From e84f8a41d4fff6ca0311f5add6acd5fe000e7c18 Mon Sep 17 00:00:00 2001
From: Kyle Fuller <kyle@fuller.li>
Date: Mon, 28 Nov 2016 02:56:04 +0000
Subject: [PATCH] feat(if): Support `and`, `or` and `not` during if expressions

Closes #73
---
 CHANGELOG.md                        |   6 +
 Sources/IfTag.swift                 | 373 ++++++++++++++++++++++++----
 Tests/StencilTests/IfNodeSpec.swift | 247 ++++++++++++------
 docs/builtins.rst                   |  45 ++++
 4 files changed, 547 insertions(+), 124 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2aa8dc4..29e5716 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,12 @@
 
 ### Enhancements
 
+- If tags can now use prefix and infix operators such as `not`, `and` and `or`.
+
+    ```html+django
+    {% if one or two and not three %}
+    ```
+
 - You may now register custom template filters which make use of arguments.
 - There is now a `default` filter.
 
diff --git a/Sources/IfTag.swift b/Sources/IfTag.swift
index 1084936..91b8753 100644
--- a/Sources/IfTag.swift
+++ b/Sources/IfTag.swift
@@ -1,61 +1,48 @@
-class IfNode : NodeType {
-  let variable:Variable
-  let trueNodes:[NodeType]
-  let falseNodes:[NodeType]
+protocol Expression: CustomStringConvertible {
+  func evaluate(context: Context) throws -> Bool
+}
 
-  class func parse(_ parser:TokenParser, token:Token) throws -> NodeType {
-    let components = token.components()
-    guard components.count == 2 else {
-      throw TemplateSyntaxError("'if' statements should use the following 'if condition' `\(token.contents)`.")
-    }
-    let variable = components[1]
-    var trueNodes = [NodeType]()
-    var falseNodes = [NodeType]()
 
-    trueNodes = try parser.parse(until(["endif", "else"]))
+protocol InfixOperator: Expression {
+  init(lhs: Expression, rhs: Expression)
+}
 
-    guard let token = parser.nextToken() else {
-      throw TemplateSyntaxError("`endif` was not found.")
-    }
 
-    if token.contents == "else" {
-      falseNodes = try parser.parse(until(["endif"]))
-      _ = parser.nextToken()
-    }
+protocol PrefixOperator: Expression {
+  init(expression: Expression)
+}
 
-    return IfNode(variable: variable, trueNodes: trueNodes, falseNodes: falseNodes)
+
+final class StaticExpression: Expression, CustomStringConvertible {
+  let value: Bool
+
+  init(value: Bool) {
+    self.value = value
   }
 
-  class func parse_ifnot(_ parser:TokenParser, token:Token) throws -> NodeType {
-    let components = token.components()
-    guard components.count == 2 else {
-      throw TemplateSyntaxError("'ifnot' statements should use the following 'ifnot condition' `\(token.contents)`.")
-    }
-    let variable = components[1]
-    var trueNodes = [NodeType]()
-    var falseNodes = [NodeType]()
-
-    falseNodes = try parser.parse(until(["endif", "else"]))
-
-    guard let token = parser.nextToken() else {
-      throw TemplateSyntaxError("`endif` was not found.")
-    }
-
-    if token.contents == "else" {
-      trueNodes = try parser.parse(until(["endif"]))
-      _ = parser.nextToken()
-    }
-
-    return IfNode(variable: variable, trueNodes: trueNodes, falseNodes: falseNodes)
+  func evaluate(context: Context) throws -> Bool {
+    return value
   }
 
-  init(variable:String, trueNodes:[NodeType], falseNodes:[NodeType]) {
-    self.variable = Variable(variable)
-    self.trueNodes = trueNodes
-    self.falseNodes = falseNodes
+  var description: String {
+    return "\(value)"
+  }
+}
+
+
+final class VariableExpression: Expression, CustomStringConvertible {
+  let variable: Variable
+
+  init(variable: Variable) {
+    self.variable = variable
   }
 
-  func render(_ context: Context) throws -> String {
+  var description: String {
+    return "(variable: \(variable.variable))"
+  }
+
+  /// Resolves a variable in the given context as boolean
+  func resolve(context: Context, variable: Variable) throws -> Bool {
     let result = try variable.resolve(context)
     var truthy = false
 
