Refinery - parser for complex data structures

This library is a refinement of Cats' Validated for representing the result of parsing some input, to add more context about where any failures occurred.

Problem

Using Either in Scala is a way to parse complex data structures and report errors. However, the first failure stops the process, and users need to fix it to see what else is wrong. Thus, failures have to be fixed one by one.
Cats' Validated can parse multiple parts of a structure in parallel, reporting all the errors at once. However, when building a complex structure (for example, using mapN), the errors do not point to the exact part that failed.
This library offers a ValidatedC data type that keeps track of context - the logical place in the structure that is currently being parsed. This context is added to errors so it's easy to pinpoint the problem.

Related libraries comparison

Library	Data parsed	Errors reported	Error context
`Either` with a `for` comprehension	✅ Arbitrary	⚠️ First	❌ None
`Validated` with `mapN`	✅ Arbitrary	✅ All	❌ None
Circe	⚠️ JSON	⚠️ First	✅ JSON path
kantan.csv	⚠️ CSV	⚠️ First	⚠️ CSV row only
scala-parser-combinators, FastParse	⚠️ Strings	⚠️ First	✅ Position
Refinery	✅ Arbitrary	✅ All	✅ User defined

Motivational example

Let's try to parse a configuration into a simple case class:

import cats.implicits._

case class Box(width: Int, height: Int, depth: Int)

val config: Map[String, String] = Map(
  "width" -> "10",
  "height" -> "oops",
)

The configuration is invalid (height is not an integer, and depth is missing). How will the errors be reported?

Either

def parseInt(value: String): Either[String, Int] =
  value.toIntOption.toRight(s"Invalid integer: $value")

def getConfig(key: String): Either[String, String] =
  config.get(key).toRight(s"No key: $key")

println(
  (
    getConfig("width").flatMap(parseInt),
    getConfig("height").flatMap(parseInt),
    getConfig("depth").flatMap(parseInt),
  ).mapN(Box.apply)
)
// => Left(Invalid integer: oops)

Only the first error is reported.

Cats' Validated

def parseInt(value: String): ValidatedNec[String, Int] =
  value.toIntOption.toValidNec(s"Invalid integer: $value")

def getConfig(key: String): ValidatedNec[String, String] =
  config.get(key).toValidNec(s"No key: $key")

println(
  (
    getConfig("width").andThen(parseInt),
    getConfig("height").andThen(parseInt),
    getConfig("depth").andThen(parseInt),
  ).mapN(Box.apply)
)
// => Invalid(Chain(Invalid integer: oops, No key: depth))

Both invalid height and missing depth errors are reported, but the height error lacks context.

Refinery

import refinery._

def parseInt(value: String): ValidatedC[String, String, Int] =
  value.toIntOption.toValidatedC(s"Invalid integer: $value")

def getConfig(key: String): ValidatedC[String, String, String] =
  config.get(key).toValidatedC("Key missing").context(key)

println(
  (
    getConfig("width").andThen(parseInt),
    getConfig("height").andThen(parseInt),
    getConfig("depth").andThen(parseInt),
  ).mapN(Box.apply)
)
// => Invalid(Chain(height: Invalid integer: oops, depth: Key missing))

Because getConfig now adds context to the validated value, the "Invalid integer" error now specifies that the bad value was height.

koterpillar / refinery 0.0.8