feat(for tag): Support where expressions (#96)

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Enhancements
 
+- `for` block now can contain `where` expression to filter array items. For example `{% for item in items where item > 1 %}` is now supported.
 - `if` blocks may now contain else if (`elif`) conditions.
 
   ```html+django
@@ -16,7 +17,6 @@
   {% endif %}
   ```
 
-
 ## 0.8.0
 
 ### Breaking

Sources/ForTag.swift

@@ -3,12 +3,14 @@ class ForNode : NodeType {
   let loopVariable:String
   let nodes:[NodeType]
   let emptyNodes: [NodeType]
+  let `where`: Expression?
 
   class func parse(_ parser:TokenParser, token:Token) throws -> NodeType {
     let components = token.components()
 
-    guard components.count == 4 && components[2] == "in" else {
+    guard components.count >= 2 && components[2] == "in" &&
-      throw TemplateSyntaxError("'for' statements should use the following 'for x in y' `\(token.contents)`.")
+        (components.count == 4 || (components.count >= 6 && components[4] == "where")) else {
+      throw TemplateSyntaxError("'for' statements should use the following 'for x in y where condition' `\(token.contents)`.")
     }
 
     let loopVariable = components[1]
@@ -28,20 +30,35 @@ class ForNode : NodeType {
     }
 
     let filter = try parser.compileFilter(variable)
-    return ForNode(resolvable: filter, loopVariable: loopVariable, nodes: forNodes, emptyNodes:emptyNodes)
+    let `where`: Expression?
+    if components.count >= 6 {
+      `where` = try parseExpression(components: Array(components.suffix(from: 5)), tokenParser: parser)
+    } else {
+      `where` = nil
+    }
+    return ForNode(resolvable: filter, loopVariable: loopVariable, nodes: forNodes, emptyNodes:emptyNodes, where: `where`)
   }
 
-  init(resolvable: Resolvable, loopVariable:String, nodes:[NodeType], emptyNodes:[NodeType]) {
+  init(resolvable: Resolvable, loopVariable:String, nodes:[NodeType], emptyNodes:[NodeType], where: Expression? = nil) {
     self.resolvable = resolvable
     self.loopVariable = loopVariable
     self.nodes = nodes
     self.emptyNodes = emptyNodes
+    self.where = `where`
   }
 
   func render(_ context: Context) throws -> String {
     let values = try resolvable.resolve(context)
 
-    if let values = values as? [Any] , values.count > 0 {
+    if var values = values as? [Any], values.count > 0 {
+      if let `where` = self.where {
+        values = try values.filter({ item -> Bool in
+          return try context.push(dictionary: [loopVariable: item]) { () -> Bool in
+            try `where`.evaluate(context: context)
+          }
+        })
+      }
+      if values.count > 0 {
         let count = values.count
         return try values.enumerated().map { index, item in
           let forContext: [String: Any] = [
@@ -55,6 +72,7 @@ class ForNode : NodeType {
           }
         }.joined(separator: "")
       }
+    }
 
     return try context.push {
       try renderNodes(emptyNodes, context)

Tests/StencilTests/ForNodeSpec.swift

@@ -63,6 +63,21 @@ func testForNode() {
       try expect(try node.render(context)) == "112233"
     }
 
+    $0.it("renders the given nodes while filtering items using where expression") {
+        let nodes: [NodeType] = [VariableNode(variable: "item"), VariableNode(variable: "forloop.counter")]
+        let `where` = try parseExpression(components: ["item", ">", "1"], tokenParser: TokenParser(tokens: [], environment: Environment()))
+        let node = ForNode(resolvable: Variable("items"), loopVariable: "item", nodes: nodes, emptyNodes: [], where: `where`)
+        try expect(try node.render(context)) == "2132"
+    }
+
+    $0.it("renders the given empty nodes when all items filtered out with where expression") {
+        let nodes: [NodeType] = [VariableNode(variable: "item")]
+        let emptyNodes: [NodeType] = [TextNode(text: "empty")]
+        let `where` = try parseExpression(components: ["item", "==", "0"], tokenParser: TokenParser(tokens: [], environment: Environment()))
+        let node = ForNode(resolvable: Variable("emptyItems"), loopVariable: "item", nodes: nodes, emptyNodes: emptyNodes, where: `where`)
+        try expect(try node.render(context)) == "empty"
+    }
+
     $0.it("can render a filter") {
       let templateString = "{% for article in ars|default:articles %}" +
         "- {{ article.title }} by {{ article.author }}.\n" +

docs/builtins.rst

@@ -19,6 +19,17 @@ A for loop allows you to iterate over an array found by variable lookup.
       {% endfor %}
     </ul>
 
+The ``for`` tag can contain optional ``where`` expression to filter out 
+elements on which this expression evaluates to false.
+
+.. code-block:: html+django
+
+    <ul>
+      {% for user in users where user.name != "Kyle" %}
+        <li>{{ user }}</li>
+      {% endfor %}
+    </ul>
+
 The ``for`` tag can take an optional ``{% empty %}`` block that will be
 displayed if the given list is empty or could not be found.