Solid Type System vs Runtime Checks and Unit Tests Vladimir Pavkin - - PowerPoint PPT Presentation

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Solid Type System vs Runtime Checks and Unit Tests Vladimir Pavkin - - PowerPoint PPT Presentation

Apr 23, 2023 564 likes •1.21k views

Solid Type System vs Runtime Checks and Unit Tests Vladimir Pavkin Plan Fail Fast concept Type Safe Patterns Fail Fast Immediate and visible failure Where can it fail? Handled runtime exceptions & assertions Unhandled runtime failure

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Solid Type System

vs

Runtime Checks and Unit Tests

Vladimir Pavkin

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Plan

Fail Fast concept Type Safe Patterns

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Fail Fast

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Immediate and visible failure

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Where can it fail?

Handled runtime exceptions & assertions Unhandled runtime failure

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Handling runtime exceptions

assert(!list.isEmpty, "List must be empty") try { str.toInt } catch { case _:Throwable => 0 }

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Where can it fail?

Runtime checks Handled runtime exceptions & assertions Unhandled runtime failure

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Runtime checks

if(container == null) if(container.isInstanceOf[ContainerA])

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Where can it fail?

Unit tests Runtime checks Handled runtime exceptions & assertions Unhandled runtime failure

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Unit tests

it should "throw NoSuchElementException for empty stack" in { val emptyStack = new Stack[Int] a [NoSuchElementException] should be thrownBy { emptyStack.pop() } } it should "not throw for empty stack" in { val stackWrapper = StackWrapper(new Stack[Int]) noException should be thrownBy stackWrapper.pop() }

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Where can it fail?

Linters Unit tests Runtime checks Handled runtime exceptions & assertions Unhandled runtime failure

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Linters

scalacOptions ++= Seq( "­Xlint", "­deprecation", "­Xfatal­warnings" ) // Wrong number of args to format() logger.error( "Failed to open %s. Error: %d" .format(file) )

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Where can it fail?

Compiler Linters Unit tests Runtime checks Handled runtime exceptions & assertions Unhandled runtime failure

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The goal

To move as much as possible to the Compiler

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How?

Just give it enough type information.

Type system to the rescue!

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Before we start...

Examples domain?

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Beefcakes!

No offense intended :)

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Ok?

def becomeAMan(douchebag: Person): Man = if(douchebag.weight > 70) new Man(douchebag.renameTo("Arny")) else null

No! Unhandled runtime failure!

becomeAMan(vpavkin).name //vpavkin.weight < 70

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NULL

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Can we handle this?

var man = becomeAMan(person) if(man != null) name else //...

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Still not nice.

code client has to clutter code with runtime checks (or fail) compiler won't complain if you forget to check

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If you control the source code, don't ever use null as a return result. It's like farting in an elevator.

Some random guy at a random Scala forum

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The problem is

insufficient type information!

Return type should be something like ManOrNull

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Option

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Option

sealed trait Option[T] case class Some[T](x: T) extends Option[T] case object None extends Option[Nothing]

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Better API

def becomeAMan(douchebag: Person): Option[Man] = if(douchebag.weight > 70) Some(new Man(douchebag.renameTo("Arny"))) else None code is documentation client has to deal with None result at compile time.

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Use wrapped value?

def firstWorkout(douchebag: Person): Option[WorkoutResult] = becomeAMan(douchebag).map(man => man.workout())

Unwrap?

def willHaveASexyGirlfriend(douchebag: Person): Boolean = becomeAMan(douchebag) match { case Some(man) => true case None => false }

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Exceptions

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Classic

def workout(man: Man): WorkoutResult = if(!man.hasShaker) throw new Error("Not enough protein!!!!111") else // do some squats or stare in the mirror for 1h

Again!

Client either uses try/catch or fails at runtime! Return type doesn't tell anything about possible failure

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Let's add some types!

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scala.Either

  • r

scalaz.\/

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Declare possible failure

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Better API

def workout(man:Man): ProteinFail \/ WorkoutResult = if(!man.hasShaker) ProteinFail("Not enough protein!!!!111").left else someWorkoutResult.right code is documentation client has to deal with errors at compile time.

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scalaz.\/

sealed trait \/[E, R] case class ­\/[E](a: E) extends (E \/ Nothing) case class \/­[R](a: R) extends (Nothing \/ R)

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Use wrapped value?

workout(man).map(result => submitToFacebook(result)) // type is // ProteinFail \/ Future[List[FacebookLike]]

Unwrap?

def tellAboutTheWorkout(w: ProteinFail \/ WorkoutResult): String = w match { case ­\/(fail) => "F**k your proteins, I can do without it" case \/­(result) => s"Dude, eat proteins, or you won't do like me: $result" }

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isInstanceOf[Man]

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isInstanceOf[T]

trait GymClient case class Man(name: String) extends GymClient case class Douchebag(name: String) extends GymClient def gymPrice(h: GymClient): Int = if(h.isInstanceOf[Man]){ val man = h.asInstanceOf[Man] if(man.name == "Arny") 0 else 100 } else { 200 }

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So runtime.

