Implicit parameters, when to use them (or not)!

Julien Truffaut
Backend Scala developer for 10+ years
FP instructor at fp-tower.com
Co-founder of scalajobs.com
2

Implicit parameters
def newBlogPost(title: String)(implicit author: User): BlogPost
class UserService(db: Connection)(implicit ec: ExecutionContext) {}
3

Implicit parameters
def newBlogPost(title: String)(implicit author: User): BlogPost
class UserService(db: Connection)(implicit ec: ExecutionContext) {}
When should we define a parameter as implicit?
4

Parameters definition
Explicit
def fullName(firstName: String, lastName: String): String =
s"$firstName $lastName"
fullName("John", "Doe")
// res0: String = "John Doe"
5

Explicit
def fullName(firstName: String, lastName: String): String =
s"$firstName $lastName"
// res0: String = "John Doe"
fullName(lastName = "Smith", firstName = "Bob")
// res1: String = "Bob Smith"
6

Austria
sealed abstract class AcademicTitle(val shortForm: String)
case object MasterOfScience extends AcademicTitle("MSc")
case object Magistrate extends AcademicTitle("Mag")
...
7

Austria
sealed abstract class AcademicTitle(val shortForm: String)
case object MasterOfScience extends AcademicTitle("MSc")
case object Magistrate extends AcademicTitle("Mag")
...
def fullName(firstName: String, lastName: String, title: Option[AcademicTitle]): String = {
val suffix = title.map(" " + _.shortForm).getOrElse("")
s"${firstName} ${lastName}${suffix}"
}
fullName("John", "Doe", None)
// res: String = "John Doe"
fullName("Bob", "Smith", Some(MasterOfScience))
// res: String = "Bob Smith MSc"
8

Default value
def fullName(firstName: String, lastName: String, title: Option[AcademicTitle] = None): String = {
val suffix = title.map(" " + _.shortForm).getOrElse("")
s"${firstName} ${lastName}${suffix}"
}
// res: String = "John Doe"
fullName("Bob", "Smith", Some(MasterOfScience))
// res: String = "Bob Smith MSc"
9

Default value
def kafkaListener(
topic : String,
bootstrapServers : String,
commitTimeout : FiniteDuration = 50.millis,
pollInterval : FiniteDuration = 15.millis,
autoOffsetReset : AutoOffsetReset = AutoOffsetReset.Latest,
)
10

Default value
def kafkaListener(
topic : String,
bootstrapServers : String,
commitTimeout : FiniteDuration = 50.millis,
pollInterval : FiniteDuration = 15.millis,
autoOffsetReset : AutoOffsetReset = AutoOffsetReset.Latest,
)
Similar options: Overloads or Builder Pattern
11

Implicit
case class BlogPost(
author : UserId,
title : String,
content : String,
)
case class UserId(value: String)
12

Implicit
def createEmptyBlogPost(title: String)(implicit requesterId: UserId): BlogPost =
BlogPost(
author = requesterId,
title = title,
content = ""
)
13

Implicit
BlogPost(
title = title,
content = ""
)
createEmptyBlogPost("Scala Implicits: The complete guide")(UserId("john_1234")) // Explicit call
// res: BlogPost = BlogPost(
// author = UserId("john_1234"),
// title = "Scala Implicits: The complete guide",
// content = "",
// )
14

Implicit
BlogPost(
title = title,
content = ""
)
createEmptyBlogPost("Scala Implicits: The complete guide") // Implicit call
// content = "",
// )
15

Implicit resolution
val ImplicitValues: Map[Type, Value] = // Maintained by the compiler
Map(
Int -> 5,
String -> "",
UserId -> UserId("john_1234"),
)
createEmptyBlogPost("Scala Implicits: The complete guide") // user code
16

Implicit resolution
Map(
Int -> 5,
String -> "",
)
|
v
createEmptyBlogPost("Scala Implicits: The complete guide")(ImplicitValues(UserId)) // lookup at compile-time
17

Implicit resolution
Map(
Int -> 5,
String -> "",
)
|
v
18

Implicit resolution
Map(
Int -> 5,
String -> "",
)
|
v
|
v
createEmptyBlogPost("Scala Implicits: The complete guide")(UserId("john_1234"))
19

