Types Working
For You
Richard Dallaway, @d6y
underscore.io
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Modern type system
with lots of power
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Two Themes
Straightforward Scala
Types Working for Us
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Progression
Part 1 Straightforward Scala
Part 2 Functional Programming
Part 3 Typelevel Programming
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Straightforward Scala
— Part 1 —
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The only problem was we had no idea what the code
was doing at first.
We came across a strange symbol we hadn’t seen in
our projects before
The spaceship operator <|*|>
Someone said out loud “what the hell is that?”
http://jimplush.com/talk/
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The only problem was we had no idea what the code
was doing at first.
We came across a strange symbol we hadn’t seen in
our projects before
The spaceship operator <|*|>
Someone said out loud “what the hell is that?”
http://jimplush.com/talk/
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The only problem was we had no idea what the code
was doing at first.
We came across a strange symbol we hadn’t seen in
our projects before
The spaceship operator <|*|>
Someone said out loud “what the hell is that?”
http://jimplush.com/talk/
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The only problem was we had no idea what the code
was doing at first.
We came across a strange symbol we hadn’t seen in
our projects before
The spaceship operator <|*|>
Someone said out loud “what the hell is that?”
http://jimplush.com/talk/
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“It’s about having a maintainable
code base where you can have
people cross projects easily and
get new hires up to speed rapidly”
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Power!
Protect the team from it
and
Get the benefit of it
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What can we do?
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1. Expressions, types, & values
2. Objects and classes
3. Algebraic data types
4. Structural recursion
5. Sequencing computation
6. Type classes
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1. Expressions, types, & values
2. Objects and classes
3. Algebraic data types
4. Structural recursion
5. Sequencing computation
6. Type classes
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Algebraic data types
Structural recursion
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Algebraic data types
data into code
Structural recursion
transformation
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Model data with logical
ors and logical ands
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A website visitor is:
• anonymous; or
• logged in
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A logged in user has:
• an ID; and
• facts we know about
them
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Two Patterns
and (product types)
or (sum types)
Sum and product together
make algebraic data types
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Structure of the code
follows the structure of
the data
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A website visitor is:
• anonymous; or
• logged in
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sealed trait Visitor
case class Anonymous()
extends Visitor
case class User()
extends Visitor
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A logged in user has:
• an ID; and
• facts we know about
them
An anonymous has:
• an ID
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sealed trait Visitor
case class Anonymous()
extends Visitor
case class User()
extends Visitor
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sealed trait Visitor
case class Anonymous(id: Id)
extends Visitor
case class User(id: Id, facts: Set[Fact])
extends Visitor
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Structural recursion
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def serveAd(v: Visitor): Advert = ???
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Structure of the code
follows the structure of
the data
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def serveAd(v: Visitor): Advert = ???
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def serveAd(v: Visitor): Advert =
v match {
case User(_, info) => relevantAd(info)
case Anonymous(id) => adRotation(id)
}
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def serveAd(v: Visitor): Advert =
v match {
case User(_, info) => relevantAd(info)
case Anonymous(id) => adRotation(id)
}
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def serveAd(v: Visitor): Advert =
v match {
case User(_, info) => relevantAd(info)
case Anonymous(id) => adRotation(id)
}
Structure
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ADT & Structural Recursion
Straightforward part of Scala.
Clear, productive, occurs frequently.
Be opinionated in what you use.
Structure helps us.
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Help from FP Ideas
— Part 2 —
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Combining lists
Concatenating strings
Union of sets
Combining things in a loop
Chaining logical operations
Adding numbers
Building up a JavaScript expression
Showing errors in a UI
...
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A combine function and
an empty value
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Addition
Empty Combine
0 +
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Set
Empty Combine
Set.empty union
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For any T
Empty Combine
A zero for T
A way to
combine two Ts
and give me
back a T
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A combine function and
an empty value
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Monoid
A combine function and
an empty value
…and laws
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The boss asks…
What’s the total visits to the web site?
def report(vs: List[Int]): Int = ???
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For any T
Empty Combine
A zero for T
A way to
combine two Ts
and give me
back a T
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For any T
trait Monoid[T] {
def empty: T
def combine(x: T, y: T): T
}
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val addition = new Monoid[Int] {
def empty = 0
def combine(x: Int, y: Int) = x+y
}
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fold
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def fold(vs: List[Int]): Int =
vs match {
case Nil => 0
case v :: rest => v + fold(rest)
}
fold(List(1,2,3))
// 6
def fold(vs: List[Int]): Int =
vs match {
case Nil => 0
case v :: rest => v + fold(rest)
}
fold(List(1,2,3))
// 6
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def fold(vs: List[Int], m: Monoid[Int]): Int =
vs match {
case Nil => 0
case v :: rest => v + fold(rest)
}
fold(List(1,2,3), addition)
// 6
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def fold(vs: List[Int], m: Monoid[Int]): Int =
vs match {
case Nil => m.empty
case v :: rest => m.combine(v, fold(rest,m))
}
fold(List(1,2,3), addition)
// 6
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def fold[T](vs: List[T], m: Monoid[T]): T =
vs match {
case Nil => m.empty
case v :: rest => m.combine(v, fold(rest,m))
}
fold(List(1,2,3), addition)
// 6
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Split on cases,
inspect values you have
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def fold[T](vs: List[T], m: Monoid[T]): T =
vs match {
case Nil => ???
case v :: rest => ???
