channingwalton / transaction-monad

A Monad that represents a transaction with success, failure, and post commit functions

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The Transaction Monad

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The Transaction Monad represents a transactional operation that may or may not be successful (much like Either), with functions to run depending on the success of the transaction.


In several projects we ended up with a monad transformer stack of the form:

final case class PostCommit(description: String, item: () => Unit) 
type IOWriter[A] = WriterT[IO, List[PostCommit], A]
type Transaction[T] = EitherT[IOWriter, String, T]

Where IO is an effect like scalaz or cats-effect IO, and PostCommit is a function to run when the Transaction runs successfully.

The PostCommit functions tend to be things like sending an email or updating another system after a workflow transition has occurred. We don't want the email sent if the workflow transition failed to be successfully committed. (e.g. "Thanks for the $1M you've deposited today." when the deposit failed.)

However, this effect stack is a little cumbersome so this project wraps that up for you.


import com.casualmiracles.transaction._

for all the functions you need to build a Transaction[F[], E, A]_ with a success value, a failure, a log, or postrun functions.

Transaction Builder

The TransactionBuilder helps you build Transaction instances for a given effect and also to run the transaction.

A note of caution. Once you've constructed this builder, you can import builder._ so that you can use its methods, its syntax, and get the list monoid so you don't need to remember to import it from cats. This last point is important because if you don't get the list monoid, then you will get complex complication errors about missing implicit instances for Monad that will mislead and frustrate you.

TransactionTest gives some examples of use.

The TransactionRunner is used to run a transaction for a given effect.

Cats Effect

The core module is supplemented by a second module, cats-effect, that provides TransactionIO to build and run a Transaction[cats.effect.IO, E, A], and a TransactionRunner[IO].

See DoobieExample

Using the Transaction Monad

libraryDependencies ++= Seq(
  "com.casualmiracles" %% "transaction-core" % "0.0.16",
  "org.typelevel" %% "cats-core" % "1.0.0")
libraryDependencies ++= Seq(
  "com.casualmiracles" %% "transaction-cats-effect" % "0.0.16",
  "org.typelevel" %% "cats-effect" % "0.5")