Add a not expectation property returning an opposite expectation

CHANGELOG.md

@@ -5,6 +5,12 @@
 ### Enhancements
 
 - Unhandled errors will now be reported from the invoked cases source map.
+- New `not` expectation method which returns a Not Expectation. This allows you
+  to run expecations such as:
+
+  ```swift
+  try expect(...).to.not.beNil()
+  ```
 
 ## 0.8.0
 

Sources/Spectre/Expectation.swift

@@ -5,6 +5,13 @@ public protocol ExpectationType {
 }
 
 
+public protocol NotExpectationType {
+  associatedtype ValueType
+  var expression: () throws -> ValueType? { get }
+  func failure(_ reason: String) -> FailureType
+}
+
+
 struct ExpectationFailure : FailureType {
   let file: String
   let line: Int
@@ -44,6 +51,30 @@ open class Expectation<T> : ExpectationType {
   }
 }
 
+public class NotExpectation<T>: NotExpectationType {
+  public typealias ValueType = T
+  public let expression: () throws -> ValueType?
+
+  let file: String
+  let line: Int
+  let function: String
+
+  public  var to: NotExpectation<T> {
+    return self
+  }
+
+  init(file: String, line: Int, function: String, expression: @escaping () throws -> ValueType?) {
+    self.file = file
+    self.line = line
+    self.function = function
+    self.expression = expression
+  }
+
+  public func failure(_ reason: String) -> FailureType {
+    return ExpectationFailure(reason: reason, file: file, line: line, function: function)
+  }
+}
+
 public func expect<T>( _ expression: @autoclosure @escaping () throws -> T?, file: String = #file, line: Int = #line, function: String = #function) -> Expectation<T> {
   return Expectation(file: file, line: line, function: function, expression: expression)
 }
@@ -52,6 +83,14 @@ public func expect<T>(_ file: String = #file, line: Int = #line, function: Strin
   return Expectation(file: file, line: line, function: function, expression: expression)
 }
 
+// MARK: Not
+
+extension Expectation {
+  public var not: NotExpectation<T> {
+    return NotExpectation(file: file, line: line, function: function, expression: expression)
+  }
+}
+
 // MARK: Equatability
 
 public func == <E: ExpectationType>(lhs: E, rhs: E.ValueType) throws where E.ValueType: Equatable {
@@ -126,6 +165,15 @@ extension ExpectationType {
   }
 }
 
+extension NotExpectationType {
+  public func beNil() throws {
+    let value = try expression()
+    if value == nil {
+      throw failure("value is nil")
+    }
+  }
+}
+
 // MARK: Boolean
 
 extension ExpectationType where ValueType == Bool {

Tests/SpectreTests/ExpectationSpec.swift

@@ -23,6 +23,23 @@ public func testExpectation() {
     $0.it("passes when value is nil") {
       try expect(name).to.beNil()
     }
+
+    $0.context("not") {
+      $0.it("does not error when value is not nil") {
+        do {
+         try expect("kyle").to.not.beNil()
+        } catch {
+          fatalError()
+        }
+      }
+
+      $0.it("errors when value is nil") {
+        do {
+          try expect(name).to.not.beNil()
+          fatalError()
+        } catch {}
+      }
+    }
   }
 
   $0.describe("comparison to type") {