@@ -75,6 +62,298 @@ class IfNode : NodeType {
       truthy = true
     }
 
+    return truthy
+  }
+
+  func evaluate(context: Context) throws -> Bool {
+    return try resolve(context: context, variable: variable)
+  }
+}
+
+
+final class NotExpression: Expression, PrefixOperator, CustomStringConvertible {
+  let expression: Expression
+
+  init(expression: Expression) {
+    self.expression = expression
+  }
+
+  var description: String {
+    return "not \(expression)"
+  }
+
+  func evaluate(context: Context) throws -> Bool {
+    return try !expression.evaluate(context: context)
+  }
+}
+
+
+final class OrExpression: Expression, InfixOperator, CustomStringConvertible {
+  let lhs: Expression
+  let rhs: Expression
+
+  init(lhs: Expression, rhs: Expression) {
+    self.lhs = lhs
+    self.rhs = rhs
+  }
+
+  var description: String {
+    return "(\(lhs) or \(rhs))"
+  }
+
+  func evaluate(context: Context) throws -> Bool {
+    let lhs = try self.lhs.evaluate(context: context)
+    if lhs {
+      return lhs
+    }
+
+    return try rhs.evaluate(context: context)
+  }
+}
+
+
+final class AndExpression: Expression, InfixOperator, CustomStringConvertible {
+  let lhs: Expression
+  let rhs: Expression
+
+  init(lhs: Expression, rhs: Expression) {
+    self.lhs = lhs
+    self.rhs = rhs
+  }
+
+  var description: String {
+    return "(\(lhs) and \(rhs))"
+  }
+
+  func evaluate(context: Context) throws -> Bool {
+    let lhs = try self.lhs.evaluate(context: context)
+    if !lhs {
+      return lhs
+    }
+
+    return try rhs.evaluate(context: context)
+  }
+}
+
+
+enum Operator {
+  case infix(String, Int, InfixOperator.Type)
+  case prefix(String, Int, PrefixOperator.Type)
+
+  var name: String {
+    switch self {
+    case .infix(let name, _, _):
+      return name
+    case .prefix(let name, _, _):
+      return name
+    }
+  }
+}
+
+
+let operators: [Operator] = [
+  .infix("or", 6, OrExpression.self),
+  .infix("and", 7, AndExpression.self),
+  .prefix("not", 8, NotExpression.self),
+]
+
+
+func findOperator(name: String) -> Operator? {
+  for op in operators {
+    if op.name == name {
+      return op
+    }
+  }
+
+  return nil
+}
+
+
+enum IfToken {
+  case infix(name: String, bindingPower: Int, op: InfixOperator.Type)
+  case prefix(name: String, bindingPower: Int, op: PrefixOperator.Type)
+  case variable(Variable)
+  case end
+
+  var bindingPower: Int {
+    switch self {
+    case .infix(_, let bindingPower, _):
+      return bindingPower
+    case .prefix(_, let bindingPower, _):
+      return bindingPower
+    case .variable(_):
+      return 0
+    case .end:
+        return 0
+    }
+  }
+
+  func nullDenotation(parser: IfExpressionParser) throws -> Expression {
+    switch self {
+    case .infix(let name, _, _):
+      throw TemplateSyntaxError("'if' expression error: infix operator '\(name)' doesn't have a left hand side")
+    case .prefix(_, let bindingPower, let op):
+      let expression = try parser.expression(bindingPower: bindingPower)
+      return op.init(expression: expression)
+    case .variable(let variable):
+      return VariableExpression(variable: variable)
+    case .end:
+      throw TemplateSyntaxError("'if' expression error: end")
+    }
+  }
+
+  func leftDenotation(left: Expression, parser: IfExpressionParser) throws -> Expression {
+    switch self {
+    case .infix(_, let bindingPower, let op):
+      let right = try parser.expression(bindingPower: bindingPower)
+      return op.init(lhs: left, rhs: right)
+    case .prefix(let name, _, _):
+      throw TemplateSyntaxError("'if' expression error: prefix operator '\(name)' was called with a left hand side")
+    case .variable(let variable):
+      throw TemplateSyntaxError("'if' expression error: variable '\(variable)' was called with a left hand side")
+    case .end:
+      throw TemplateSyntaxError("'if' expression error: end")
+    }
+  }
+
+  var isEnd: Bool {
+    switch self {
+    case .end:
+      return true
+    default:
+      return false
+    }
+  }
