feat(filter): Support filter tag

Sources/FilterTag.swift

@@ -0,0 +1,35 @@
+class FilterNode : NodeType {
+  let resolvable: Resolvable
+  let nodes: [NodeType]
+
+  class func parse(_ parser: TokenParser, token: Token) throws -> NodeType {
+    let bits = token.components()
+
+    guard bits.count == 2 else {
+      throw TemplateSyntaxError("'filter' tag takes one argument, the filter expression")
+    }
+
+    let blocks = try parser.parse(until(["endfilter"]))
+
+    guard let token = parser.nextToken() else {
+      throw TemplateSyntaxError("`endfilter` was not found.")
+    }
+
+    let resolvable = try parser.compileFilter("filter_value|\(bits[1])")
+    return FilterNode(nodes: blocks, resolvable: resolvable)
+  }
+
+  init(nodes: [NodeType], resolvable: Resolvable) {
+    self.nodes = nodes
+    self.resolvable = resolvable
+  }
+
+  func render(_ context: Context) throws -> String {
+    let value = try renderNodes(nodes, context)
+
+    return try context.push(dictionary: ["filter_value": value]) {
+      return try VariableNode(variable: resolvable).render(context)
+    }
+  }
+}
+

Sources/Namespace.swift

@@ -42,6 +42,7 @@ public class Namespace {
     registerTag("include", parser: IncludeNode.parse)
     registerTag("extends", parser: ExtendsNode.parse)
     registerTag("block", parser: BlockNode.parse)
+    registerTag("filter", parser: FilterNode.parse)
   }
 
   fileprivate func registerDefaultFilters() {