Merge pull request #325 from stencilproject/feature/resolvable-expressions

Resolvable boolean expressions

CHANGELOG.md

@@ -36,6 +36,11 @@
 - You can now access outer loop's scope by labeling it: `{% outer: for ... %}... {% for ... %} {{ outer.counter }} {% endfor %}{% endfor %}`.  
   [Ilya Puchka](https://github.com/ilyapuchka)
   [#175](https://github.com/stencilproject/Stencil/pull/175)
+- Boolean expressions can now be rendered, i.e `{{ name == "John" }}` will render `true` or `false` depending on the evaluation result.  
+  [Ilya Puchka](https://github.com/ilyapuchka)
+  [David Jennes](https://github.com/djbe)
+  [#164](https://github.com/stencilproject/Stencil/pull/164)
+  [#325](https://github.com/stencilproject/Stencil/pull/325)
 
 ### Deprecations
 

Sources/Stencil/Expression.swift

@@ -1,7 +1,13 @@
-public protocol Expression: CustomStringConvertible {
+public protocol Expression: CustomStringConvertible, Resolvable {
   func evaluate(context: Context) throws -> Bool
 }
 
+extension Expression {
+  func resolve(_ context: Context) throws -> Any? {
+    try "\(evaluate(context: context))"
+  }
+}
+
 protocol InfixOperator: Expression {
   init(lhs: Expression, rhs: Expression)
 }
@@ -37,8 +43,12 @@ final class VariableExpression: Expression, CustomStringConvertible {
     "(variable: \(variable))"
   }
 
+  func resolve(_ context: Context) throws -> Any? {
+    try variable.resolve(context)
+  }
+
   /// Resolves a variable in the given context as boolean
-  func resolve(context: Context, variable: Resolvable) throws -> Bool {
+  func evaluate(context: Context) throws -> Bool {
     let result = try variable.resolve(context)
     var truthy = false
 
@@ -58,10 +68,6 @@ final class VariableExpression: Expression, CustomStringConvertible {
 
     return truthy
   }
-
-  func evaluate(context: Context) throws -> Bool {
-    try resolve(context: context, variable: variable)
-  }
 }
 
 final class NotExpression: Expression, PrefixOperator, CustomStringConvertible {

Sources/Stencil/Node.swift

@@ -93,30 +93,36 @@ public class VariableNode: NodeType {
     func hasToken(_ token: String, at index: Int) -> Bool {
       components.count > (index + 1) && components[index] == token
     }
+    func compileResolvable(_ components: [String], containedIn token: Token) throws -> Resolvable {
+      try (try? parser.compileExpression(components: components, token: token)) ??
+        parser.compileFilter(components.joined(separator: " "), containedIn: token)
+    }
 
+    let variable: Resolvable
     let condition: Expression?
     let elseExpression: Resolvable?
 
     if hasToken("if", at: 1) {
+      variable = try compileResolvable([components[0]], containedIn: token)
+
       let components = components.suffix(from: 2)
       if let elseIndex = components.firstIndex(of: "else") {
         condition = try parser.compileExpression(components: Array(components.prefix(upTo: elseIndex)), token: token)
-        let elseToken = components.suffix(from: elseIndex.advanced(by: 1)).joined(separator: " ")
+        let elseToken = Array(components.suffix(from: elseIndex.advanced(by: 1)))
-        elseExpression = try parser.compileResolvable(elseToken, containedIn: token)
+        elseExpression = try compileResolvable(elseToken, containedIn: token)
       } else {
         condition = try parser.compileExpression(components: Array(components), token: token)
         elseExpression = nil
       }
-    } else {
+    } else if !components.isEmpty {
+      variable = try compileResolvable(components, containedIn: token)
       condition = nil
       elseExpression = nil
-    }
+    } else {
-
-    guard let resolvable = components.first else {
       throw TemplateSyntaxError(reason: "Missing variable name", token: token)
     }
-    let filter = try parser.compileResolvable(resolvable, containedIn: token)
+
-    return VariableNode(variable: filter, token: token, condition: condition, elseExpression: elseExpression)
+    return VariableNode(variable: variable, token: token, condition: condition, elseExpression: elseExpression)
   }
 
   public init(variable: Resolvable, token: Token? = nil) {

Tests/StencilTests/ExpressionSpec.swift

@@ -115,12 +115,12 @@ final class ExpressionsTests: XCTestCase {
 
   func testNotExpression() {
     it("returns truthy for positive expressions") {
-      let expression = NotExpression(expression: StaticExpression(value: true))
+      let expression = NotExpression(expression: VariableExpression(variable: Variable("true")))
       try expect(expression.evaluate(context: Context())).to.beFalse()
     }
 
     it("returns falsy for negative expressions") {
-      let expression = NotExpression(expression: StaticExpression(value: false))
+      let expression = NotExpression(expression: VariableExpression(variable: Variable("false")))
       try expect(expression.evaluate(context: Context())).to.beTrue()
     }
   }

