Functional Programming Patterns for the Pragmatic Programmer

Functional Programming
Patterns
(for the pragmatic programmer)
~
@raulraja CTO @47deg

Acknowledgment
• Scalaz
• Rapture : Jon Pretty
• Miles Sabin : Shapeless
• Rúnar Bjarnason : Compositional Application
Architecture With Reasonably Priced Monads
• Noel Markham : A purely functional approach
to building large applications
• Jan Christopher Vogt : Tmaps

Functions are ﬁrst class citizens in FP
Architecture

I want my main app services to strive for
• Composability
• Dependency Injection
• Interpretation
• Fault Tolerance

Composability
Composition gives us the power
to easily mix simple functions
to achieve more complex workﬂows.

Composability
We can achieve monadic function composition
with Kleisli Arrows
A M[B]
In other words a function that
for a given input it returns a type constructor…
List[B], Option[B], Either[B], Task[B],
Future[B]…

Composability
When the type constructor M[_] it's a Monad it
can be composed and sequenced in
for comprehensions
val composed = for {
a <- Kleisli((x : String) Option(x.toInt + 1))
b <- Kleisli((x : String) Option(x.toInt * 2))
} yield a + b

Composability
The deferred injection of the input parameter
enables
Dependency Injection
b <- Kleisli((x : String) Option(x.toInt * 2))
} yield a + b
composed.run("1")

Composability : Kleisli
What about when the args are not of the same
type?
b <- Kleisli((x : Int) Option(x * 2))
} yield a + b

Composability : Kleisli
By using Kleisli we just achieved
• Composability
• Interpretation
• Fault Tolerance

Interpretation : Free Monads
What is a Free Monad?
-- A monad on a custom ADT that can be run
through an Interpreter

sealed trait Op[A]
case class Ask[A](a: () A) extends Op[A]
case class Async[A](a: () A) extends Op[A]
case class Tell(a: () Unit) extends Op[Unit]

What can you achieve with a custom ADT and
Free Monads?
def ask[A](a: A): OpMonad[A] = Free.liftFC(Ask(() a))
def async[A](a: A): OpMonad[A] = Free.liftFC(Async(() a))
def tell(a: Unit): OpMonad[Unit] = Free.liftFC(Tell(() a))

Functors and Monads for Free
(No need to manually implement map, ﬂatMap,
etc...)
type OpMonad[A] = Free.FreeC[Op, A]
implicit val MonadOp: Monad[OpMonad] =
Free.freeMonad[({type λ[α] = Coyoneda[Op, α]})#λ]

At this point a program like this is nothing but
Data
describing the sequence of execution but FREE
of it's runtime interpretation.
val program = for {
a <- ask(1)
b <- async(2)
_ <- tell(println("log something"))
} yield a + b

We isolate interpretations
via Natural transformations AKA Interpreters.
In other words with map over
the outer type constructor Op
object ProdInterpreter extends (Op ~> Task) {
def apply[A](op: Op[A]) = op match {
case Ask(a) Task(a())
case Async(a) Task.fork(Task.delay(a()))
case Tell(a) Task.delay(a())
}
}

We can have different interpreters for our
production / test / experimental code.
object TestInterpreter extends (Op ~> Id.Id) {
def apply[A](op: Op[A]) = op match {
case Ask(a) a()
case Async(a) a()
case Tell(a) a()
}
}

Requirements
• Composability
• Interpretation
• Fault Tolerance

Fault Tolerance
Most containers and patterns generalize to the
most common super-type or simply Throwable
loosing type information.
val f = scala.concurrent.Future.failed(new NumberFormatException)
val t = scala.util.Try(throw new NumberFormatException)
val d = for {
a <- 1.right[NumberFormatException]
b <- (new RuntimeException).left[Int]
} yield a + b

Fault Tolerance
We don't have to settle for Throwable!!!
We could use instead…
• Nested disjunctions
• Coproducts
• Delimited, Monadic, Dependently-typed,
Accumulating Checked Exceptions

Fault Tolerance : Dependently-
typed Acc Exceptions
Introducing rapture.core.Result

Result is similar to / but has 3 possible
outcomes
(Answer, Errata, Unforeseen)
val op = for {
a <- Result.catching[NumberFormatException]("1".toInt)
b <- Result.errata[Int, IllegalArgumentException](
new IllegalArgumentException("expected"))
} yield a + b

Result uses dependently typed monadic
exception accumulation
val op = for {
a <- Result.catching[NumberFormatException]("1".toInt)
b <- Result.errata[Int, IllegalArgumentException](
new IllegalArgumentException("expected"))
} yield a + b

You may recover by resolving errors to an
Answer.
op resolve (
each[IllegalArgumentException](_ 0),
each[NumberFormatException](_ 0),
each[IndexOutOfBoundsException](_ 0))

Or reconcile exceptions into a new custom
one.
case class MyCustomException(e : Exception) extends Exception(e.getMessage)
op reconcile (
each[IllegalArgumentException](MyCustomException(_)),
each[NumberFormatException](MyCustomException(_)),
each[IndexOutOfBoundsException](MyCustomException(_)))

Requirements
We have all the pieces we need
Let's put them together!
• Composability
• Interpretation
• Fault Tolerance

Solving the Puzzle
How do we assemble a type that is:
Kleisli + Custom ADT + Result
for {
a <- Kleisli((x : String) ask(Result.catching[NumberFormatException](x.toInt)))
b <- Kleisli((x : String) ask(Result.catching[IllegalArgumentException](x.toInt)))
} yield a + b
We want a and b to be seen as Int but this won't
compile
because there are 3 nested monads

Solving the Puzzle : Monad
Transformers
Monad Transformers to the rescue!
type ServiceDef[D, A, B <: Exception] =
ResultT[({type λ[α] = ReaderT[OpMonad, D, α]})#λ, A, B]

Solving the Puzzle : Services
Two services with different dependencies
case class Converter() {
def convert(x: String): Int = x.toInt
}
case class Adder() {
def add(x: Int): Int = x + 1
}
case class Config(converter: Converter, adder: Adder)
val system = Config(Converter(), Adder())

def service1(x : String) = Service { converter: Converter
ask(Result.catching[NumberFormatException](converter.convert(x)))
}
def service2 = Service { adder: Adder
ask(Result.catching[IllegalArgumentException](adder.add(22) + " added "))
}

a <- service1("1").liftD[Config]
b <- service2.liftD[Config]
} yield a + b
composed.exec(system)(TestInterpreter)
composed.exec(system)(ProdInterpreter)

Conclusion
• Composability : Kleisli
• Dependency Injection : Kleisli
• Interpretation : Free monads
• Fault Tolerance : Dependently typed
checked exceptions

Thanks!
@raulraja
@47deg
http://github.com/47deg/func-architecture

Functional Programming Patterns for the Pragmatic Programmer

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Functional Programming Patterns for the Pragmatic Programmer