Introduction to Monads in Scala (1)

Monads
Part 1
functional programming with scala

Thinking in Use Cases

 get the coffee
 meet with colleagues on the kitchen
 read emails
 review social media
 update the workspace
 write the code

Thinking in Java

CompanyHolder.getStuffFactoryBuilder().
buildCurrentState().
find(Rooms.KITCHEN, CoffeMachine.Any).
get(0).
run(new CoffeeJob( me.getHabitsStore().find("coffe").
mapToJobMachnie(), null));

Thinking in Java

Need an update:

new SVNUpdateJob(myProject).go();

• execute
• process
• run
• start
• doIt

Thinking in Java

Check Gmail

MySmartProxyHack.go();

go via Hack or Job – action name
isn't important

Q

How many times u were not lucky with
fact that do is the keyword?

ProjectManager.do() vs manage
SecurityProcessor.do() vs process

Guava from G

Bad library: it makes me feeling smell

Preconditions.checkArgument(...)
vs
checkArgument(...)

with
static import... but this is a way for nonlazy dudes
in Java

Patriots in Java

First action is create, get, find, build, ...?

Use a Spring, Guice!!!

Other ways of thinking

C and C++
Python

Evil?

We r smart – look into Python,
Ruby, Java Script or Perl – gun meat.

Im smart enough to understand
AbstractProxyMediator or
NotificationStrategyFactory
– they even don't have it!

Monad is
Usually articles that start with words like
"monad" and "functor" quickly devolve into
soup of Greek letters. That's because both are
abstract concepts in a branch of mathematics
called category theory and explaining them
completely is a mathematical exercise.
JAMES IRY

JEE Session Facade stack trace

Functions …

F1(F2(F3(F4(Xinput))))

Jump to Scala
Few syntax details to understand examples

val, List

val act1 = List ("Leonardo" , "Raphael")

List operations, Nil

val act2 = “April” :: "Donatello" :: Nil
val act3 = act1 ::: act2

Var, no sugar

var act4:List[String] = act3
act4 = "Michelangelo" :: act4

Function as value

val double = (x: Int) => x * 2
double(10)

Sugar
List(1,2,3).foreach ( (x:Int) => println ( x ) )
List(1,2,3).foreach ( x => println ( x ) )
List(1,2,3).foreach ( println _ )
List(1,2,3).foreach ( println )

Yield
val result = for (i <- 1 to 5) yield i

Vector(1, 2, 3, 4, 5)

Trait
trait Manager extends Lead{
private var workSchedule:Schedule = …
override def schedule = workSchedule
}

class College extends Worker with Manager with
Wellpaid with HardToFind

1. Monads are Container Types

Option, List, …

Where is my manager?

Company company = getCompany();
Manager manager = company.findManager();

if (manager != null) {
manager.getBonusFor(me);
} else {
System.out.println("As always...");
}

Schrödinger's State
public interface Option<T> {
public T value();
public boolean hasValue();
}

Option<Manager> manager = company.findManager();

if (manager.hasValue()) {
manager.getBonusFor(me);
} else {
System.out.println(”Aren't news!");
}

Option
trait Manager {
def getBonusFor(id: Long) :Option[Double]
}

trait Company {
def manager:Manager
}

val sum = getCompany().manager. getBonusFor(myId)
val bonus = sum match {
case Some(bonus) => bonus
case None => 0
}

0 on default

val myBonus = manager.getOrElse(0)

If in If
if (company != null) {
Manager manager = company.findManager();

if (manager != null) {
double bonus = manager.getBonusFor(me);

...
} else {
System.out.println("Isn't news!");
}
}

Or Else
val m =
company.getManager.getOrElse(ManagmentFac
tory.newOne)
m.getBonusFor(me).getOrElse(…)

Option is a monad
for (
c <- getCompany;
m <- c.getManagerFor(me);
b <- m.getBonusFor(me)
) b spend

2. Monads Support Higher Order
Functions
val list = List(1, 2, 3)
def neg (elem: Int) = -elem
val mapResult = list map neg

List(1,2,3) map {-_}

Doesn’t change the kind of monad, but
may change its parameterized type...
val one = Some(1)
val oneString = one map {_.toString}
assert (oneString == Some("1"))

3. Monads are Combinable
val opMan : Option[Manager] = company
getManager
def extractBonus(m:Manager) : Option[Double]
= ...
val result = opMan map extractBonus

Option[Option[Double]]

List[List[Int]]]

List[List[List[Int]]]]

Flatten
def flatten[A]
(outer:Option[Option[A]]) : Option[A] =
outer match {
case None => None
case Some(inner) => inner
}

If the outer option is None, then result is None.
Otherwise the result is the inner Option.

Join, Flatten etc
Scala does not require you to write flatten explicitly.
But it does require that each monad have a method
called flatMap.

class M[A] {
private def flatten[B](x:M[M[B]]) : M[B] = ...
def map[B](f: A => B) : M[B] = ...
def flatMap[B](f: A => M[B]) : M[B] = flatten(map(f))
}

Review
val opMan : Option[Manager] = company
getManager
def extractBonus(m:Manager) : Option[Double]
= ...
val result = opMan flatMap extractBonus

Option[Double]

4. Monads Can Be Built In Different
Ways
”unit,” in Haskell it's called “return”

single argument “constructor” or ”actory”

A become a monad of type M[A]
For List: unit(x) == List(x)
For Option: unit(x) == Some(x)

class M[A](value: A) {
private def unit[B] (value : B) = new M(value)
…
}

Map based on flatMap
Scala does not require a separate "unit" function or
method

class M[A](value: A) {
private def unit[B] (value : B) = new M(value)
def map[B](f: A => B) : M[B] =
flatMap {x => unit(f(x))}
def flatMap[B](f: A => M[B]) : M[B] = ...
}

Basis 1
Generic Haskell Scala
M data M a class M[A]
or newtype M a or case class M[A]
or instance Monad (M a) or trait M[A]

Ma Ma M[A]
unit v return v new M(v)
or M(v)

map f m fmap f m m map f
bind f m m >>= f m flatMap f
or f =<< m
join join flatten
do for

Introduction to Monads in Scala (1)

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