// Add another client type case class PrettyGirl(name:String) extends GymClient

It still compiles. And we charge girls as much as douchebags! It's an unhandled runtime failure!

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isInstanceOf[T]

trait GymClient case class Man(name: String) extends GymClient case class Douchebag(name: String) extends GymClient case class PrettyGirl(name:String) extends GymClient def gymPrice(h: GymClient): Int = if(h.isInstanceOf[Man]){ val man = h.asInstanceOf[Man] if(man.name == "Arny") 0 else 100 } else { 200 }

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sealed ADT

+

pattern matching

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sealed = can't be extended in

  • ther files
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Algebraic Data Type

1) Product types 2) Sum types

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Compiler knows all the possible class/trait children.

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Sealed ADT + pattern matching

sealed trait GymClient case class Man(name: String) extends GymClient case class Douchebag(name: String) extends GymClient def gymPrice(h: GymClient): Int = h match { case Man("Arny") => 0 case _: Man => 100 case _: Douchebag => 200 } // compiler checks, that match is exhaustive

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What if we add girls now?

sealed trait GymClient case class Man(name: String) extends GymClient case class Douchebag(name: String) extends GymClient case class PrettyGirl(name:String) extends GymClient def gymPrice(h: GymClient): Int = h match { case Man("Arny") => 0 case _: Man => 100 case _: Douchebag => 200 } // COMPILE ERROR! Match fails for PrettyGirl.

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Compiler saved us again!

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Tagging

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Gym DB

case class Beefcake(id: String, name: String) case class GymPass(id: String,

  • wnerId: String)
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Safer: Tags

trait JustTag def onlyTagged(value: String @@ JustTag): String = s"Tagged string: $value" // can use as plain String

  • nlyTagged("plain string") // Compiler error

val tagged = tag[JustTag]("tagged")

  • nlyTagged(tagged) // OK
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Gym DB: safer keys

case class Beefcake(id: String @@ Beefcake, name: String) case class GymPass(id: String @@ GymPass,

  • wnerId: String @@ Beefcake)
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Phantom Types

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PullUp

sealed trait PullUpState final class Up extends PullUpState final class Down extends PullUpState

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PullUp

class Beefcake[S <: PullUpState] private () { def pullUp[T >: S <: Down]() = this.asInstanceOf[Beefcake[Up]] def pullDown[T >: S <: Up]() = this.asInstanceOf[Beefcake[Down]] }

  • bject Beefcake {

def create() = new Beefcake[Down] }

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PullUp

val fresh = Beefcake.create() //Beefcake[Down] val heDidIt = fresh.pullUp() //Beefcake[Up] val notAgainPlease = heDidIt.pullUp() // CompileError: // inferred type arguments [Up] do not conform // to method pullUp's type parameter bounds

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Path Dependent Types

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The Two Gyms

class Gym(val name: String) class Beefcake(val gym: Gym){ def talkTo(other: Beefcake): Unit = println("Wazzup, Hetch!") } val normalGym = new Gym("nicefitness") val swagGym = new Gym("kimberly") val normalGuy = new Beefcake(normalGym) val swagGuy = new Beefcake(swagGym) normalGuy.talkTo(swagGuy) // we don't want that

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The Two Gyms Runtime solution

class Beefcake(val gym: Gym){ def talkTo(other: Beefcake): Unit = { // throws IllegalArgumentException if false require(this.gym == other.gym) println("Wazzup, Hetch!") } }

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Path Dependent Types

class A { class B } val a1 = new A val a2 = new A var b = new a1.B // type is a1.B b = new a2.B // Compile Error: types don't match

Type depends on the value it belongs to.

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Type safe solution

class Gym(val name: String){ class Beefcake(val gym: Gym){ def talkTo(other: Beefcake): Unit = println("Wazzup, Hetch!") } } val normalGym = new Gym("nicefitness") val swagGym = new Gym("kimberly") val normalGuy = new normalGym.Beefcake(normalGym) val swagGuy = new swagGym.Beefcake(swagGym) normalGuy.talkTo(swagGuy) // doesn't compile, Yay!

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This is not a talk about Scala type system. Not covered:

Trait composition Existential types Macros Type Classes Shapeless ...

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Q & A

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Thank you!

goo.gl/U0WYAB

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