Implicit resolution
Map(
Int -> 5,
String -> "",
// No UserId
)
|
v
|
v
???
20

Implicit resolution
Map(
Int -> 5,
String -> "",
// No UserId
)
|
v
|
v
error: could not find implicit value for parameter requesterId: UserId // compile-time error
21

Implicit Definition
implicit val requesterId: UserId = UserId("john_1234")
createEmptyBlogPost("Scala Implicits: The complete guide")
// content = "",
// )
22

Implicit Scope
class BlogPostTest extends AnyFunSuite {
test("createEmptyBlogPost gets the author implicitly") {
val result = createEmptyBlogPost("Scala Implicits: The complete guide") // ✅ Compile
assert(result.author == requesterId)
}
test("createEmptyBlogPost has no content") {
val result = createEmptyBlogPost("Scala Implicits: The complete guide") // ❌ could not find implicit value
assert(result.content.isEmpty)
}
}
23

Implicit Scope
}
val result = createEmptyBlogPost("Scala Implicits: The complete guide") // ❌ could not find implicit value
}
}
24

Implicit Scope
}
}
}
25

Summary
1. The compiler keeps track of all the implicits available within a scope
2. At compile-time, the compiler injects all implicit parameters
3. If an implicit is missing, we get a compiled-time error
26

Summary
1. The compiler keeps track of all the implicits available within a scope
2. At compile-time, the compiler injects all implicit parameters
3. If an implicit is missing, we get a compiled-time error
4. If there are 2 or more implicit values of the same type in scope, we also
get a compiled-time error
27

Within a Scope, there must be Only One implicit value per Type
28

Environment Pattern
val httpService = {
case req @ POST -> Root / "blog" => // create a blog
case req @ PUT -> Root / "blog" / id => // update a blog
case req @ DELETE -> Root / "blog" / id => // delete a blog
}
30

Environment Pattern
case req @ POST -> Root / "blog" =>
implicit val requesterId: UserId = extractRequesterId(req)
for {
payload <- req.parseBodyAs[NewBlog]
_ <- blogAPI.create(payload.title)
} yield Ok()
31

Environment Pattern
for {
} yield Ok()
32

Environment Pattern
for {
} yield Ok()
class BlogAPI(db: DB) {
def create(title: String)(implicit requesterId: UserId): Future[Unit] = {
val newBlog = createEmptyBlogPost(title)
db.save(newBlog)
}
}
33

Environment Pattern
for {
} yield Ok()
db.save(newBlog)
}
}
BlogPost(
title = title,
content = "",
)
34

Environment Pattern
for {
} yield Ok()
db.save(newBlog)
}
}
BlogPost(
title = title,
content = "",
)
35

Environment Pattern
author : UserId,
title : String,
content : String,
createAt: Instant,
)
36

Environment Pattern
author : UserId,
title : String,
content : String,
createAt: Instant,
)
BlogPost(
title = title,
content = "",
createAt = Instant.now(),
)
37

Environment Pattern
test("create blog post") {
val result = createEmptyBlogPost("Test")
assert(result == BlogPost(requesterId, "Test", "", ???))
}
38

Environment Pattern
trait Clock {
def now(): Instant
}
39

Environment Pattern
trait Clock {
def now(): Instant
}
object Clock {
val real: Clock = new Clock {
def now(): Instant = Instant.now()
}
def static(timestamp: Instant): Clock = new Clock {
def now(): Instant = timestamp
}
}
40

Environment Pattern
trait Clock {
def now(): Instant
}
object Clock {
val real: Clock = new Clock {
def now(): Instant = Instant.now()
}
def static(timestamp: Instant): Clock = new Clock {
def now(): Instant = timestamp
}
}
41

Environment Pattern
def createEmptyBlogPost(title: String)(implicit requesterId: UserId, clock: Clock): BlogPost =
BlogPost(
title = title,
content = "",
createAt = clock.now(),
)
42