+}
+
+
+final class IfExpressionParser {
+  let tokens: [IfToken]
+  var position: Int = 0
+
+  init(components: [String]) {
+    self.tokens = components.map { component in
+      if let op = findOperator(name: component) {
+        switch op {
+        case .infix(let name, let bindingPower, let cls):
+          return .infix(name: name, bindingPower: bindingPower, op: cls)
+        case .prefix(let name, let bindingPower, let cls):
+          return .prefix(name: name, bindingPower: bindingPower, op: cls)
+        }
+      }
+
+      return .variable(Variable(component))
+    }
+  }
+
+  var currentToken: IfToken {
+    if tokens.count > position {
+      return tokens[position]
+    }
+
+    return .end
+  }
+
+  var nextToken: IfToken {
+    position += 1
+    return currentToken
+  }
+
+  func parse() throws -> Expression {
+    let expression = try self.expression()
+
+    if !currentToken.isEnd {
+      throw TemplateSyntaxError("'if' expression error: dangling token")
+    }
+
+    return expression
+  }
+
+  func expression(bindingPower: Int = 0) throws -> Expression {
+    var token = currentToken
+    position += 1
+
+    var left = try token.nullDenotation(parser: self)
+
+    while bindingPower < currentToken.bindingPower {
+      token = currentToken
+      position += 1
+      left = try token.leftDenotation(left: left, parser: self)
+    }
+
+    return left
+  }
+}
+
+
+func parseExpression(components: [String]) throws -> Expression {
+  let parser = IfExpressionParser(components: components)
+  return try parser.parse()
+}
+
+
+class IfNode : NodeType {
+  let expression: Expression
+  let trueNodes: [NodeType]
+  let falseNodes: [NodeType]
+
+  class func parse(_ parser: TokenParser, token: Token) throws -> NodeType {
+    var components = token.components()
+    guard components.count == 2 else {
+      throw TemplateSyntaxError("'if' statements should use the following 'if condition' `\(token.contents)`.")
+    }
+    components.removeFirst()
+    var trueNodes = [NodeType]()
+    var falseNodes = [NodeType]()
+
+    trueNodes = try parser.parse(until(["endif", "else"]))
+
+    guard let token = parser.nextToken() else {
+      throw TemplateSyntaxError("`endif` was not found.")
+    }
+
+    if token.contents == "else" {
+      falseNodes = try parser.parse(until(["endif"]))
+      _ = parser.nextToken()
+    }
+
+    let expression = try parseExpression(components: components)
+    return IfNode(expression: expression, trueNodes: trueNodes, falseNodes: falseNodes)
+  }
+
+  class func parse_ifnot(_ parser: TokenParser, token: Token) throws -> NodeType {
+    var components = token.components()
+    guard components.count == 2 else {
+      throw TemplateSyntaxError("'ifnot' statements should use the following 'ifnot condition' `\(token.contents)`.")
+    }
+    components.removeFirst()
+    var trueNodes = [NodeType]()
+    var falseNodes = [NodeType]()
+
+    falseNodes = try parser.parse(until(["endif", "else"]))
+
+    guard let token = parser.nextToken() else {
+      throw TemplateSyntaxError("`endif` was not found.")
+    }
+
+    if token.contents == "else" {
+      trueNodes = try parser.parse(until(["endif"]))
+      _ = parser.nextToken()
+    }
+
+    let expression = try parseExpression(components: components)
+    return IfNode(expression: expression, trueNodes: trueNodes, falseNodes: falseNodes)
+  }
+
+  init(expression: Expression, trueNodes: [NodeType], falseNodes: [NodeType]) {
+    self.expression = expression
+    self.trueNodes = trueNodes
+    self.falseNodes = falseNodes
+  }
+
+  func render(_ context: Context) throws -> String {
+    let truthy = try expression.evaluate(context: context)
+
     return try context.push {
       if truthy {
         return try renderNodes(trueNodes, context)
diff --git a/Tests/StencilTests/IfNodeSpec.swift b/Tests/StencilTests/IfNodeSpec.swift
index c048864..ca3dc45 100644
--- a/Tests/StencilTests/IfNodeSpec.swift
+++ b/Tests/StencilTests/IfNodeSpec.swift
@@ -3,6 +3,172 @@ import Spectre
 
