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Bastion

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Bastion is a library to convert types, using when needed custom defined smart constructors. This library should be well suited for Domain Driven Designed applications.

Based on Magnolia for typeclass derivation, and parts of uPickle for JSON serialization/deserialization in bastion-ujson module.

Disclaimer

This project is a way for me to learn typeclass derivation using Magnolia. It's not ready for a production use, and it's too early too know if it will (but I hope so !).

Table of content

Concepts

Converting from a data type A to another data type B could have been represented as a typeclass Convert[A,B]{ def to(a:A):B}.

Derive this typeclass on two type parameters at the same time seems not trivial with Shapeless, and I just don't find how with Magnolia. As I was trying to implement it, the obvious way for me was to encode the data type A in a simplistic representation, for which the contract is known by any decoder. Since the majority of the translating labor is to convert fields from a product type A to fields of the same name of a type B, I just use the dynamics feature of Scala to achieve this translation.

So there are four types that you will manipulate through this library :

  • DynamicRepr, which represents the encoding of the type you want to convert from
  • DynamicReprEncode, which is the contract for instances of encoders from a specific type to DynamicRepr
  • Decoder, which is the contract for instances of decoders from any DynamicRepr to a specific type
  • DecodingState, that you may need if you implement your own decoder, which represents the current state of a decoder.

It should be enough the majority of time to make the imports below and call the convert method that will use instance of Encode and Decode in the implicit scope :

import bastion._
import bastion.derivation.dynamicrepr.auto._
import bastion.derivation.decode.auto._
//...
val instanceOfTypeB:TypeB = instanceOfTypeA.convert[TypeB]

For more advanced usage, please see below, and the examples here.

Installation

Releases Snapshots
Sonatype Nexus (Releases) Sonatype Nexus (Snapshots)
Sonatype Nexus (Releases) Sonatype Nexus (Snapshots)

Sbt

libraryDependencies ++= Seq(
  "dev.scarisey" %% "bastion-core" % "X.Y.Z",
  "dev.scarisey" %% "bastion-ujson" % "X.Y.Z"
)

Usages

Simple mapping

import bastion._
import bastion.derivation.dynamicrepr.auto._
import bastion.derivation.decode.auto._

final case class PersonExternal(
    id: String,
    firstName: String,
    lastName: String,
    birthdate: LocalDate,
    posX: Double,
    posY: Double
  )
final case class Person(id: String, firstName: String, lastName: String, birthdate: LocalDate)

val person = PersonExternal("anId", "firstName", "lastName", LocalDate.parse("1985-01-12"), 44.846565, -0.567351).convert[Person]
//Right(Person(anId,firstName,lastName,1985-01-12))

Convert to an ADT

import bastion._
import bastion.derivation.dynamicrepr.auto._
import bastion.derivation.decode.auto._

case class A1(aField1: String)
case class A2(aField2: Int)
case class A3(aDouble: Double)
  
sealed trait B
case class B0(aBoolean: Boolean) extends B
case class B1(aField1: String)   extends B
case class B2(aField2: Int)      extends B

A1("foo").convert[B] // Right(B1("foo"))
A2(42).convert[B] // Right(B2(42))
A3(2.0).convert[B] //Left(No matching subtypes of B for ProductDynamicRepr(A3(2.0)))

Use your smart constructors

import bastion._
import bastion.derivation.dynamicrepr.auto._
import bastion.derivation.decode.auto._

case class A(aString:String,anInt:Int)

case class Wrapped private(aField: String)
object Wrapped{
  def apply(s:String):Either[String,Wrapped] = Either.cond(s.length>2,new Wrapped(s),"length > 2")
}
case class B(aString:Wrapped,anInt:Int)

implicit val decodeWrapped:Decoder[Wrapped] = Decoder.wrapE(Wrapped.apply)

A("foo",42).convert[B] //Right(B(Wrapped("foo"),42))
A("fo",42).convert[B] //Left(WrappedError("length > 2"))

See this more complete example : SmartConstructors

Lenient case

import bastion._
import bastion.derivation.dynamicrepr.configured.auto._
import bastion.derivation.decode.auto._
import bastion.derivation.dynamicrepr.Configuration.lenient

case class Source(aString: String, anInt: Int, aBoolean: Boolean)
case class Target(an_int: Int, A_String: String)

Source("foo", 42, true).convert[Target] //Target(42,foo)

Conversions from string

import java.time.LocalDateTime
import bastion._
import derivation.dynamicrepr.auto._
import derivation.decode.auto._

case class A(aLocalDateTime: String, aDouble: String, anInteger: String)
case class B(aLocalDateTime: LocalDateTime, aDouble: Double, anInteger: Int)

A("2020-06-04T13:45:00.000", "3.14", "42").convert[B] //Right(B(2020-06-04T13:45,3.14,42))

You can check Decoder for an exhaustive view.

Conversions between numeric types

The basic idea is that conversions are allowed from narrower numeric types to wider numeric types. For example, you can convert from an Int to a Long, Double, BigInt or BigDecimal, but you can't convert from a Long to an Int (you should implement your own decoder, where you will decide how to handle limits).

import bastion._
import derivation.dynamicrepr.auto._
import derivation.decode.auto._

case class SomeLong(aField:Long)
case class SomeFloat(aField:Float)

SomeLong(Long.MaxValue).convert[SomeFloat] //Right(SomeFloat(9.223372E18))

You can check Decoder for an exhaustive view.

Your own converter for non matching types

import bastion._
import derivation.dynamicrepr.auto._
import bastion.derivation.decode.auto._

case class SubSource1(aString: String)
case class SubSource2(anInt: Int)
case class Source(sub1: SubSource1, sub2: SubSource2)
case class Target(field1: Int, field2: String)

implicit val decoderTarget: Decoder[Target] = 
  Decoder.instance(g => (g.sub2.anInt, g.sub1.aString).apply(Target.apply))

Source(SubSource1("foo"), SubSource2(42)).convert[Target] //Right(Target(42,foo))
//...

implicit val decoderTarget: Decoder[Target] = 
  Decoder.instance(g => (g.sub32.anInt, g.sub1.aString).apply(Target.apply))

Source(SubSource1("foo"), SubSource2(42)).convert[Target]
    //Left(IncorrectPath: applying root.sub32.anInt on ProductDynamicRepr(Source(SubSource1(foo),SubSource2(42))) produces NilDynamicRepr)

Json serialization

import bastion._,json._,derivation.json._

case class FooString(foo: String)
case class Foo(bar: Double, baz: FooString, items: List[String])

val foo = Foo(42.0, FooString("aFoo"), List("baz", "bar"))
foo.asJson// {"bar":42,"baz":{"foo":"aFoo"},"items":["baz","bar"]}
foo.asJsonAst //the uJson Value tree

Json deserialization

import bastion._
import json.decode
import derivation.decode.auto._

case class Foo(aField1:String,aField2:Double)
val aJson = """{"aField2":33.0,"aField1":"foo"}"""

decodeJson[Foo](aJson) //Right(Foo(foo,33.0))

Things to do, and perspectives

  • There are still too much imports to do before being able to convert types.
  • I would love to verify at compile time the shape of a DynamicRepr, but I still don't know how to do unless using Shapeless (and replace DynmicRepr by HList). I also guess the issue about inlining combine and dispatch could adress this problem.

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License

bastion 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.