Implement Alternative for Array

thoughtbot · Dec 5, 2016 · 637a360 · 637a360
1 parent 31430d5
commit 637a360
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diff --git a/Runes.xcodeproj/project.pbxproj b/Runes.xcodeproj/project.pbxproj
@@ -39,6 +39,10 @@
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+		F8DDC6EF1DD67D2600E173E0 /* Array+Alternative.swift in Sources */ = {isa = PBXBuildFile; fileRef = F8DDC6EE1DD67D2600E173E0 /* Array+Alternative.swift */; };
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@@ -149,6 +153,7 @@
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@@ -335,6 +340,7 @@
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 				F8E09D161DCF83F200816E60 /* Array+Functor.swift */,
 				F8E09D171DCF83F200816E60 /* Array+Monad.swift */,
@@ -640,6 +646,7 @@
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@@ -665,6 +672,7 @@
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@@ -680,6 +688,7 @@
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@@ -715,6 +724,7 @@
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 				F8E09D321DCF83F200816E60 /* Optional+Monad.swift in Sources */,
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diff --git a/Sources/Runes/Array/Array+Alternative.swift b/Sources/Runes/Array/Array+Alternative.swift
@@ -0,0 +1,22 @@
+/**
+  Return the result of concatenating two arrays
+
+  - parameter lhs: A value of type `[T]`
+  - parameter rhs: A value of type `[T]`
+
+  - returns: The result of concatenating `lhs` and `rhs`
+*/
+public func <|> <T>(lhs: [T], rhs: @autoclosure () -> [T]) -> [T] {
+  return lhs + rhs()
+}
+
+/**
+  Return an empty context of `[]`
+
+  This is the dual of `pure`.
+
+  - returns: An instance of `[]` of the type `[T]`
+*/
+public func empty<T>() -> [T] {
+  return []
+}
diff --git a/Tests/RunesTests/ArraySpec.swift b/Tests/RunesTests/ArraySpec.swift
@@ -90,6 +90,29 @@ class ArraySpec: XCTestCase {
     }
   }
 
+  func testAlternative() {
+    property("alternative operator - left empty") <- forAll { (x: Int) in
+      let lhs: [Int] = empty() <|> pure(x)
+      let rhs: [Int] = pure(x)
+
+      return lhs == rhs
+    }
+
+    property("alternative operator - right empty") <- forAll { (x: Int) in
+      let lhs: [Int] = pure(x) <|> empty()
+      let rhs: [Int] = pure(x)
+
+      return lhs == rhs
+    }
+
+    property("alternative operator - neither empty") <- forAll { (x: Int, y: Int) in
+      let lhs: [Int] = pure(x) <|> pure(y)
+      let rhs: [Int] = pure(x) + pure(y)
+
+      return lhs == rhs
+    }
+  }
+
   func testMonad() {
     // return x >>= f = f x
     property("left identity law") <- forAll { (x: Int, fa: ArrowOf<Int, Int>) in