 
 func testIfNode() {
+  describe("Expression") {
+    $0.describe("VariableExpression") {
+      let expression = VariableExpression(variable: Variable("value"))
+
+      $0.it("evaluates to true when value is not nil") {
+        let context = Context(dictionary: ["value": "known"])
+        try expect(try expression.evaluate(context: context)).to.beTrue()
+      }
+
+      $0.it("evaluates to false when value is unset") {
+        let context = Context()
+        try expect(try expression.evaluate(context: context)).to.beFalse()
+      }
+
+      $0.it("evaluates to true when array variable is not empty") {
+        let items: [[String: Any]] = [["key":"key1","value":42],["key":"key2","value":1337]]
+        let context = Context(dictionary: ["value": [items]])
+        try expect(try expression.evaluate(context: context)).to.beTrue()
+      }
+
+      $0.it("evaluates to false when array value is empty") {
+        let emptyItems = [[String: Any]]()
+        let context = Context(dictionary: ["value": emptyItems])
+        try expect(try expression.evaluate(context: context)).to.beFalse()
+      }
+
+      $0.it("evaluates to false when dictionary value is empty") {
+        let emptyItems = [String:Any]()
+        let context = Context(dictionary: ["value": emptyItems])
+        try expect(try expression.evaluate(context: context)).to.beFalse()
+      }
+
+      $0.it("evaluates to false when Array<Any> value is empty") {
+        let context = Context(dictionary: ["value": ([] as [Any])])
+        try expect(try expression.evaluate(context: context)).to.beFalse()
+      }
+
+      $0.it("evaluates to true when integer value is above 0") {
+        let context = Context(dictionary: ["value": 1])
+        try expect(try expression.evaluate(context: context)).to.beTrue()
+      }
+
+      $0.it("evaluates to false when integer value is below 0 or below") {
+        let context = Context(dictionary: ["value": 0])
+        try expect(try expression.evaluate(context: context)).to.beFalse()
+
+        let negativeContext = Context(dictionary: ["value": 0])
+        try expect(try expression.evaluate(context: negativeContext)).to.beFalse()
+      }
+
+      $0.it("evaluates to true when float value is above 0") {
+        let context = Context(dictionary: ["value": Float(0.5)])
+        try expect(try expression.evaluate(context: context)).to.beTrue()
+      }
+
+      $0.it("evaluates to false when float is 0 or below") {
+        let context = Context(dictionary: ["value": Float(0)])
+        try expect(try expression.evaluate(context: context)).to.beFalse()
+      }
+
+      $0.it("evaluates to true when double value is above 0") {
+        let context = Context(dictionary: ["value": Double(0.5)])
+        try expect(try expression.evaluate(context: context)).to.beTrue()
+      }
+
+      $0.it("evaluates to false when double is 0 or below") {
+        let context = Context(dictionary: ["value": Double(0)])
+        try expect(try expression.evaluate(context: context)).to.beFalse()
+      }
+    }
+
+    $0.describe("NotExpression") {
+      $0.it("returns truthy for positive expressions") {
+        let expression = NotExpression(expression: StaticExpression(value: true))
+        try expect(expression.evaluate(context: Context())).to.beFalse()
+      }
+
+      $0.it("returns falsy for negative expressions") {
+        let expression = NotExpression(expression: StaticExpression(value: false))
+        try expect(expression.evaluate(context: Context())).to.beTrue()
+      }
+    }
+
+    $0.describe("expression parsing") {
+      $0.it("can parse a variable expression") {
