Close #87 - Add Writer constructor for (W, A)

src/main/scala/just/fp/package.scala

@@ -11,6 +11,8 @@ package object fp extends IdInstance {
   type Writer[L, V] = WriterT[Id, L, V]
 
   object Writer {
-    def apply[W, A](w: W, a: A): WriterT[Id, W, A] = WriterT[Id, W, A]((w, a))
+    def apply[W, A](w: W, a: A): Writer[W, A] = Writer.writer((w, a))
+
+    def writer[W, A](wa: (W, A)): Writer[W, A] = WriterT[Id, W, A](wa)
   }
 }

src/test/scala/just/fp/WriterTSpec.scala

@@ -16,6 +16,7 @@ object WriterTSpec extends Properties {
   , property("testWriterFunctorLaws", WriterFunctorLaws.laws)
   , property("testWriterApplicativeLaws", WriterApplicativeLaws.laws)
   , property("testWriterMonadLaws", WriterMonadLaws.laws)
+  , property("test Writer(w, a) should be equal to Writer.writer((w, a))", testWriterApplyAndWriter)
   )
 
   object WriterTFunctorLaws {
@@ -98,4 +99,16 @@ object WriterTSpec extends Properties {
         , Gens.genAToMonadA(Gens.genIntToInt)
       )
   }
+
+  def testWriterApplyAndWriter: Property = for {
+    w <- Gens.genUnicodeString.log("w")
+    a <- Gens.genIntFromMinToMax.log("a")
+    wa = (w, a)
+  } yield {
+    val writer1 = Writer.writer(wa)
+    val writer2 = Writer(w, a)
+
+    import just.fp.syntax._
+    Result.diffNamed("=== Not Equal ===", writer1, writer2)(_ === _)
+  }
 }