Merge pull request twitter#511 from NathanHowell/identity

Identity class and Monad instance

algebird-core/src/main/scala/com/twitter/algebird/Identity.scala

@@ -0,0 +1,71 @@
+/*
+Copyright 2016 GoDaddy, Inc.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package com.twitter.algebird
+
+case class Identity[T](get: T)
+
+object IdentityMonad extends Monad[Identity] {
+  override def map[T, U](m: Identity[T])(fn: T => U): Identity[U] = {
+    Identity(fn(m.get))
+  }
+
+  override def apply[T](v: T): Identity[T] = {
+    Identity(v)
+  }
+
+  override def join[T, U](mt: Identity[T], mu: Identity[U]): Identity[(T, U)] = {
+    Identity((mt.get, mu.get))
+  }
+
+  override def flatMap[T, U](m: Identity[T])(fn: T => Identity[U]): Identity[U] = {
+    fn(m.get)
+  }
+
+  override def sequence[T](ms: Seq[Identity[T]]): Identity[Seq[T]] = {
+    Identity(ms.map(_.get))
+  }
+
+  override def joinWith[T, U, V](mt: Identity[T], mu: Identity[U])(fn: (T, U) => V): Identity[V] = {
+    Identity(fn(mt.get, mu.get))
+  }
+
+  override def join[T1, T2, T3](m1: Identity[T1], m2: Identity[T2], m3: Identity[T3]): Identity[(T1, T2, T3)] = {
+    Identity((m1.get, m2.get, m3.get))
+  }
+
+  override def join[T1, T2, T3, T4](m1: Identity[T1], m2: Identity[T2], m3: Identity[T3], m4: Identity[T4]): Identity[(T1, T2, T3, T4)] = {
+    Identity((m1.get, m2.get, m3.get, m4.get))
+  }
+
+  override def join[T1, T2, T3, T4, T5](m1: Identity[T1], m2: Identity[T2], m3: Identity[T3], m4: Identity[T4], m5: Identity[T5]): Identity[(T1, T2, T3, T4, T5)] = {
+    Identity((m1.get, m2.get, m3.get, m4.get, m5.get))
+  }
+}
+
+object Identity {
+  implicit def identityFunctor: Functor[Identity] = {
+    IdentityMonad
+  }
+
+  implicit def identityApplicative: Applicative[Identity] = {
+    IdentityMonad
+  }
+
+  implicit def identityMonad: Monad[Identity] = {
+    IdentityMonad
+  }
+}

algebird-test/src/main/scala/com/twitter/algebird/MonadLaws.scala

@@ -65,4 +65,7 @@ object MonadLaws {
 
   implicit def someA[T](implicit arbl: Arbitrary[T]): Arbitrary[Some[T]] =
     Arbitrary { arbl.arbitrary.map { l => Some(l) } }
+
+  implicit def identityA[T](implicit arbl: Arbitrary[T]): Arbitrary[Identity[T]] =
+    Arbitrary { arbl.arbitrary.map { l => Identity(l) } }
 }

algebird-test/src/test/scala/com/twitter/algebird/ApplicativeProperties.scala

@@ -37,16 +37,23 @@ class ApplicativeProperties extends CheckProperties {
   property("indexedseq") {
     applicativeLaws[IndexedSeq, Int, String, Long]()
   }
+
   property("vector") {
     applicativeLaws[Vector, Int, String, Long]()
   }
 
   property("set") {
     applicativeLaws[Set, Int, String, Long]()
   }
+
   property("seq") {
     applicativeLaws[Seq, Int, String, Long]()
   }
+
+  property("identity") {
+    applicativeLaws[Identity, Int, String, Long]()
+  }
+
   property("sequenceGen") {
     // This follows from the laws, so we are just testing
     // the implementation of sequenceGen against sequence here

algebird-test/src/test/scala/com/twitter/algebird/FunctorProperties.scala

@@ -18,7 +18,8 @@ package com.twitter.algebird
 
 class FunctorProperties extends CheckProperties {
   import com.twitter.algebird.FunctorLaws._
-  import com.twitter.algebird.Monad._ // for Arbitrary instances
+  import com.twitter.algebird.Monad._ // for Functor instances
+  import com.twitter.algebird.MonadLaws._ // for Arbitrary instances
 
   property("list") {
     functorLaws[List, Int, String, Long]()
@@ -44,4 +45,7 @@ class FunctorProperties extends CheckProperties {
     functorLaws[Seq, Int, String, Long]()
   }
 
+  property("identity") {
+    functorLaws[Identity, Int, String, Long]()
+  }
 }

algebird-test/src/test/scala/com/twitter/algebird/MonadProperties.scala

@@ -48,4 +48,8 @@ class MonadProperties extends CheckProperties {
   property("seq") {
     monadLaws[Seq, Int, String, Long]()
   }
+
+  property("identity") {
+    monadLaws[Identity, Int, String, Long]()
+  }
 }