From 989d72857a9b3ec6c3fdad562a141be681c507e2 Mon Sep 17 00:00:00 2001
From: Dave Gurnell <dave@untyped.com>
Date: Tue, 8 Mar 2016 22:28:07 +0000
Subject: [PATCH] Add .tell and .writer syntax for creating Writers. Fixes
 #920.

---
 core/src/main/scala/cats/syntax/all.scala     |  1 +
 core/src/main/scala/cats/syntax/package.scala |  1 +
 core/src/main/scala/cats/syntax/writer.scala  | 13 ++++++++++
 .../test/scala/cats/tests/WriterTests.scala   | 26 +++++++++++++++++++
 4 files changed, 41 insertions(+)
 create mode 100644 core/src/main/scala/cats/syntax/writer.scala
 create mode 100644 tests/src/test/scala/cats/tests/WriterTests.scala

diff --git a/core/src/main/scala/cats/syntax/all.scala b/core/src/main/scala/cats/syntax/all.scala
index 2a2f68faef..b99ed8785f 100644
--- a/core/src/main/scala/cats/syntax/all.scala
+++ b/core/src/main/scala/cats/syntax/all.scala
@@ -37,3 +37,4 @@ trait AllSyntax
     with XorSyntax
     with ValidatedSyntax
     with CoproductSyntax
+    with WriterSyntax
diff --git a/core/src/main/scala/cats/syntax/package.scala b/core/src/main/scala/cats/syntax/package.scala
index 154a9e94cf..26ec7fd4f5 100644
--- a/core/src/main/scala/cats/syntax/package.scala
+++ b/core/src/main/scala/cats/syntax/package.scala
@@ -35,4 +35,5 @@ package object syntax {
   object traverse extends TraverseSyntax
   object xor extends XorSyntax
   object validated extends ValidatedSyntax
+  object writer extends WriterSyntax
 }
diff --git a/core/src/main/scala/cats/syntax/writer.scala b/core/src/main/scala/cats/syntax/writer.scala
new file mode 100644
index 0000000000..41dab81a63
--- /dev/null
+++ b/core/src/main/scala/cats/syntax/writer.scala
@@ -0,0 +1,13 @@
+package cats
+package syntax
+
+import cats.data.Writer
+
+trait WriterSyntax {
+  implicit def writerIdSyntax[A](a: A): WriterIdSyntax[A] = new WriterIdSyntax(a)
+}
+
+final class WriterIdSyntax[A](val a: A) extends AnyVal {
+  def tell: Writer[A, Unit] = Writer(a, ())
+  def writer[W](w: W): Writer[W, A] = Writer(w, a)
+}
diff --git a/tests/src/test/scala/cats/tests/WriterTests.scala b/tests/src/test/scala/cats/tests/WriterTests.scala
new file mode 100644
index 0000000000..6b93c1ef72
--- /dev/null
+++ b/tests/src/test/scala/cats/tests/WriterTests.scala
@@ -0,0 +1,26 @@
+package cats
+package tests
+
+import cats.data.Writer
+import cats.laws.discipline.eq._
+
+class WriterTests extends CatsSuite {
+  test("pure syntax creates a writer with an empty log"){
+    forAll { (result: String) =>
+      type Logged[A] = Writer[List[Int], A]
+      result.pure[Logged] should === (Writer(List.empty[Int], result))
+    }
+  }
+
+  test("tell syntax creates a writer with a unit result"){
+    forAll { (log: List[Int]) =>
+      log.tell should === (Writer(log, ()))
+    }
+  }
+
+  test("writer syntax creates a writer with the specified result and log") {
+    forAll { (result: String, log: List[Int]) =>
+      result.writer(log) should === (Writer(log, result))
+    }
+  }
+}