in `input`
+    current,     // index of current chunk, in `input`
     inputLength;
 
 function peek(regex) {
@@ -119,8 +119,9 @@ less.parser = {
     // Parse an input string into an abstract syntax tree
     //
     parse: function (str) {
-        var tree;
+        var tree, start, end, zone, line;
 
+        i = j = current = 0;
         input = str;
         inputLength = input.length;
 
@@ -134,8 +135,22 @@ less.parser = {
         tree = new(node.Ruleset)([], $(this.parsers.primary, []));
         tree.root = true;
 
+        // If `i` is smaller than the input length - 1,
+        // it means the parser wasn't able to parse the whole
+        // string, so we've got a parsing error.
         if (i < input.length - 1) {
-            throw new(Error)("Parse Error: " + input.slice(0, i));
+            start = (function () {
+                for (var n = i; n > 0; n--) {
+                    if (input[n] === '\n') { break }
+                }
+                return n;
+            })() + 1;
+            line = input.slice(0, i).match(/\n/g).length + 1;
+            end = input.slice(i).indexOf('\n') + i;
+            zone = stylize(input.slice(start, i), 'green') +
+                   stylize(input.slice(i, end), 'yellow');
+
+            throw new(Error)("Parse Error on line " + line + ":\n" + zone);
         }
         return tree;
     },
@@ -435,3 +450,16 @@ less.parser = {
     }
 };
 
+// Stylize a string
+function stylize(str, style) {
+    var styles = {
+        'bold'      : [1,  22],
+        'underline' : [4,  24],
+        'yellow'    : [33, 39],
+        'green'     : [32, 39],
+        'red'       : [31, 39]
+    };
+    return '\033[' + styles[style][0] + 'm' + str +
+           '\033[' + styles[style][1] + 'm';
+}
+