Add .tell and .writer syntax for creating Writers. Fixes #920.

core/src/main/scala/cats/syntax/all.scala

@@ -37,3 +37,4 @@ trait AllSyntax
     with XorSyntax
     with ValidatedSyntax
     with CoproductSyntax
+    with WriterSyntax

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
 }

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)
+}

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))
+    }
+  }
+}