Added support for brackets in boolean expressions (#165)

* added support for brackets in boolean expressions

* more descriptive error messages

* use array slices

* added test for nested expressions

* removed brackets validation step

* address code review comments

* added doc comment

* simplify expression spec

* fixed docs

CHANGELOG.md

@@ -16,6 +16,9 @@
 - Now requires Swift 4.1 or newer.  
   [Yonas Kolb](https://github.com/yonaskolb)
   [#228](https://github.com/stencilproject/Stencil/pull/228)
+- You can now use parentheses in boolean expressions to change operator precedence.  
+  [Ilya Puchka](https://github.com/ilyapuchka)
+  [#165](https://github.com/stencilproject/Stencil/pull/165)
 
 ### New Features
 

Sources/IfTag.swift

@@ -38,10 +38,11 @@ func findOperator(name: String) -> Operator? {
 }
 
 
-enum IfToken {
+indirect enum IfToken {
-  case infix(name: String, bindingPower: Int, op: InfixOperator.Type)
+  case infix(name: String, bindingPower: Int, operatorType: InfixOperator.Type)
-  case prefix(name: String, bindingPower: Int, op: PrefixOperator.Type)
+  case prefix(name: String, bindingPower: Int, operatorType: PrefixOperator.Type)
   case variable(Resolvable)
+  case subExpression(Expression)
   case end
 
   var bindingPower: Int {
@@ -52,6 +53,8 @@ enum IfToken {
       return bindingPower
     case .variable(_):
       return 0
+    case .subExpression(_):
+      return 0
     case .end:
       return 0
     }
@@ -66,6 +69,8 @@ enum IfToken {
       return op.init(expression: expression)
     case .variable(let variable):
       return VariableExpression(variable: variable)
+    case .subExpression(let expression):
+      return expression
     case .end:
       throw TemplateSyntaxError("'if' expression error: end")
     }
@@ -80,6 +85,8 @@ enum IfToken {
       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 .subExpression(_):
+      throw TemplateSyntaxError("'if' expression error: sub expression was called with a left hand side")
     case .end:
       throw TemplateSyntaxError("'if' expression error: end")
     }
@@ -100,21 +107,71 @@ final class IfExpressionParser {
   let tokens: [IfToken]
   var position: Int = 0
 
-  init(components: [String], tokenParser: TokenParser, token: Token) throws {
+  private init(tokens: [IfToken]) {
-    self.tokens = try components.map { component in
+    self.tokens = tokens
-      if let op = findOperator(name: component) {
+  }
-        switch op {
+  
-        case .infix(let name, let bindingPower, let cls):
+  static func parser(components: [String], tokenParser: TokenParser, token: Token) throws -> IfExpressionParser {
-          return .infix(name: name, bindingPower: bindingPower, op: cls)
+    return try IfExpressionParser(components: ArraySlice(components), tokenParser: tokenParser, token: token)
-        case .prefix(let name, let bindingPower, let cls):
+  }
-          return .prefix(name: name, bindingPower: bindingPower, op: cls)
-        }
-      }
 
-      return .variable(try tokenParser.compileResolvable(component, containedIn: token))
+  private init(components: ArraySlice<String>, tokenParser: TokenParser, token: Token) throws {
+    var parsedComponents = Set<Int>()
+    var bracketsBalance = 0
+    self.tokens = try zip(components.indices, components).flatMap { (index, component) in
+      guard !parsedComponents.contains(index) else { return nil }
+
+      if component == "(" {
+        bracketsBalance += 1
+        let (expression, parsedCount) = try IfExpressionParser.subExpression(
+          from: components.suffix(from: index + 1),
+          tokenParser: tokenParser,
+          token: token
+        )
+        parsedComponents.formUnion(Set(index...(index + parsedCount)))
+        return .subExpression(expression)
+      } else if component == ")" {
+        bracketsBalance -= 1
+        if bracketsBalance < 0 {
+          throw TemplateSyntaxError("'if' expression error: missing opening bracket")
+        }
+        parsedComponents.insert(index)
+        return nil
+      } else {
+        parsedComponents.insert(index)
+        if let op = findOperator(name: component) {
+          switch op {
+          case .infix(let name, let bindingPower, let operatorType):
+            return .infix(name: name, bindingPower: bindingPower, operatorType: operatorType)
+          case .prefix(let name, let bindingPower, let operatorType):
+            return .prefix(name: name, bindingPower: bindingPower, operatorType: operatorType)
+          }
+        }
+        return .variable(try tokenParser.compileResolvable(component, containedIn: token))
+      }
     }
   }
 