+        let expression = try parseExpression(components: ["value"])
+        try expect(expression.evaluate(context: Context())).to.beFalse()
+        try expect(expression.evaluate(context: Context(dictionary: ["value": true]))).to.beTrue()
+      }
+
+      $0.it("can parse a not expression") {
+        let expression = try parseExpression(components: ["not", "value"])
+        try expect(expression.evaluate(context: Context())).to.beTrue()
+        try expect(expression.evaluate(context: Context(dictionary: ["value": true]))).to.beFalse()
+      }
+
+      $0.describe("and expression") {
+        let expression = try! parseExpression(components: ["lhs", "and", "rhs"])
+
+        $0.it("evaluates to false with lhs false") {
+          try expect(expression.evaluate(context: Context(dictionary: ["lhs": false, "rhs": true]))).to.beFalse()
+        }
+
+        $0.it("evaluates to false with rhs false") {
+          try expect(expression.evaluate(context: Context(dictionary: ["lhs": true, "rhs": false]))).to.beFalse()
+        }
+
+        $0.it("evaluates to false with lhs and rhs false") {
+          try expect(expression.evaluate(context: Context(dictionary: ["lhs": false, "rhs": false]))).to.beFalse()
+        }
+
+        $0.it("evaluates to true with lhs and rhs true") {
+          try expect(expression.evaluate(context: Context(dictionary: ["lhs": true, "rhs": true]))).to.beTrue()
+        }
+      }
+
+      $0.describe("or expression") {
+        let expression = try! parseExpression(components: ["lhs", "or", "rhs"])
+
+        $0.it("evaluates to true with lhs true") {
+          try expect(expression.evaluate(context: Context(dictionary: ["lhs": true, "rhs": false]))).to.beTrue()
+        }
+
+        $0.it("evaluates to true with rhs true") {
+          try expect(expression.evaluate(context: Context(dictionary: ["lhs": false, "rhs": true]))).to.beTrue()
+        }
+
+        $0.it("evaluates to true with lhs and rhs true") {
+          try expect(expression.evaluate(context: Context(dictionary: ["lhs": true, "rhs": true]))).to.beTrue()
+        }
+
+        $0.it("evaluates to false with lhs and rhs false") {
+          try expect(expression.evaluate(context: Context(dictionary: ["lhs": false, "rhs": false]))).to.beFalse()
+        }
+      }
+
+      $0.describe("multiple expression") {
+        let expression = try! parseExpression(components: ["one", "or", "two", "and", "not", "three"])
+
+        $0.it("evaluates to true with one") {
+          try expect(expression.evaluate(context: Context(dictionary: ["one": true]))).to.beTrue()
+        }
+
+        $0.it("evaluates to true with one and three") {
+          try expect(expression.evaluate(context: Context(dictionary: ["one": true, "three": true]))).to.beTrue()
+        }
+
+        $0.it("evaluates to true with two") {
+          try expect(expression.evaluate(context: Context(dictionary: ["two": true]))).to.beTrue()
+        }
+
+        $0.it("evaluates to false with two and three") {
+          try expect(expression.evaluate(context: Context(dictionary: ["two": true, "three": true]))).to.beFalse()
+        }
+
+        $0.it("evaluates to false with two and three") {
+          try expect(expression.evaluate(context: Context(dictionary: ["two": true, "three": true]))).to.beFalse()
+        }
+
+        $0.it("evaluates to false with nothing") {
+          try expect(expression.evaluate(context: Context())).to.beFalse()
+        }
+      }
+    }
+  }
+
   describe("IfNode") {
     $0.describe("parsing") {
       $0.it("can parse an if block") {
@@ -21,7 +187,6 @@ func testIfNode() {
         let falseNode = node?.falseNodes.first as? TextNode
 