Environment Pattern
def createEmptyBlogPost(title: String)(implicit requesterId: UserId, clock: Clock): BlogPost =
BlogPost(
title = title,
content = "",
createAt = clock.now(),
)
object Main extends App {
implicit val clock: Clock = Clock.real
...
}
implicit val clock: Clock = Clock.static(Instant.EPOCH)
...
}
43

Environment Pattern
object Main extends App {
implicit val ec: ExecutionContext = ExecutionContext.global
...
}
class BlogDatabaseTest extends AnyFunSuite {
implicit val ec: ExecutionContext = fixedSizeExecutionContext(1)
...
}
44

Environment parameters are Static in a given Context
45

Context: Per request
RequesterId
TraceId
Context: Prod / Test
Clock
ExecutionContext (for Future)
RandomGenerator
46

Advice
1. Use precise types
def createQueue[A](implicit size: Int): Queue[A] =
...
def connect(hostname: String)(implicit port: Int): Unit =
...
47

Advice
BlogPost(
title = title,
content = "",
)
def shareBlog(blog: BlogPost, targetUser: UserId)(implicit requesterId: UserId): Future[Unit] =
...
48

Advice
case class RequesterId(userId: UserId)
def createEmptyBlogPost(title: String)(implicit requesterId: RequesterId): BlogPost =
BlogPost(
author = requesterId.userId,
title = title,
content = "",
)
49

Advice
2. Use clear and simple Context
50

The value of implicit parameters must be Obvious
51

Environment Pattern Alternatives
Reader
def createBlogPost(title: String) : Reader[(RequesterId, Clock), BlogPost] = ...
def createProject(projectName: String): Reader[(RequesterId, Clock), Project] = ...
for {
blog <- createBlogPost("Implicits for the noob")
project <- createProject("tutorial")
} yield ...
52

Reader
def createBlogPost(title: String) : Reader[(RequesterId, Clock), BlogPost] = ...
def createProject(projectName: String): Reader[RequesterId , Project] = ...
for {
blog <- createBlogPost("Implicits for the noob")
project <- createProject("tutorial") ❌
} yield ...
53

ZIO
trait ZIO[-R, +E, +A]
54

Typeclass Pattern
val user : User = ...
val userIds: List[UserId] = ...
Ok(user)
// res: HttpResponse(200, {"userId" : "1234", "name" : "John Doe"})
Ok(userIds)
// res: HttpResponse(200, ["1234", "9464", "0582"])
56

Typeclass Pattern
Ok(user)
Ok(userIds)
def Ok[A](value: A): HttpResponse =
HttpResponse(200, value.toJson)
57

Typeclass Pattern
Ok(user)
Ok(userIds)
58

Typeclass Pattern
Ok(user)
Ok(userIds)
Ok((x: Int) => x + 1) // ???
59

Typeclass Pattern
trait JsonEncoder[A] {
def encode(value: A): Json
}
60

Typeclass Pattern
}
def Ok[A](value: A, encoder: JsonEncoder[A]): HttpResponse =
HttpResponse(200, encoder.encode(value))
61

Typeclass Pattern
}
def Ok[A](value: A, encoder: JsonEncoder[A]): HttpResponse =
val userEncoder : JsonEncoder[User] = ...
val listUserIdsEncoder: JsonEncoder[List[UserId]] = ...
Ok(user, userEncoder)
Ok(userIds, listUserIdsEncoder)
62

Typeclass Pattern
}
def Ok[A](value: A)(implicit encoder: JsonEncoder[A]): HttpResponse =
implicit val userEncoder : JsonEncoder[User] = ...
implicit val listUserIdsEncoder: JsonEncoder[List[UserId]] = ...
Ok(user)
Ok(userIds)
63

Typeclass Pattern
object JsonSerializer {
implicit val userEncoder : JsonEncoder[User] = ...
implicit val listUserIdsEncoder: JsonEncoder[List[UserId]] = ...
}
import JsonSerializer._
Ok(user)
64

Implicit Search
Ok(user)
Search for JsonEncoder[User] in User companion object
case class User(userId: UserId, email: String)
object User {
implicit val encoder: JsonEncoder[User] = ...
}
65