+  private static func subExpression(from components: ArraySlice<String>, tokenParser: TokenParser, token: Token) throws -> (Expression, Int) {
+    var bracketsBalance = 1
+    let subComponents = components
+      .prefix(while: {
+        if $0 == "(" {
+            bracketsBalance += 1
+        } else if $0 == ")" {
+            bracketsBalance -= 1
+        }
+        return bracketsBalance != 0
+      })
+    if bracketsBalance > 0 {
+      throw TemplateSyntaxError("'if' expression error: missing closing bracket")
+    }
+
+    let expressionParser = try IfExpressionParser(components: subComponents, tokenParser: tokenParser, token: token)
+    let expression = try expressionParser.parse()
+    return (expression, subComponents.count)
+  }
+  
   var currentToken: IfToken {
     if tokens.count > position {
       return tokens[position]
@@ -154,13 +211,11 @@ final class IfExpressionParser {
   }
 }
 
-
 func parseExpression(components: [String], tokenParser: TokenParser, token: Token) throws -> Expression {
-  let parser = try IfExpressionParser(components: components, tokenParser: tokenParser, token: token)
+  let parser = try IfExpressionParser.parser(components: components, tokenParser: tokenParser, token: token)
   return try parser.parse()
 }
 
-
 /// Represents an if condition and the associated nodes when the condition
 /// evaluates
 final class IfCondition {

Tests/StencilTests/ExpressionSpec.swift

@@ -6,6 +6,11 @@ func testExpressions() {
   describe("Expression") {
     let parser = TokenParser(tokens: [], environment: Environment())
 
+    func parseExpression(components: [String]) throws -> Expression {
+      let parser = try IfExpressionParser.parser(components: components, tokenParser: parser, token: .text(value: "", at: .unknown))
+      return try parser.parse()
+    }
+
     $0.describe("VariableExpression") {
       let expression = VariableExpression(variable: Variable("value"))
 
@@ -105,19 +110,19 @@ func testExpressions() {
 
     $0.describe("expression parsing") {
       $0.it("can parse a variable expression") {
-        let expression = try parseExpression(components: ["value"], tokenParser: parser, token: .text(value: "", at: .unknown))
+        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"], tokenParser: parser, token: .text(value: "", at: .unknown))
+        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"], tokenParser: parser, token: .text(value: "", at: .unknown))
+        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()
@@ -137,7 +142,7 @@ func testExpressions() {
       }
 
       $0.describe("or expression") {
-        let expression = try! parseExpression(components: ["lhs", "or", "rhs"], tokenParser: parser, token: .text(value: "", at: .unknown))
+        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()
@@ -157,7 +162,7 @@ func testExpressions() {
       }
 
       $0.describe("equality expression") {
-        let expression = try! parseExpression(components: ["lhs", "==", "rhs"], tokenParser: parser, token: .text(value: "", at: .unknown))
+        let expression = try! parseExpression(components: ["lhs", "==", "rhs"])
 
         $0.it("evaluates to true with equal lhs/rhs") {
           try expect(expression.evaluate(context: Context(dictionary: ["lhs": "a", "rhs": "a"]))).to.beTrue()
@@ -193,7 +198,7 @@ func testExpressions() {
       }
 
       $0.describe("inequality expression") {
-        let expression = try! parseExpression(components: ["lhs", "!=", "rhs"], tokenParser: parser, token: .text(value: "", at: .unknown))
+        let expression = try! parseExpression(components: ["lhs", "!=", "rhs"])
 
         $0.it("evaluates to true with inequal lhs/rhs") {
           try expect(expression.evaluate(context: Context(dictionary: ["lhs": "a", "rhs": "b"]))).to.beTrue()
@@ -205,7 +210,7 @@ func testExpressions() {
       }
 
       $0.describe("more than expression") {
-        let expression = try! parseExpression(components: ["lhs", ">", "rhs"], tokenParser: parser, token: .text(value: "", at: .unknown))
+        let expression = try! parseExpression(components: ["lhs", ">", "rhs"])
 
         $0.it("evaluates to true with lhs > rhs") {
           try expect(expression.evaluate(context: Context(dictionary: ["lhs": 5.0, "rhs": 4]))).to.beTrue()
@@ -217,7 +222,7 @@ func testExpressions() {
       }
 
       $0.describe("more than equal expression") {
-        let expression = try! parseExpression(components: ["lhs", ">=", "rhs"], tokenParser: parser, token: .text(value: "", at: .unknown))
+        let expression = try! parseExpression(components: ["lhs", ">=", "rhs"])
 
         $0.it("evaluates to true with lhs == rhs") {
           try expect(expression.evaluate(context: Context(dictionary: ["lhs": 5.0, "rhs": 5]))).to.beTrue()
@@ -229,7 +234,7 @@ func testExpressions() {
       }
 