         try expect(nodes.count) == 1
-        try expect(node?.variable.variable) == "value"
         try expect(node?.trueNodes.count) == 1
         try expect(trueNode?.text) == "true"
         try expect(node?.falseNodes.count) == 1
@@ -44,7 +209,6 @@ func testIfNode() {
         let falseNode = node?.falseNodes.first as? TextNode
 
         try expect(nodes.count) == 1
-        try expect(node?.variable.variable) == "value"
         try expect(node?.trueNodes.count) == 1
         try expect(trueNode?.text) == "true"
         try expect(node?.falseNodes.count) == 1
@@ -74,84 +238,13 @@ func testIfNode() {
 
     $0.describe("rendering") {
       $0.it("renders the truth when expression evaluates to true") {
-        let context = Context(dictionary: ["items": true])
-        let node = IfNode(variable: "items", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
-        try expect(try node.render(context)) == "true"
+        let node = IfNode(expression: StaticExpression(value: true), trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
+        try expect(try node.render(Context())) == "true"
       }
 
       $0.it("renders the false when expression evaluates to false") {
-        let context = Context(dictionary: ["items": false])
-        let node = IfNode(variable: "items", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
-        try expect(try node.render(context)) == "false"
-      }
-
-      $0.it("renders the truth when expression is not nil") {
-        let context = Context(dictionary: ["known": "known"])
-        let node = IfNode(variable: "known", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
-        try expect(try node.render(context)) == "true"
-      }
-
-      $0.it("renders the false when expression is nil") {
-        let context = Context(dictionary: [:])
-        let node = IfNode(variable: "unknown", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
-        try expect(try node.render(context)) == "false"
-      }
-
-      $0.it("renders the truth when array expression is not empty") {
-        let items: [[String: Any]] = [["key":"key1","value":42],["key":"key2","value":1337]]
-        let arrayContext = Context(dictionary: ["items": [items]])
-        let node = IfNode(variable: "items", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
-        try expect(try node.render(arrayContext)) == "true"
-      }
-
-      $0.it("renders the false when array expression is empty") {
-        let emptyItems = [[String: Any]]()
-        let arrayContext = Context(dictionary: ["items": emptyItems])
-        let node = IfNode(variable: "items", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
-        try expect(try node.render(arrayContext)) == "false"
-      }
-
-      $0.it("renders the false when dictionary expression is empty") {
-        let emptyItems = [String:Any]()
-        let arrayContext = Context(dictionary: ["items": emptyItems])
-        let node = IfNode(variable: "items", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
-        try expect(try node.render(arrayContext)) == "false"
-      }
-
-      $0.it("renders the false when Array<Any> variable is empty") {
-        let arrayContext = Context(dictionary: ["items": ([] as [Any])])
-        let node = IfNode(variable: "items", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
-        try expect(try node.render(arrayContext)) == "false"
-      }
-
-      $0.it("renders the false when integer is below 1") {
-        let context = Context(dictionary: ["value": 0])
-        let node = IfNode(variable: "value", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
-        try expect(try node.render(context)) == "false"
-
-        let negativeContext = Context(dictionary: ["value": -5])
-        let negativeNode = IfNode(variable: "value", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
-        try expect(try negativeNode.render(negativeContext)) == "false"
-      }
-
-      $0.it("renders the false when float is below 1") {
-        let context = Context(dictionary: ["value": Float(0)])
-        let node = IfNode(variable: "value", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
-        try expect(try node.render(context)) == "false"
-
-        let negativeContext = Context(dictionary: ["value": Float(-5)])
-        let negativeNode = IfNode(variable: "value", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
-        try expect(try negativeNode.render(negativeContext)) == "false"
-      }
-
-      $0.it("renders the false when double is below 1") {
-        let context = Context(dictionary: ["value": Double(0)])
-        let node = IfNode(variable: "value", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
-        try expect(try node.render(context)) == "false"
-
-        let negativeContext = Context(dictionary: ["value": Double(-5)])
-        let negativeNode = IfNode(variable: "value", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
-        try expect(try negativeNode.render(negativeContext)) == "false"
+        let node = IfNode(expression: StaticExpression(value: false), trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
+        try expect(try node.render(Context())) == "false"
       }
     }
   }
diff --git a/docs/builtins.rst b/docs/builtins.rst
index dab8a41..f2afb3b 100644
--- a/docs/builtins.rst
+++ b/docs/builtins.rst
@@ -41,6 +41,14 @@ The for block sets a few variables available within the loop:
 ``if``
 ~~~~~~
 
+The ``{% if %}`` tag evaluates a variable, and if that variable evaluates to
+true the contents of the block are processed. Being true is defined as:
+
+* Present in the context
+* Being non-empty (dictionaries or arrays)
+* Not being a false boolean value
+* Not being a numerical value of 0 or below
+
 .. code-block:: html+django
 
     {% if variable %}
@@ -49,6 +57,43 @@ The for block sets a few variables available within the loop:
       The variable was not found.
     {% endif %}
 
+Operators
+^^^^^^^^^
+
+``if`` tags may combine ``and``, ``or`` and ``not`` to test multiple variables
+or to negate a variable.
+
+.. code-block:: html+django
+
+    {% if one and two %}
+        Both one and two evaluate to true.
+    {% endif %}
+
+    {% if not one %}
+        One evaluates to false
+    {% endif %}
+
+    {% if one or two %}
+        Either one or two evaluates to true.
+    {% endif %}
+
+    {% if not one or two %}
+        One does not evaluate to false or two evaluates to true.
+    {% endif %}
+
+You may use ``and``, ``or`` and ``not`` multiple times together. ``not`` has
+higest prescidence followed by ``and``. For example:
+
+.. code-block:: html+django
+
+    {% if one or two and three %}
+
+Will be treated as:
+
+.. code-block:: text
+
+    one or (two and three)
+
 ``ifnot``
 ~~~~~~~~~