Add field type, parsing, and failing test.

lib/carto/index.js

@@ -51,7 +51,7 @@ var carto = {
 };
 
 [ 'call', 'color', 'comment', 'definition', 'dimension',
-  'directive', 'element', 'expression', 'filterset', 'filter',
+  'directive', 'element', 'expression', 'filterset', 'filter', 'field',
   'keyword', 'layer', 'operation', 'quoted', 'imagefilter',
   'reference', 'rule', 'ruleset', 'selector', 'style', 'url', 'value',
   'variable', 'zoom', 'invalid', 'fontset'

lib/carto/parser.js

@@ -423,6 +423,20 @@ carto.Parser = function Parser(env) {
                     }
                 },
 
+                // A reference to a Mapnik field, like
+                //
+                //     [NAME]
+                //
+                // Behind the scenes, this has the same representation, but Carto
+                // needs to be careful to warn when unsupported operations are used.
+                field: function() {
+                    if (! $('[')) return;
+                    var field_name = $(/(^[a-zA-Z0-9\-_]+)/);
+                    if (! $(']')) return;
+                    if (field_name) return new tree.Field(field_name[1]);
+                },
+
+                // This is a comparison operator
                 comparison: function() {
                     var str = $(/^=~|=|!=|<=|>=|<|>/);
                     if (str) {
@@ -553,7 +567,9 @@ carto.Parser = function Parser(env) {
             variable: function() {
                 var name;
 
-                if (input.charAt(i) === '@' && (name = $(/^(@[\w-]+)\s*:/))) { return name[1]; }
+                if (input.charAt(i) === '@' && (name = $(/^(@[\w-]+)\s*:/))) {
+                    return name[1];
+                }
             },
 
             //
@@ -561,8 +577,12 @@ carto.Parser = function Parser(env) {
             // and can be found inside a rule's value.
             //
             entity: function() {
-                return $(this.entities.literal) || $(this.entities.variable) || $(this.entities.url) ||
-                       $(this.entities.call) || $(this.entities.keyword);
+                return $(this.entities.literal) ||
+                    $(this.entities.field) ||
+                    $(this.entities.variable) ||
+                    $(this.entities.url) ||
+                    $(this.entities.call) ||
+                    $(this.entities.keyword);
             },
 
             //

lib/carto/tree/field.js

@@ -0,0 +1,16 @@
+(function(tree) {
+
+tree.Field = function Field(content) {
+    this.value = content || '';
+    this.is = 'field';
+};
+tree.Field.prototype = {
+    toString: function(quotes) {
+        return '[' + this.value + ']';
+    },
+    'eval': function() {
+        return this;
+    }
+};
+
+})(require('../tree'));

test/rendering/field.mml

@@ -0,0 +1,23 @@
+{
+    "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
+    "Stylesheet": [
+        "field.mss"
+    ],
+    "Layer": [{
+        "name": "world",
+        "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
+        "Datasource": {
+            "file": "http://tilemill-data.s3.amazonaws.com/test_data/shape_demo.zip",
+            "type": "shape"
+        }
+    },
+    {
+        "class": "new",
+        "name": "countries",
+        "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
+        "Datasource": {
+            "file": "http://tilemill-data.s3.amazonaws.com/test_data/shape_demo.zip",
+            "type": "shape"
+        }
+    }]
+}

test/rendering/field.mss

@@ -0,0 +1,6 @@
+#world {
+  text-name: [NAME];
+  text-size: 11;
+  text-face-name: "Georgia Regular", "Arial Italic";
+}
+