First stab at thinking in terms of composable programs.

Author: kryptt

algebraic-data-types/src/main/tut/index.html

@@ -210,6 +210,40 @@
 
 ---
 
+# Equational reasoning at the program level
+
+<small>
+```scala
+def create(ca: CreateAccount) = Kleisli { (s: Session) =>
+  (for {
+    uid  <- ca.owner.fold(
+      id    => id.point[BaseResultTaskET],
+      cu    => EitherT(UserService.create(cu, webUrl).run(s)).map(_._1))
+    _     = logger.trace(s"createUser uid:$uid")
+    bid  <- ca.billing_address.fold(
+      bid   => bid.point[BaseResultTaskET],
+      fa    => EitherT(AddressService.create(fa).run(s)))
+    _     = logger.trace(s"createAddress bid:$bid")
+    cap   = ca.parsed(uid, bid, ca.ig.fold(_.some, _ => none, _ => none))
+    aid  <- createAccount(cap).leftMap[BaseResultTask](BadRequest(_))
+    _    <- ca.ig.fold(
+      _     => ().point[BaseResultTaskET],
+      csc   => EitherT(SchoolService.create(csc.createSchool(aid.some, bid.left)).run(s)),
+      csg   => EitherT(InstitutionGroupService.create(csg.createIG(aid.some)).run(s)))
+    _     = logger.trace(s"createAccount aid:$aid")
+  } yield (aid, "Account created successfully")).run
+}
+```
+
+Given a `CreateAccount`, we can construct a program that goes from a user
+`Session` to a `Program[A] = ConnectionIO[BaseResultTask \/ A]`. Lets name it
+```
+type SessionResult[A] = Kleisli[ConnectionIO, Session, BaseResultTask \/ A]
+```
+</small>
+
+---
+
 # References
 
 * [Algebraic Data Types](http://tpolecat.github.io/presentations/algebraic_types.html) by Rob Norris (@tpolecat)
@@ -220,8 +254,11 @@
 
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