Allow conditions in variable node (#243)

* use condition in variable node

* added support for else expression

* addressing code review comments

Sources/Node.swift

@@ -58,18 +58,64 @@ public protocol Resolvable {
 public class VariableNode : NodeType {
   public let variable: Resolvable
   public var token: Token?
+  let condition: Expression?
+  let elseExpression: Resolvable?
+
+  class func parse(_ parser:TokenParser, token:Token) throws -> NodeType {
+    var components = token.components()
+
+    func hasToken(_ token: String, at index: Int) -> Bool {
+      return components.count > (index + 1) && components[index] == token
+    }
+
+    let condition: Expression?
+    let elseExpression: Resolvable?
+
+    if hasToken("if", at: 1) {
+      let components = components.suffix(from: 2)
+      if let elseIndex = components.index(of: "else") {
+        condition = try parseExpression(components: Array(components.prefix(upTo: elseIndex)), tokenParser: parser, token: token)
+        let elseToken = components.suffix(from: elseIndex.advanced(by: 1)).joined(separator: " ")
+        elseExpression = try parser.compileResolvable(elseToken, containedIn: token)
+      } else {
+        condition = try parseExpression(components: Array(components), tokenParser: parser, token: token)
+        elseExpression = nil
+      }
+    } else {
+      condition = nil
+      elseExpression = nil
+    }
+
+    let filter = try parser.compileResolvable(components[0], containedIn: token)
+    return VariableNode(variable: filter, token: token, condition: condition, elseExpression: elseExpression)
+  }
 
   public init(variable: Resolvable, token: Token? = nil) {
     self.variable = variable
     self.token = token
+    self.condition = nil
+    self.elseExpression = nil
+  }
+
+  init(variable: Resolvable, token: Token? = nil, condition: Expression?, elseExpression: Resolvable?) {
+    self.variable = variable
+    self.token = token
+    self.condition = condition
+    self.elseExpression = elseExpression
   }
 
   public init(variable: String, token: Token? = nil) {
     self.variable = Variable(variable)
     self.token = token
+    self.condition = nil
+    self.elseExpression = nil
   }
 
   public func render(_ context: Context) throws -> String {
+    if let condition = self.condition, try condition.evaluate(context: context) == false {
+      return try elseExpression?.resolve(context).map(stringify) ?? ""
+    }
+
     let result = try variable.resolve(context)
     return stringify(result)
   }