travisbrown · April 28, 2017 02:34 · Jul 24, 2015 · Jul 24, 2015 · Jul 23, 2015 · Jul 23, 2015
diff --git a/indexed-example.scala b/indexed-example.scala
@@ -7,16 +7,9 @@ case object Third extends Status
 
 type StringOr[A] = String \/ A
 
-def f = IndexedStateT.constantIndexedStateT[StringOr, Unit, First.type, Unit](
-  ()
-)(First)
-
-def g = IndexedStateT[StringOr, First.type, Second.type, Unit](s =>
-  (Second, ()).right
-)
-def h = IndexedStateT[StringOr, Second.type, Third.type, Unit](s =>
-  (Third, ()).right
-)
+def f = IndexedStateT.constantIndexedStateT[StringOr, Unit, First.type, Unit](())(First)
+def g = IndexedStateT[StringOr, First.type, Second.type, Unit](_ => (Second, ()).right)
+def h = IndexedStateT[StringOr, Second.type, Third.type, Unit](_ => (Third, ()).right)
 
 for {
   a <- f

diff --git a/indexed-example.scala b/indexed-example.scala
@@ -0,0 +1,34 @@
+import scalaz._, Scalaz._
+
+sealed trait Status
+case object First extends Status
+case object Second extends Status
+case object Third extends Status
+
+type StringOr[A] = String \/ A
+
+def f = IndexedStateT.constantIndexedStateT[StringOr, Unit, First.type, Unit](
+  ()
+)(First)
+
+def g = IndexedStateT[StringOr, First.type, Second.type, Unit](s =>
+  (Second, ()).right
+)
+def h = IndexedStateT[StringOr, Second.type, Third.type, Unit](s =>
+  (Third, ()).right
+)
+
+for {
+  a <- f
+  b <- g
+  c <- h
+} yield c
+
+/* Doesn't compile:
+for {
+  a <- g
+  b <- f
+  c <- h
+} yield c
+*/
+
diff --git a/MonadADT.scala b/MonadADT.scala
@@ -29,7 +29,7 @@ res4: scala.util.Either[String,Status] = Right(Three)
 // make sense per this domain (where all we know is One -> Two -> Three),
 // but of course One, Two and Three constitute Status.
 
-// Can I do better here? I wrote a service in this model,
-// but I'm finding myself adding tests (with a test service)
+// Can I do better here? I wrote a service that's similar to 
+// this example, but I'm finding myself adding tests (with a test service)
 // where `g` returns Right(One). As a result, `h` will return
 // Left("Invalid state. Got One, but expected Two.")
diff --git a/MonadADT.scala b/MonadADT.scala
@@ -29,4 +29,7 @@ res4: scala.util.Either[String,Status] = Right(Three)
 // make sense per this domain (where all we know is One -> Two -> Three),
 // but of course One, Two and Three constitute Status.
 
-Can I do better here? 
+// Can I do better here? I wrote a service in this model,
+// but I'm finding myself adding tests (with a test service)
+// where `g` returns Right(One). As a result, `h` will return
+// Left("Invalid state. Got One, but expected Two.")
diff --git a/MonadADT.scala b/MonadADT.scala
@@ -0,0 +1,32 @@
+// Given the following ADT:
+
+sealed trait Status
+case object One extends Status
+case object Two extends Status
+case object Three extends Status
+
+// They represent states. The natural progression, for this
+// example, is : One -> Two -> Three
+
+// Then, the functions that'll be used in the below monadic chaining.
+// Observe that, it's possible, for an error to occur during f, g, or h
+
+def f: Either[String, Status] = Right(One)
+def g(x: Status): Either[String, Status] = Right(Two)
+def h(x: Status): Either[String, Status] = Right(Three)
+
+// Lastly, a for-comprehension that sequences through f, g and h. 
+// Please ignore the fact that g and h both discard their input
+
+scala> for { 
+     |   a <- f.right
+     |   b <- g(a).right
+     |   c <- h(b).right
+     | } yield c
+res4: scala.util.Either[String,Status] = Right(Three)
+
+// From a type perspectice, `g` could return `Right(One)`. It would not
+// make sense per this domain (where all we know is One -> Two -> Three),
+// but of course One, Two and Three constitute Status.
+
+Can I do better here?