Formify is a Scala 3 utility library built to convert case classes into the x-www-form-urlencoded data format. This format is sometimes required by various APIs (notably the Stripe API and Twilio API) when transmitting data. This library offers a simple method of transforming your algebraic data types into strings compliant with this content type.

libraryDependencies += "com.melvinlow" %% "formify" % <version>

To better understand the functionality of this library, let's take a look at a sample Stripe API request and payload, as per the official documentation:

curl https://api.stripe.com/v1/checkout/sessions \
  -u sk_test_4eC39HqLyjWDarjtT1zdp7dc: \
  --data-urlencode success_url="https://example.com/success" \
  -d "line_items[0][price]"=price_H5ggYwtDq4fbrJ \
  -d "line_items[0][quantity]"=2 \
  -d mode=payment

This payload can be modeled in Scala as follows:

final case class Payload(line_items: List[LineItem], mode: String)
final case class LineItem(price: String, quantity: Int)

val data = Payload(List(LineItem("price_H5ggYwtDq4fbrJ", 2)), "payment")

Formify facilitates the transformation of such a representation back to its original form:

FormDataEncoder.encode(data).compile.toList
// res0: List[Tuple2[String, String]] = List(
//   ("line_items[0][price]", "price_H5ggYwtDq4fbrJ"),
//   ("line_items[0][quantity]", "2"),
//   ("mode", "payment")
// )

FormDataEncoder.encode(data).serialize
// res1: String = "line_items%5B0%5D%5Bprice%5D=price_H5ggYwtDq4fbrJ&line_items%5B0%5D%5Bquantity%5D=2&mode=payment"

The compiled version (before .toList) is a Chain[(String, String)], which can be passed directly to http4s's UrlForm.

Start by including the following imports:

import com.melvinlow.formify.*
import com.melvinlow.formify.instances.auto.given

Following this, you need to provide a method to merge nested field names into a single string using a FormFieldComposer. This step is necessary because x-www-form-urlencoded payloads lack a standard approach for this.

Here is an example of accomplishing this for Stripe's API, where each field name not at the top level is enclosed within brackets "[]":

given FormFieldComposer = FormFieldComposer.make { fragments =>
  fragments.head + fragments.tail.map(f => s"[$f]").toList.mkString
}

That's it! With the prior auto imports, you're now equipped to automatically convert your ADTs:

final case class Cat(owner: Option[String], favorite_foods: Array[String])

val mirai = Cat(None, Array("sushi", "taco bell"))
// mirai: Cat = Cat(owner = None, favorite_foods = Array("sushi", "taco bell"))

FormDataEncoder.encode(mirai).compile.toList
// res3: List[Tuple2[String, String]] = List(
//   ("favorite_foods[0]", "sushi"),
//   ("favorite_foods[1]", "taco bell")
// )

Finally, if you'd like, you can import the syntax package to gain access to the asFormData extension shortcut:

import com.melvinlow.formify.syntax.all.*

mirai.asFormData.compile.toList
// res4: List[Tuple2[String, String]] = List(
//   ("favorite_foods[0]", "sushi"),
//   ("favorite_foods[1]", "taco bell")
// )

Besides the FormFieldComposer, there are two important typeclasses for encoding custom types like java.time.Instant.

The FormValueEncoder[T] typeclass converts the leaf nodes of your ADT into form values. If you're working with a custom type such as java.time.Instant, it's likely you'll need to provide your own instance of this typeclass.

A Contravariant typeclass instance from cats is provided, meaning you only need to determine how to convert your custom type to a type already supported by a FormValueEncoder instance. For instance, java.time.Instant could be encoded into epoch seconds by contramapping it to a Long:

import cats.syntax.all.*
import java.time.Instant

given FormValueEncoder[Instant] = FormValueEncoder[Long].contramap(_.getEpochSecond)

After this, you can conveniently use java.time.Instant in your ADTs:

final case class Person(created_at: Instant)

val jay = Person(Instant.now)
// jay: Person = Person(created_at = 2023-08-07T06:16:50.648187Z)

FormDataEncoder.encode(jay).compile.toList
// res5: List[Tuple2[String, String]] = List(("created_at", "1691389010"))

The FormDataEncoder[T] typeclass is the one you've been interacting with from the beginning. It is responsible for converting the non-leaf, branch parts of your ADT. If you need to generate field names rather than just values, you will need to provide two instances of this typeclass: one for when your branch type contains leaf nodes and another for when it contains another branch.

Once again, a Contravariant typeclass instance from cats is provided, so you simply need to convert your custom type to a type that already has a FormDataEncoder instance. Generally, this will be a Map[String, T] or a List[T], depending on whether your custom data type has named fields or not.

For example, you could encode a Set[T] by sorting its elements first and then converting it into a List[T]:

// When T is a branch node
given [T: Ordering: FormDataEncoder]: FormDataEncoder[Set[T]] =
  FormDataEncoder[List[T]].contramap(_.toList.sorted)

// When T is a leaf node
given [T: Ordering: FormValueEncoder]: FormDataEncoder[Set[T]] =
  FormDataEncoder[List[T]].contramap(_.toList.sorted)

After this, you can use Set[T] in your ADTs:

final case class Puppy(favorite_words: Set[String])

val aya = Puppy(Set("woof", "wan", "bark", "bitcoin"))
// aya: Puppy = Puppy(favorite_words = Set("woof", "wan", "bark", "bitcoin"))

FormDataEncoder.encode(aya).compile.toList
// res6: List[Tuple2[String, String]] = List(
//   ("favorite_words[0]", "bark"),
//   ("favorite_words[1]", "bitcoin"),
//   ("favorite_words[2]", "wan"),
//   ("favorite_words[3]", "woof")
// )

In practice, a type can serve both as a branch node and a leaf node. For instance, you might want to encode a List as a String instead of indexing into it.

To manage this ambiguity, the auto derivation implementation gives priority to FormValueEncoder over FormDataEncoder. You can thus manually prompt the auto derivation to halt and encode your type as a leaf node (instead of further recursion) by providing a FormValueEncoder instance for it.

As an example, let's force a List[Int] to be encoded as a String while keeping the default behavior for List[String]:

given FormValueEncoder[List[Int]] =
  FormValueEncoder[String].contramap(_.mkString(" and "))

final case class Bird(ages: List[Int], colors: List[String])

val mai = Bird(List(1, 2, 3), List("red", "blue", "green"))
// mai: Bird = Bird(
//   ages = List(1, 2, 3),
//   colors = List("red", "blue", "green")
// )

FormDataEncoder.encode(mai).compile.toList
// res7: List[Tuple2[String, String]] = List(
//   ("ages", "1 and 2 and 3"),
//   ("colors[0]", "red"),
//   ("colors[1]", "blue"),
//   ("colors[2]", "green")
// )

Finally, it's worth noting that x-www-form-urlencoded payloads are fundamentally just key-value pairs. As such, there is no inherent notion of nesting. However, the definition of FormData underneath the hood is just a Chain[(NonEmptyChain[String], Option[String])] (i.e., key-value pairs), which means that it is flexible enough to support any encoding scheme.

Simply go through instances.scala and import whatever default converters you need. You can then implement the rest as you see fit.

As mentioned earlier, FormData is really just an opaque type alias to Chain[(NonEmptyChain[String], Option[String])]. The NonEmptyChain[String] represents a field name that is possibly split into fragments due to nesting, while the Option[String] represents a value which may or may not exist.

The compile method simply converts this type into a Chain[(String, String)] by merging the fragments using the provided FormFieldComposer and discarding any fields with missing values.