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https://github.com/kailuowang/henkan

A small library for converting between case classes.
https://github.com/kailuowang/henkan

conversion scala shapeless transformation typelevel

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A small library for converting between case classes.

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# Henkan [変換]



A small library for converting between case classes. 



## Modules



### `henkan.convert`



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



  *Features*:



 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



### `henkan.optional`



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



Henkan is available on Scala 2.12, 2.13 as well as Scala.js

 

```scala



 libraryDependencies += "com.kailuowang" %% "henkan-convert" % "0.6.5"



 libraryDependencies += "com.kailuowang" %% "henkan-optional" % "0.6.5"

```



## Examples



### Transform between case classes



```scala

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`

```scala

import henkan.convert.Syntax._

// import henkan.convert.Syntax._



employee.to[UnionMember]()

// res0: UnionMember = UnionMember(George,123 E 86 St,1963-03-12)



unionMember.to[Employee]()

// res1: Employee = Employee(Micheal,41 Dunwoody St,1994-07-29,50000.0)



unionMember.to[Employee].set(salary = 60000.0)

// res2: Employee = Employee(Micheal,41 Dunwoody St,1994-07-29,60000.0)

```

Missing fields will fail the compilation

```scala

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

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

:20: error: 

    You have not provided enough arguments to convert from People to Employee.

    shapeless.HNil



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

                          ^

```

Wrong argument types will fail the compilation

```scala

scala> unionMember.to[Employee].set(salary = 60) //salary was input as Int rather than Double

:20: error: 

    You have not provided enough arguments to convert from UnionMember to Employee.

    shapeless.labelled.FieldType[Symbol @@ String("salary"),Int] :: shapeless.HNil



error after rewriting to henkan.convert.Syntax.henkanSyntaxConvert[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



```scala

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`



```scala

import cats.data.Validated

import cats.implicits._

import henkan.optional.all._

```



```scala

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

```scala



case class MessageWithMissingField(a: Option[String])

```



```scala

scala> validate(MessageWithMissingField(Some("a"))).to[Domain]

: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)

       validate(MessageWithMissingField(Some("a"))).to[Domain]

                                                      ^

```



You can convert in the opposite direction as well

```scala

from(Domain("a", 2)).toOptional[Message]

// res3: 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`



```scala

case class DomainWithMissingField(a: String)

```

```scala

scala> from(DomainWithMissingField("a")).toOptional[Message]

res4: 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](examples/src/main/scala/henkan/) including a typesafe config transformer



## Contributors and participation



henkan is currently maintained by [Kailuo Wang][kailuowang].



Any form of contribution (issue report, PR, etc) is more than welcome.



The henkan project supports the [Typelevel][typelevel] [code of conduct][typelevel-coc]

and wants all of its channels (Gitter, GitHub, etc.) to be welcoming environments for

everyone.



## License



henkan is licensed under the [Apache License, Version 2.0][apache2]

(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.



## Special Thanks



The `convert` was originally adapted from this [gist](https://github.com/kailuowang/henkan/edit/master/docs/src/main/tut/README.md) 

by @joprice. 



[apache2]: http://www.apache.org/licenses/LICENSE-2.0

[kailuowang]: http://twitter.com/kailuowang

[typelevel]: http://typelevel.org/

[typelevel-coc]: http://typelevel.org/conduct.html

[kittens]: http://github.com/milessabin/kittens

[shapeless]: http://github.com/milessabin/shapeless

[cats]: http://github.com/typelevel/cats