kailuowang / henkan

A small library for converting between case classes.

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Henkan [変換]

A small library for converting between case classes.

Contributors and participation

henkan is currently maintained by Kailuo Wang.

The henkan project supports the Typelevel code of conduct and wants all of its channels (Gitter, GitHub, etc.) to be welcoming environments for everyone.


henkan is licensed under the Apache License, Version 2.0 (the "License"); you may not use this software except in compliance with the License.

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.



Transform between case classes, which minimize the need to manually using constructor to transform information from one case class to another.


  1. quick transformation when the source case class has all the fields the target case class has: e.g. a.to[B]()

  2. supplement (if source case class doesn't have a field) or override field values. e.g. a.to[B].set(foo = "bar")

  3. use the default values of the target case classes if needed


Conversion between case classes with optional fields and case class with required fields. One of the use cases for such conversions is conversion between scalaPB generated classes where most fields are Options and internal case classes where you have required fields.

Get started

 libraryDependencies += "com.kailuowang" %% "henkan-convert" % "0.2.10"

 libraryDependencies += "com.kailuowang" %% "henkan-optional" % "0.2.10"


Transform between case classes

import java.time.LocalDate

case class Employee(name: String, address: String, dateOfBirth: LocalDate, salary: Double = 50000d)

case class UnionMember(name: String, address: String, dateOfBirth: LocalDate)

val employee = Employee("George", "123 E 86 St", LocalDate.of(1963, 3, 12), 54000)

val unionMember = UnionMember("Micheal", "41 Dunwoody St", LocalDate.of(1994, 7, 29))

Now use the henkan magic to transform between UnionMember and Employee

import henkan.convert.Syntax._
// import henkan.convert.Syntax._

// res4: UnionMember = UnionMember(George,123 E 86 St,1963-03-12)

// res5: Employee = Employee(Micheal,41 Dunwoody St,1994-07-29,50000.0)

unionMember.to[Employee].set(salary = 60000.0)
// res6: Employee = Employee(Micheal,41 Dunwoody St,1994-07-29,60000.0)

Missing fields will fail the compilation

case class People(name: String, address: String)
// defined class People

val people = People("John", "49 Wall St.")
// people: People = People(John,49 Wall St.)
scala> people.to[Employee]() //missing DoB
<console>:20: error: 
    You have not provided enough arguments to convert from People to Employee.

       people.to[Employee]() //missing DoB

Wrong argument types will fail the compilation

scala> unionMember.to[Employee].set(salary = 60) //salary was input as Int rather than Double
<console>:20: error: One or more fields in shapeless.::[shapeless.labelled.FieldType[shapeless.tag.@@[Symbol,String("salary")],Int],shapeless.HNil] is not in Employee
error after rewriting to henkan.convert.Syntax.convert[UnionMember](unionMember).to[Employee].set.applyDynamicNamed("apply")(scala.Tuple2("salary", 60))
possible cause: maybe a wrong Dynamic method signature?
       unionMember.to[Employee].set(salary = 60) //salary was input as Int rather than Double

Transform between case classes with optional field

cats.optional provides some facility to transform between case classes with optional fields and ones with required fields. Suppose you have two case classes: Message whose fields are optional and Domain whose fields are required

case class Message(a: Option[String], b: Option[Int])
case class Domain(a: String, b: Int)

You can validate an instance of Message to a Validated Domain

import cats.data.Validated
import cats.implicits._
import henkan.optional.all._
validate(Message(Some("a"), Some(2))).to[Domain]
// res0: henkan.optional.ValidateFromOptional.Result[Domain] = Valid(Domain(a,2))

validate(Message(Some("a"), None)).to[Domain]
// res1: henkan.optional.ValidateFromOptional.Result[Domain] = Invalid(NonEmptyList(RequiredFieldMissing(b)))

The compilation will fail if the from case class doesn't have all fields the target case class needs

case class MessageWithMissingField(a: Option[String])
scala> validate(MessageWithMissingField(Some("a"))).to[Domain]
<console>:24: error: Cannot build validate function from MessageWithMissingField to Domain, possibly due to missing fields in MessageWithMissingField or missing cats instances (`Traverse` instances are needed to convert fields in containers)

You can convert in the opposite direction as well

from(Domain("a", 2)).toOptional[Message]
// res4: Message = Message(Some(a),Some(2))

Note that if you from case class does not have all the fields the target class has, they will be set as None

case class DomainWithMissingField(a: String)
scala> from(DomainWithMissingField("a")).toOptional[Message]
res5: Message = Message(Some(a),None)

henkan.optional supports nested case classes as well.

Note that if you are converting scalaPB generated case class, it generates Seq for repeated items, although the underlying implementation is actually List. henkan.optional.all has a Traverse instance for Seq but only works fine when the underlying implementation is either a List or Vector

Other examples can be found in examples including a typesafe config transformer