Implicit Search
Ok(userIds)
Search for JsonEncoder[List[UserId]] in List companion object
sealed trait List[A]
object List {
// There is no JsonEncoder here
}
66

Implicit Search
Ok(userIds)
Search for JsonEncoder[List[UserId]] in JsonEncoder companion object
object JsonEncoder {
implicit val string: JsonEncoder[String] = ...
implicit val int : JsonEncoder[Int] = ...
implicit val date : JsonEncoder[LocalDate] = ...
implicit val list : JsonEncoder[List[???]] = ...
}
67

Implicit Derivation
Ok(userIds)
implicit def list[A]: JsonEncoder[List[A]] = ...
}
68

Implicit Derivation
Ok(userIds)
implicit def list[A](implicit valueEncoder: A): JsonEncoder[List[A]] = ...
}
69

Implicit Search for JsonEncoder[List[UserId]]
1. Search current scope ❌
2. Search companion object of List ❌
3. Search companion object of JsonEncoder (Maybe)
4. Search companion object of UserId ✅
70

Typeclass Pattern
def Ok[A](value: A)(implicit encoder: JsonEncoder[A]): HttpResponse
Ok(user)
Ok(userIds)
Ok((x: Int) => x + 1) // ❌ compile-time error: could not find implicit value for parameter
71

Typeclass Pattern
def Ok[A: JsonEncoder](value: A): HttpResponse
Ok(user)
Ok(userIds)
Ok((x: Int) => x + 1) // ❌ compile-time error: could not find implicit value for parameter
72

Typeclass Pattern
}
73

Typeclass Pattern
For types we own, define instances in the companion object
object User {
implicit val encoder: JsonEncoder[User] = ...
}
object UserId {
implicit val encoder: JsonEncoder[UserId] = ...
}
For other types, define instances in the companion object of the typeclass
}
74

Typeclass Pattern
Use typeclass derivation to automatically create instances
implicit def list[A](implicit valueEncoder: A): JsonEncoder[List[A]] = ...
}
75

Typeclass Pattern
// shapeless, magnolia
implicit def caseClass[A: JsonEncoder, B: JsonEncoder]: JsonEncoder[CaseClass[A, B]] = ...
}
case class User(userId: UserId, email: String)
object User {
implicit val encoder: JsonEncoder[User] = JsonEncoder.caseClass
}
76

Typeclass Pattern (Scala 3)
case class User(userId: UserId, email: String) derives JsonEncoder
77

Advice
Don't define instances in custom objects
object Client1Encoder {
implicit val userEncoder: JsonEncoder[User] = ...
}
object Client2Encoder {
implicit val userEncoder: JsonEncoder[User] = ...
}
import Client1Encoder._
// lots of code
Ok(user)
78

79

Ordering
trait Ordering[A] {
def compare(left: A, right: A): Int
}
80

Ordering
trait Ordering[A] {
}
trait List[A] {
def sorted(implicit ordering: Ordering[A]): List[A] = ...
}
81

Ordering
trait Ordering[A] {
}
trait List[A] {
}
List(8,1,9).sorted
// ???
82

Ordering
trait Ordering[A] {
}
trait List[A] {
}
List(8,1,9).sorted
// res3: List[Int] = List(1, 8, 9)
83

Custom Ordering
implicit val descendingIntOrdering: Ordering[Int] = Ordering.Int.reverse
List(8,1,9).sorted
84

Custom Ordering
implicit val descendingIntOrdering: Ordering[Int] = Ordering.Int.reverse
List(8,1,9).sorted
List(8,1,9).sorted(Ordering.Int.reverse)
85

Better Ordering
trait List[A] {
def sorted(ordering: Ordering[A]): List[A] = ...
}
List(8,1,9).sorted(Ordering.Int.ascending)
// res: List[Int] = List(1, 8, 9)
List(8,1,9).sorted(Ordering.Int.descending)
// res: List[Int] = List(9, 8, 1)
86

Review
Explicit
Simple
Great tooling
Default value
When one value is much more common than the others
Implicit
When some parameters value are static within a context
When writing generic functions
87

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Implicit parameters, when to use them (or not)!

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