Subscript syntax for Variables (#215)

* Implement variable indirect resolution

* Add some tests

* Changelog entry

* Update documentation

* Rework the syntax to use brackets instead of a $

* Move the lookup parser into it's own file

* Add invalid syntax tests

* Swift 3 support

* Rename some things + extra test

CHANGELOG.md

@@ -6,7 +6,11 @@
 
 - Added an optional second parameter to the `include` tag for passing a sub context to the included file.  
   [Yonas Kolb](https://github.com/yonaskolb)
-  [#394](https://github.com/stencilproject/Stencil/pull/214)
+  [#214](https://github.com/stencilproject/Stencil/pull/214)
+- Variables now support the subscript notation. For example, if you have a variable `key = "name"`, and an
+  object `item = ["name": "John"]`, then `{{ item[key] }}` will evaluate to "John".  
+  [David Jennes](https://github.com/djbe)
+  [#215](https://github.com/stencilproject/Stencil/pull/215)
 
 - Adds support for using spaces in filter expression
   [Ilya Puchka](https://github.com/yonaskolb)
@@ -14,8 +18,8 @@
 
 ### Bug Fixes
 
-- Fixed using quote as a filter parameter
-  [Ilya Puchka](https://github.com/yonaskolb)
+- Fixed using quote as a filter parameter.  
+  [Ilya Puchka](https://github.com/ilyapuchka)
   [#210](https://github.com/stencilproject/Stencil/pull/210)
 
 

Sources/KeyPath.swift

@@ -0,0 +1,112 @@
+import Foundation
+
+/// A structure used to represent a template variable, and to resolve it in a given context.
+final class KeyPath {
+  private var components = [String]()
+  private var current = ""
+  private var partialComponents = [String]()
+  private var subscriptLevel = 0
+
+  let variable: String
+  let context: Context
+
+  // Split the keypath string and resolve references if possible
+  init(_ variable: String, in context: Context) {
+    self.variable = variable
+    self.context = context
+  }
+
+  func parse() throws -> [String] {
+    defer {
+      components = []
+      current = ""
+      partialComponents = []
+      subscriptLevel = 0
+    }
+
+    for c in variable.characters {
+      switch c {
+      case "." where subscriptLevel == 0:
+        try foundSeparator()
+      case "[":
+        try openBracket()
+      case "]":
+        try closeBracket()
+      default:
+        try addCharacter(c)
+      }
+    }
+    try finish()
+
+    return components
+  }
+
+  private func foundSeparator() throws {
+    if !current.isEmpty {
+      partialComponents.append(current)
+    }
+
+    guard !partialComponents.isEmpty else {
+      throw TemplateSyntaxError("Unexpected '.' in variable '\(variable)'")
+    }
+
+    components += partialComponents
+    current = ""
+    partialComponents = []
+  }
+
+  // when opening the first bracket, we must have a partial component
+  private func openBracket() throws {
+    guard !partialComponents.isEmpty || !current.isEmpty else {
+      throw TemplateSyntaxError("Unexpected '[' in variable '\(variable)'")
+    }
+
+    if subscriptLevel > 0 {
+      current.append("[")
+    } else if !current.isEmpty {
+      partialComponents.append(current)
+      current = ""
+    }
+
+    subscriptLevel += 1
+  }
+
+  // for a closing bracket at root level, try to resolve the reference
+  private func closeBracket() throws {
+    guard subscriptLevel > 0 else {
+      throw TemplateSyntaxError("Unbalanced ']' in variable '\(variable)'")
+    }
+
+    if subscriptLevel > 1 {
+      current.append("]")
+    } else if !current.isEmpty,
+      let value = try Variable(current).resolve(context) {
+      partialComponents.append("\(value)")
+      current = ""
+    } else {
+      throw TemplateSyntaxError("Unable to resolve subscript '\(current)' in variable '\(variable)'")
+    }
+
+    subscriptLevel -= 1
+  }
+
+  private func addCharacter(_ c: Character) throws {
+    guard partialComponents.isEmpty || subscriptLevel > 0 else {
+      throw TemplateSyntaxError("Unexpected character '\(c)' in variable '\(variable)'")
+    }
+
+    current.append(c)
+  }
+
+  private func finish() throws {
+    // check if we have a last piece
+    if !current.isEmpty {
+      partialComponents.append(current)
+    }
+    components += partialComponents
+
+    guard subscriptLevel == 0 else {
+      throw TemplateSyntaxError("Unbalanced subscript brackets in variable '\(variable)'")
+    }
+  }
+}