}
fold(List(1,2,3), addition)
// 6
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def fold[T](vs: List[T], m: Monoid[T]): T =
vs match {
case Nil => m.empty
case v :: rest => ???
}
fold(List(1,2,3), addition)
// 6
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But back to Monoids…
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The boss asks…
What’s the total visits to the web site?
def report(vs: List[Int]): Int =
fold(vs, addition)
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Benefits
Composition
Flexibility
Problem Solving
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The boss asks…
How many distinct visitors?
def report(vs: List[Visitor]): Int = ???
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Set
Empty Combine
Set.empty union
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The boss says…
Argh!
The servers are OutOfMemory
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HyperLogLog
Empty Combine
new HLL() HLL.plus
Armon Dadgar (Papers We Love, 2015)
“Bloom Filters and HyperLogLog”
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The boss asks…
Who are the really keen
visitors to the site?
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Count-Min Sketch
Empty Combine
new CMS() CMS.plus
Laura Bledaite (Scala eXchange 2015)
“Count-Min Sketch in Real Data Applications”
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We can safely run
a parallel version
of fold
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Laws
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a + 0 = a
(a + b) + c = a + (b + c)
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Identity & Associativity
a combine empty = a
(a combine b) combine c
= a combine (b combine c)
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a combine b
combine combine
c d e f
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No content
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Errors: 10 Warnings: 0
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Its a monoid
I know this
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…so we fold
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Summary
Types and laws give us flexibility &
help lead us to solutions.
They help us every day.
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A Taste of Typelevel
— Part 3 —
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Date Metric
Mon Low
Tue High
csv(
List(“Date”, “Metric”),
List(
List(“Mon”, “Low”),
List(“Tue”, “High”) )
)
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Date
Mon Low
Tue High
csv(
List(“Date”),
List(
List(“Mon”, “Low”),
List(“Tue”, “High”) )
)
sealed trait Nat
trait Succ[P <: Nat] extends Nat
trait Zero extends Nat
type One = Succ[Zero]
type Two = Succ[One]
implicitly[Succ[Zero] =:= One]
implicitly[Succ[One] =:= Succ[Succ[Zero]]]
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sealed trait Nat
trait Succ[P <: Nat] extends Nat
trait Zero extends Nat
type One = Succ[Zero]
type Two = Succ[One]
implicitly[Succ[Zero] =:= One]
implicitly[Succ[One] =:= Succ[Succ[Zero]]]
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sealed trait Nat
trait Succ[P <: Nat] extends Nat
trait Zero extends Nat
type One = Succ[Zero]
type Two = Succ[One]
implicitly[Succ[Zero] =:= One]
implicitly[Succ[One] =:= Succ[Succ[Zero]]]
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sealed trait Nat
trait Succ[P <: Nat] extends Nat
trait Zero extends Nat
type One = Succ[Zero]
type Two = Succ[One]
implicitly[Succ[Zero] =:= Two]
error:
Cannot prove that Succ[Zero] =:= Two.
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Merging Fields
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case class User(
id : Long,
name : String,
email : Option[String])
val user = User(
123L,
“Bruce Wayne”,
Some(“[email protected]”))
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PATCH /user/123
{
“name” : “Batman”
}
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case class User(
id : Long,
name : String,
email : Option[String])
case class Update(
name : Option[String],
email : Option[Option[String]])
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val user = User(
123L,
“Bruce Wayne”,
Some(“[email protected]”))
val update = Update(
Some(“Batman”),
None)
How do we get to…
User(
123L,
“Batman”,
Some(“[email protected]”))
How?
User String Option[String] …
Option[String]
Option[
Option[String]
]
…
Update
Head
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How?
User String Option[String] …
Option[String]
Option[
Option[String]
]
…
Update
Head The Rest…
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How?
Type constraints
Implicit methods
HLists
Labelled generic
Macros
…
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val user = User(
123L,
"Bruce Wayne”,
Some(“[email protected]”))
val update = Update(
Some(“Batman”),
None)
import bulletin._
val updated = user.merge(update)
// User(
// 123L,
// “Batman”,
// Some(“[email protected]”))
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val user = User(
123L,
"Bruce Wayne”,
Some(“[email protected]”))
val update = Update(
Some(“Batman”),
None)
import bulletin._
val updated = user.merge(update)
// User(
// 123L,
// “Batman”,
// Some(“[email protected]”))
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Summary
The compiler can help (maybe more than
you thought).
Reduce boilerplate code.
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Using Power Tools
Can go one of two ways…
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Using Power Tools
Can go one of two ways…
What the hell
is that?
It’s a monoid!
I know this
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Simple
Types
Power
Share
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2008
‘The name Scala stands for
“scalable language.”
The language is so named
because it was designed
to grow with the demands of its
users.’
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What have we seen?
Some straightforward parts of Scala
—Clear, maintainable, helpful
Encoding ideas in types
—flexibility, leads us to solutions
Let the compiler do it
—when it make sense for your demands
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Summary
Scala scaling with your needs
—be opinionated in what you use, more when needed
Types working for us, not stopping us
—functional programming, share what you learn
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Thanks!
Richard Dallaway, @d6y
underscore.io
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Thanks!
Richard Dallaway, @d6y
underscore.io
Amanda Laucher
Wesley Reisz
Noel Welsh
Dave Gurnell
Miles Sabin
Jono Ferguson
Julio Capote
Alessandro Zoffoli