Functional IO and Effects

Functional I/O and Effects
Or, if a Monad folds in a functional forest, does it make an effect?
Dylan Forciea
Oseberg
February 28, 2017

Topics
• What does it mean for a language to be functional?
• Creating a type to represent IO operations
• Adding monoid operations to the IO type
• Adding monad operations to the IO type
• How a naïve approach for executing the monad is insufficient, and how
trampolines can fix it

What is a functional programming language?
• Functions as first class objects?
• Declarative rather than imperative?
• Recursion rather than loops?

ReferentialTransparency
• Yes!
• But ReferentialTransparency is also key to some advantages of functional
programming
• Any expression can be replaced with its value without changing the
meaning of the program
• This alludes to the fact that you should avoid side effects

Example of ReferentialTransparency
def average(x: Int, y: Int, z: Int): Int = (x + y + z) / 3
average(1, 2, 3)
 (1 + 2 + 3) / 3
 (6) / 3
 2
Running this a second time with the same input values will always return the
same result

Counterexample of ReferentialTransparency
var cumulativeValue: Int = 0
var numberSummed: Int = 0
def cumulativeAverage(x: Int): Int = {
numberSummed = numberSummed + 1
cumulativeValue += x
cumulativeValue / numberSummed
}
cumulativeAverage(10)
 cumulativeValue = 10, numberSummed = 1, result = (10/1) = 10

What does ReferentialTransparency buy you?
• Testability
• Promotes parallel programming
• Can enable optimization of code

ReferentialTransparency and IO
def addOneToUserInput(): Int = scala.io.StdIn.readInt() + 1
The return value of executing this function will be different depending on what
the user inputs!

Wrapping IO in aType
• Separate IO into a type and define actions inside
• Denotes which pieces of the code are actions
• Defer running an action until later
• Describe a program containing effects purely functionally, and then run it

Wrapping IO in aType
trait IO[A] { def run: A }
def PrintLine(msg: String): IO[Unit] = new IO[Unit] { def run: Unit =
println(msg) }
PrintLine(“Hello, world!”).run
 Hello, world!

How do we run a sequence of these actions?
• Make it a monoid, of course!
• What was a monoid, again?

Monoid Refresher
• A monoid is a combination of:
• Some type, A
• A seed (or "zero") for that type
• An associative binary operator.
• Concretely:
trait Monoid[A] {
def op(a1:A, a2: A): A
def zero: A
}
• In addition to the associativity of op, for any A the following must hold:
• op(a, zero) == a == op(zero, a)

Monoid Refresher Example
• For integer addition:
object IntMonoid extends Monoid[Int] {
def op(a: Int, b: Int): Int = a + b
def zero: Int = 0
}
• You can use operations like fold to combine these:
• List(1, 2, 3).fold(IntMonoid.zero)(IntMonoid.op)
• (((0 + 1) + 2) + 3) = 6

IO as a Monoid
trait IO[A] { self =>
def run:A
def ++[B](io: IO[B]): IO[B] = new IO[B] { def run: B = { self.run; io.run } }
}
object IO {
def zero: IO[Unit] = new IO[Unit] { def run: Unit = { } }
def ++[A, B](io1: IO[A], io2: IO[B]): IO[B] = io1 ++ io2
}

IO Monoid Example Usage
(PrintLine("Hello!") ++ PrintLine("World!")).run
Hello!
World!
val ioList = List(PrintLine("One"), PrintLine("Two"), PrintLine("Three"))
ioList.fold(IO.zero)(IO.++).run
One
Two
Three

That’s great, but…
• We can chain together output effects
• But, how can we actually perform any operations on input effects?
• Monads!
• Wait, another one of those big words…

Monad refresher
• Monads provide two operations:
• def map[B](f:A => B): IO[B]
• def flatMap[B](f:A => IO[B]): IO[B]

Monad refresher
• map transforms a value from domainA to a value from domain B inside the
monad
• flatMap transforms a value from domain A into a Monad containing a value
from domain B, and then returns a monad containing a value from domain B

How does this help?
• map takes the result of an IO monad and perform computations on the value
• flatMap takes the result of an IO monad, and then perform IO for output
• Like control flow for the IO “program”

IO as a Monad
trait IO[A] { self =>
def run:A
def ++[B](io: IO[B]): IO[B] = new IO[B] { def run: B = { self.run; io.run } }
def map[B](f: A => B): IO[B] = new IO[B] { def run: B = f(self.run) }
def flatMap[B](f: A => IO[B]): IO[B] =
new IO[B] { def run: B = f(self.run).run }
}
def PrintLine(msg: String): IO[Unit] =
new IO[Unit] { def run: Unit = println(msg) }
def GetLine(): IO[String] =
new IO[String] { def run: String = scala.io.StdIn.readLine() }

Lets put it together!
PrintLine("Type a number")
.flatMap(_ => GetLine())
.map(s => s.toInt)
.map(i => i + 1)
.flatMap(r => PrintLine(r.toString))
.run
Type a number
 1
2

Can’t mix and match
GetLine().toInt + 1
<console>:15: error: value toInt is not a member of IO[String]
GetLine().toInt + 1

Some useful operations
def doWhile[A](a: IO[A])(cond: A => IO[Boolean]): IO[Unit] = for {
a1 <- a
ok <- cond(a1)
_ <- if (ok) doWhile(a)(cond) else IO.zero
} yield ()
def forever[A, B](a: IO[A]): IO[B] = a flatMap(_ => forever(a))

Houston, we have a problem!
forever(PrintLine("Hello")).run
Hello
Hello
…
<Stack Overflow!>
• What happened?!