Tests/StencilTests/IfNodeSpec.swift

@@ -200,8 +200,8 @@ final class IfNodeTests: XCTestCase {
   func testRendering() {
     it("renders a true expression") {
       let node = IfNode(conditions: [
-        IfCondition(expression: StaticExpression(value: true), nodes: [TextNode(text: "1")]),
+        IfCondition(expression: VariableExpression(variable: Variable("true")), nodes: [TextNode(text: "1")]),
-        IfCondition(expression: StaticExpression(value: true), nodes: [TextNode(text: "2")]),
+        IfCondition(expression: VariableExpression(variable: Variable("true")), nodes: [TextNode(text: "2")]),
         IfCondition(expression: nil, nodes: [TextNode(text: "3")])
       ])
 
@@ -210,8 +210,8 @@ final class IfNodeTests: XCTestCase {
 
     it("renders the first true expression") {
       let node = IfNode(conditions: [
-        IfCondition(expression: StaticExpression(value: false), nodes: [TextNode(text: "1")]),
+        IfCondition(expression: VariableExpression(variable: Variable("false")), nodes: [TextNode(text: "1")]),
-        IfCondition(expression: StaticExpression(value: true), nodes: [TextNode(text: "2")]),
+        IfCondition(expression: VariableExpression(variable: Variable("true")), nodes: [TextNode(text: "2")]),
         IfCondition(expression: nil, nodes: [TextNode(text: "3")])
       ])
 
@@ -220,8 +220,8 @@ final class IfNodeTests: XCTestCase {
 
     it("renders the empty expression when other conditions are falsy") {
       let node = IfNode(conditions: [
-        IfCondition(expression: StaticExpression(value: false), nodes: [TextNode(text: "1")]),
+        IfCondition(expression: VariableExpression(variable: Variable("false")), nodes: [TextNode(text: "1")]),
-        IfCondition(expression: StaticExpression(value: false), nodes: [TextNode(text: "2")]),
+        IfCondition(expression: VariableExpression(variable: Variable("false")), nodes: [TextNode(text: "2")]),
         IfCondition(expression: nil, nodes: [TextNode(text: "3")])
       ])
 
@@ -230,8 +230,8 @@ final class IfNodeTests: XCTestCase {
 
     it("renders empty when no truthy conditions") {
       let node = IfNode(conditions: [
-        IfCondition(expression: StaticExpression(value: false), nodes: [TextNode(text: "1")]),
+        IfCondition(expression: VariableExpression(variable: Variable("false")), nodes: [TextNode(text: "1")]),
-        IfCondition(expression: StaticExpression(value: false), nodes: [TextNode(text: "2")])
+        IfCondition(expression: VariableExpression(variable: Variable("false")), nodes: [TextNode(text: "2")])
       ])
 
       try expect(try node.render(Context())) == ""

Tests/StencilTests/NodeSpec.swift

@@ -90,4 +90,22 @@ final class NodeTests: XCTestCase {
       try expect(try renderNodes(nodes, self.context)).toThrow(TemplateSyntaxError("Custom Error"))
     }
   }
+
+  func testRenderingBooleans() {
+    it("can render true & false") {
+      try expect(Template(templateString: "{{ true }}").render()) == "true"
+      try expect(Template(templateString: "{{ false }}").render()) == "false"
+    }
+
+    it("can resolve variable") {
+      let template = Template(templateString: "{{ value == \"known\" }}")
+      try expect(template.render(["value": "known"])) == "true"
+      try expect(template.render(["value": "unknown"])) == "false"
+    }
+
+    it("can render a boolean expression") {
+      try expect(Template(templateString: "{{ 1 > 0 }}").render()) == "true"
+      try expect(Template(templateString: "{{ 1 == 2 }}").render()) == "false"
+    }
+  }
 }

docs/templates.rst

@@ -50,6 +50,17 @@ For example, if you have the following context:
 
     The result of {{ item[key] }} will be the same as {{ item.name }}. It will first evaluate the result of {{ key }}, and only then evaluate the lookup expression.
 
+Boolean expressions
+-------------------
+
+Boolean expressions can be rendered using ``{{ ... }}`` tag.
+For example, this will output string `true` if variable is equal to 1 and `false` otherwise:
+
+.. code-block:: html+django
+
+    {{ variable == 1 }}
+
+
 Filters
 ~~~~~~~