Sources/Variable.swift

@@ -50,8 +50,10 @@ public struct Variable : Equatable, Resolvable {
     self.variable = variable
   }
 
-  fileprivate func lookup() -> [String] {
-    return variable.characters.split(separator: ".").map(String.init)
+  // Split the lookup string and resolve references if possible
+  fileprivate func lookup(_ context: Context) throws -> [String] {
+    var keyPath = KeyPath(variable, in: context)
+    return try keyPath.parse()
   }
 
   /// Resolve the variable in the given context
@@ -75,7 +77,7 @@ public struct Variable : Equatable, Resolvable {
       return bool
     }
 
-    for bit in lookup() {
+    for bit in try lookup(context) {
       current = normalize(current)
 
       if let context = current as? Context {

Tests/StencilTests/VariableSpec.swift

@@ -188,6 +188,98 @@ func testVariable() {
       let result = try variable.resolve(context) as? Int
       try expect(result) == 2
     }
+
+    $0.describe("Subrscripting") {
+      $0.it("can resolve a property subscript via reflection") {
+        try context.push(dictionary: ["property": "name"]) {
+          let variable = Variable("article.author[property]")
+          let result = try variable.resolve(context) as? String
+          try expect(result) == "Kyle"
+        }
+      }
+
+      $0.it("can subscript an array with a valid index") {
+        try context.push(dictionary: ["property": 0]) {
+          let variable = Variable("contacts[property]")
+          let result = try variable.resolve(context) as? String
+          try expect(result) == "Katie"
+        }
+      }
+
+      $0.it("can subscript an array with an unknown index") {
+        try context.push(dictionary: ["property": 5]) {
+          let variable = Variable("contacts[property]")
+          let result = try variable.resolve(context) as? String
+          try expect(result).to.beNil()
+        }
+      }
+
+#if os(OSX)
+      $0.it("can resolve a subscript via KVO") {
+        try context.push(dictionary: ["property": "name"]) {
+          let variable = Variable("object[property]")
+          let result = try variable.resolve(context) as? String
+          try expect(result) == "Foo"
+        }
+      }
+#endif
+
+      $0.it("can resolve an optional subscript via reflection") {
+        try context.push(dictionary: ["property": "featuring"]) {
+          let variable = Variable("blog[property].author.name")
+          let result = try variable.resolve(context) as? String
+          try expect(result) == "Jhon"
+        }
+      }
+
+      $0.it("can resolve multiple subscripts") {
+        try context.push(dictionary: [
+          "prop1": "articles",
+          "prop2": 0,
+          "prop3": "name"
+        ]) {
+          let variable = Variable("blog[prop1][prop2].author[prop3]")
+          let result = try variable.resolve(context) as? String
+          try expect(result) == "Kyle" 
+        }
+      }
+
+      $0.it("can resolve nested subscripts") {
+        try context.push(dictionary: [
+          "prop1": "prop2",
+          "ref": ["prop2": "name"]
+        ]) {
+          let variable = Variable("article.author[ref[prop1]]")
+          let result = try variable.resolve(context) as? String
+          try expect(result) == "Kyle" 
+        }
+      }
+
+      $0.it("throws for invalid keypath syntax") {
+        try context.push(dictionary: ["prop": "name"]) {
+          let samples = [
+            ".",
+            "..",
+            ".test",
+            "test..test",
+            "[prop]",
+            "article.author[prop",
+            "article.author[[prop]",
+            "article.author[prop]]",
+            "article.author[]",
+            "article.author[[]]",
+            "article.author[prop][]",
+            "article.author[prop]comments",
+            "article.author[.]"
+          ]
+
+          for lookup in samples {
+            let variable = Variable(lookup)
+            try expect(variable.resolve(context)).toThrow()  
+          }
+        }
+      }
+    }
   }
 
   describe("RangeVariable") {

docs/templates.rst

@@ -31,6 +31,24 @@ For example, if `people` was an array:
     There are {{ people.count }} people. {{ people.first }} is the first
     person, followed by {{ people.1 }}.
 
+You can also use the subscript operator for indirect evaluation. The expression
+between brackets will be evaluated first, before the actual lookup will happen.
+
+For example, if you have the following context:
+
+.. code-block:: swift
+
+    [
+      "item": [
+        "name": "John"
+      ],
+      "key": "name"
+    ]
+
+.. code-block:: html+django
+
+    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.
+
 Filters
 ~~~~~~~