ScalaValidationWrite.md

Serializing data

Introduction

To serialize data, the validation API provides the Write type. A Write[I, O] defines a way to transform data, from type I to type O. It's basically a function I => O, where I is the type of the input to serialize, and O is the expected output type.

A simple example

Let's say you want to serialize a Float to String. All you need to do is to define a Write from Float to String:

import jto.validation._
def floatToString: Write[Float, String] = ???

For now we'll not implement floatToString, actually, the validation API comes with a number of built-in Writes, including Writes.floatW[T].

All you have to do is import the default Writes.

object Writes extends NumericTypes2StringWrites
Writes.floatW

Let's now test it against different Float values:

Writes.floatW.writes(12.8F)
Writes.floatW.writes(12F)

Defining your own Write

Creating a new Write is almost as simple as creating a new function. This example creates a new Write serializing a Float with a custom format.

val currency = Write[Double, String]{ money =>
  import java.text.NumberFormat
  import java.util.Locale
  val f = NumberFormat.getCurrencyInstance(Locale.FRANCE)
  f.format(money)
}

Testing it:

currency.writes(9.99)

Composing Writes

Writes composition is very important in this API. Write composition means that given two writes a: Write[I, J] and b: Write[J, O], we can create a new write c: Write[I, O].

Example

Let's see we're working on an e-commerce website. We have defined a Product class. Each product has a name and a price:

case class Product(name: String, price: Double)

Now we'd like to create a Write[Product, String] that serializes a product to a String of it price: Product("demo", 123) becomes 123,00 €

We have already defined currency: Write[Double, String], so we'd like to reuse that. First, we'll create a Write[Product, Double] extracting the price of the product:

val productPrice: Write[Product, Double] = Write[Product, Double](_.price)

Now we just have to compose it with currency:

val productAsPrice: Write[Product,String] = productPrice andThen currency

Let's test our new Write:

productAsPrice.writes(Product("Awesome product", 9.99))