Not tail recursive
def flatMap[B](f: A => IO[B]): IO[B] = new IO[B] { def run: B = f(self.run).run }
def forever[B](a: IO[A]): IO[B] = a flatMap(_ => forever(a))
• forever keeps on calling flatMap
• Every time flatMap is called, we end up one level lower in recursion
• Since the function definition says we need to keep track of the results of
f(this.run) to call run on it, we keep on adding on to the call stack every time

How do we fix this?
• Use Trampolining
• Create a series ofAlgebraic DataTypes (ADTs) to describe how the
operation will run
• We can make running our IO monad operations tail recursive
• First things first…
• What is tail recursion?
• What is anADT?

Tail Recursion
• Don’t want to keep track of state after calling function recursively
• If the last function run is the recursive call, then it is in tail position
• We can skip adding a stack frame

Tail recursion example
def factorial(n: BigInt): BigInt = if (n == 0) 1 else n * factorial(n-1)
def factorial_tailrec(n: BigInt): BigInt = {
def factorial1(n: BigInt, acc: BigInt): BigInt =
if (n == 0)
acc
else
factorial1(n - 1, n * acc)
factorial1(n, 1)
}

Algebraic DataTypes
• This is a data type that can be one of several things, and each thing can
contain a defined set of data
• We can use pattern matching to perform operations on the ADT
• This is probably more apparent if we just give an example…

List ADT definition
sealed trait List[+A]
case class Cons[+A](head: A, tail: List[A]) extends List[A]
case object Nil extends List[Nothing]

List ADT Pattern Matching
val a = Cons(1, Nil)
val b = Cons(2, a)
val c = Cons(3, b)
def add(list: List[Int]): Int =
list match {
case Cons(head, tail) => head + add(tail)
case Nil => 0
}
add(c)
6 3 2 1

Describing IO monad operations with an ADT
sealed trait IO[A] { self =>
def flatMap[B](f: A => IO[B]): IO[B] = FlatMap(self, f)
def map[B](f: A => B): IO[B] = flatMap(a => (Return(f(a))))
}
case class Return[A](a: A) extends IO[A]
case class Suspend[A](resume: () =>A) extends IO[A]
case class FlatMap[A, B](sub: IO[A], k: A => IO[B]) extends IO[B]
def PrintLine(s: String): IO[Unit] = Suspend(() => Return(println(s)))
def GetLine: IO[String] = Suspend(() => scala.io.StdIn.readLine())

Our example from earlier!
.flatMap(_ => GetLine)
.map(s => s.toInt)
.map(i => i + 1)
Suspend
PrintLine
Prompt

.map(s => s.toInt)
.map(i => i + 1)
Suspend
PrintLine
Prompt
FlatMap
Suspend
GetLine

.map(s => s.toInt)
.map(i => i + 1)
Suspend
PrintLine
Prompt
FlatMap
Suspend
GetLine
Return
s.toInt
FlatMap

.map(s => s.toInt)
.map(i => i + 1)
Suspend
PrintLine
Prompt
FlatMap
Suspend
GetLine
Return
s.toInt
Return
i + 1
FlatMap
FlatMap

.map(s => s.toInt)
.map(i => i + 1)
Suspend
PrintLine
Prompt
FlatMap
Suspend
GetLine
Return
s.toInt
Return
i + 1
Suspend
PrintLine
Result
FlatMap
FlatMap
FlatMap

Now we need the Interpreter…
def run[A](io: IO[A]): A = io match {
case Return(a) => a
case Suspend(r) => r()
case FlatMap(x, f) => x match {
case Return(a) => run(f(a))
case Suspend(r) => run(f(r()))
case FlatMap(y, g) => run(y.flatMap(a => g(a).flatMap(f)))
}
}

Run the example
Suspend
PrintLine
Prompt
FlatMap
Suspend
GetLine
Return
s.toInt
Return
i + 1
Suspend
PrintLine
Result
FlatMap
FlatMap
FlatMap
.map(s => s.toInt)
.map(i => i + 1)

Run the example
Suspend
PrintLine
Prompt
FlatMap
Suspend
GetLine
Return
s.toInt
Return
i + 1
Suspend
PrintLine
Result
FlatMap
FlatMap FlatMap
.map(s => s.toInt)
.map(i => i + 1)

Run the example
Suspend
GetLine
Return
s.toInt
Return
i + 1
Suspend
PrintLine
Result
FlatMap
FlatMap
FlatMap
.map(s => s.toInt)
.map(i => i + 1)

Run the example
Return
s.toInt
Return
i + 1
Suspend
PrintLine
Result
FlatMap
FlatMap
.map(s => s.toInt)
.map(i => i + 1)

Run the example
Return
i + 1
Suspend
PrintLine
Result
FlatMap
.map(s => s.toInt)
.map(i => i + 1)

Run the example
Suspend
PrintLine
Result
.map(s => s.toInt)
.map(i => i + 1)

How about forever?
def forever[B](a: IO[A]): IO[B] = a flatMap(_ => forever(a))
run(forever(PrintLine("Hello")))
Suspend
PrintLine
FlatMap

Now it works!
run(forever(PrintLine("Hello")))
Hello
Hello
…
<Keeps going!>

Takeaways
• Like a Free Monad! (A story for another day…)
• You don’t have to use the same interpreter... you can represent IO as ADTs
and not perform IO at all
• Effects are purposeful and partitioned off within a type
• Bonus – it happens to be a monad!

Other libraries
• Slick
• Describe SQL operations
• Execute them once they are prepared
• Akka Streams
• Describe a dataflow with a graph
• Push data through the graph components
• In Scala, this is an advanced flavor of the IO monad described here

References
• Functional Programming in Scala, Chiusano and Bjarnason
• https://www.slideshare.net/InfoQ/purely-functional-io - Nice presentation
by Runar Bjarnason
• http://blog.higher-order.com/assets/scalaio.pdf

Functional IO and Effects

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