       $0.describe("less than expression") {
-        let expression = try! parseExpression(components: ["lhs", "<", "rhs"], tokenParser: parser, token: .text(value: "", at: .unknown))
+        let expression = try! parseExpression(components: ["lhs", "<", "rhs"])
 
         $0.it("evaluates to true with lhs < rhs") {
           try expect(expression.evaluate(context: Context(dictionary: ["lhs": 4, "rhs": 4.5]))).to.beTrue()
@@ -241,7 +246,7 @@ func testExpressions() {
       }
 
       $0.describe("less than equal expression") {
-        let expression = try! parseExpression(components: ["lhs", "<=", "rhs"], tokenParser: parser, token: .text(value: "", at: .unknown))
+        let expression = try! parseExpression(components: ["lhs", "<=", "rhs"])
 
         $0.it("evaluates to true with lhs == rhs") {
           try expect(expression.evaluate(context: Context(dictionary: ["lhs": 5.0, "rhs": 5]))).to.beTrue()
@@ -253,7 +258,7 @@ func testExpressions() {
       }
 
       $0.describe("multiple expression") {
-        let expression = try! parseExpression(components: ["one", "or", "two", "and", "not", "three"], tokenParser: parser, token: .text(value: "", at: .unknown))
+        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()
@@ -281,7 +286,7 @@ func testExpressions() {
       }
       
       $0.describe("in expression") {
-        let expression = try! parseExpression(components: ["lhs", "in", "rhs"], tokenParser: parser, token: .text(value: "", at: .unknown))
+        let expression = try! parseExpression(components: ["lhs", "in", "rhs"])
         
         $0.it("evaluates to true when rhs contains lhs") {
           try expect(expression.evaluate(context: Context(dictionary: ["lhs": 1, "rhs": [1, 2, 3]]))).to.beTrue()
@@ -299,6 +304,41 @@ func testExpressions() {
           try expect(expression.evaluate(context: Context(dictionary: ["lhs": 3, "rhs": 1..<3]))).to.beFalse()
         }
       }
+
+      $0.describe("sub expression") {
+        $0.it("evaluates correctly") {
+          let context = Context(dictionary: ["one": false, "two": false, "three": true, "four": true])
+
+          let expression = try! parseExpression(components: ["one", "and", "two", "or", "three", "and", "four"])
+          let expressionWithBrackets = try! parseExpression(components: ["one", "and", "(", "(", "two", ")", "or", "(", "three", "and", "four", ")", ")"])
+
+          try expect(expression.evaluate(context: context)).to.beTrue()
+          try expect(expressionWithBrackets.evaluate(context: context)).to.beFalse()
+
+          let notExpression = try! parseExpression(components: ["not", "one", "or", "three"])
+          let notExpressionWithBrackets = try! parseExpression(components: ["not", "(", "one", "or", "three", ")"])
+
+          try expect(notExpression.evaluate(context: context)).to.beTrue()
+          try expect(notExpressionWithBrackets.evaluate(context: context)).to.beFalse()
+        }
+
+        $0.it("fails when brackets are not balanced") {
+          try expect(parseExpression(components: ["(", "lhs", "and", "rhs"]))
+            .toThrow(TemplateSyntaxError("'if' expression error: missing closing bracket"))
+          try expect(parseExpression(components: [")", "lhs", "and", "rhs"]))
+            .toThrow(TemplateSyntaxError("'if' expression error: missing opening bracket"))
+          try expect(parseExpression(components: ["lhs", "and", "rhs", ")"]))
+            .toThrow(TemplateSyntaxError("'if' expression error: missing opening bracket"))
+          try expect(parseExpression(components: ["(", "lhs", "and", "rhs", ")", "("]))
+            .toThrow(TemplateSyntaxError("'if' expression error: missing closing bracket"))
+          try expect(parseExpression(components: ["(", "lhs", "and", "rhs", ")", ")"]))
+            .toThrow(TemplateSyntaxError("'if' expression error: missing opening bracket"))
+          try expect(parseExpression(components: ["(", "lhs", "and", ")"]))
+            .toThrow(TemplateSyntaxError("'if' expression error: end"))
+          try expect(parseExpression(components: ["(", "and", "rhs", ")"]))
+            .toThrow(TemplateSyntaxError("'if' expression error: infix operator 'and' doesn't have a left hand side"))
+        }
+      }
     }
   }
 }

docs/builtins.rst

@@ -149,6 +149,19 @@ Will be treated as:
 
     one or (two and three)
 
+You can use parentheses to change operator precedence. For example:
+
+.. code-block:: html+django
+
+    {% if (one or two) and three %}
+
+Will be treated as:
+
+.. code-block:: text
+
+    (one or two) and three
+
+
 ``==`` operator
 """""""""""""""