acornsgrow / reeds

Reads typeclass for Scala



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Reeds is a small Scala library with one goal: provide a standard Reads[T] typeclass and standard instances for it.

  1. Reads[T]
  2. Usage
  3. Included instances
  4. Defining instances
  5. Subprojects
    1. reeds-shapeless
    2. reeds-circe
  6. Contributing
  7. Code of conduct
  8. License
  9. Contributors


Most projects will at one point need to read simple values from strings. In Scala, this often ends up as some variation of a typeclass that captures the operation String => T (or String => F[T] for some functor F[_]). The goal of Reeds is to provide this typeclass as an alternative to proliferating one-off isomorphisms of it.

Reads is defined thusly:

trait Reads[T] extends (String => ValidatedNel[Throwable, T])

ValidatedNel comes from cats and captures a validated value, which is either a value or a list of errors. Validated is different from Xor in that it is typically used to aggregate all failures, rather than short-circuiting at the first failure.


To simply read a value of type T from a String:

val validResult = Reads.apply[UUID]("deadbeef-dead-dead-beef-deaddeadbeef")
//validResult: Valid[UUID]

val invalidResult = Reads.apply[UUID]("not a valid UUID")
//invalidResult: Invalid[Throwable]

This requires that an instance Reads[T] is available (in the above example, Reads[UUID]).

A lifted typeclass FReads[F[_], T] is also provided. This allows any F[String] to be read into F[ValidatedNel[Throwable, T]] if F has a Functor instance and T has a Reads instance:

val results = FReads.apply[Seq, UUID](Seq("deadbeef-dead-dead-beef-deaddeadbeef", "not a valid UUID"))
//results: Seq[ValidatedNel[Throwable, UUID]]

val result = results.sequenceU
//result: Invalid[Throwable]

Included instances

Several commonly-used instances are provided on the Reads companion object. It is done this way so that you (and users of your library) don't have to know about Reeds or import a bunch of junk from Reeds to use your code. This goes against the somewhat common practice of requiring users (and users' users, and users' users' users etc) to import the instances into scope.

The instances currently provided are:

  • Short, Int, Long, Byte, Float, Double, Bool - these are pretty self-explanatory; each delegates to _.toX
  • BigDecimal, BigInt - delegate to the built-in parser for each.
  •, - these behave pretty much as you would expect - by simply passing the string to new URL(_) and new URI(_).
  • - this instance expects a hostname or address (IPv4 or [IPv6]) and a port number, with a colon separating the host and port.
  • java.util.UUID - this instance attempts to delegate the UUID parsing to UUID.fromString, but falls back to stripping all dashes and separating the digits into the format that UUID.fromString expects (8-4-4-4-12).
  • java.util.Currency - delegates to java.util.Currency.getInstance(_.toUpperCase)
  • java.time.LocalDate, java.time.LocalDateTime - expect a date or date/time, respectively, each in ISO-8601 format without a time zone.
  • java.time.OffsetDateTime, java.time.ZonedDateTime, java.time.Instant, java.util.Date - all expect a date/time in ISO-8601 format with a timezone offset. In the case of java.time.Instant, the given result is the Instant at UTC for the given time with its zone (i.e. the same result as OffsetDateTime with _.toInstant applied).
  • java.sql.Date - expects a date in common SQL format YYYY-MM-DD (which happily coincides with ISO-8601 for dates).
  • java.sql.Timestamp - expects a date/time in common SQL format YYYY-MM-DD Delegates to java.sql.Timestamp.valueOf(_).

Defining instances

If you need instances that aren't provided, use Reads.instance, i.e.:

import, Validated._
implicit val MyTypeReads : Reads[MyType] = Reads.instance[MyType] {
  str => if(str == "foo") Valid(MyType()) else Invalid(Reads.SimpleError(s"$str is not a valid MyType")).toValidatedNel


Reads by itself isn't all that interesting; it's what you can do with it as a typeclass that make it useful. For example, take a look at the two tiny subprojects:


reeds-shapeless can derive conversions from String products to any case class which has a Reads instance for each of its product types. For example, I could define an ADT:

case class Sale(id: UUID, createdAt: OffsetDateTime, amount: BigDecimal, currency: Currency)

And if I have a list of strings (say, from a CSV or command line arguments) I can automatically marshall it:

import reeds.generic._
val result = Seq(
//result: Valid(Sale(...))

Similarly, if I have a Map[String, String], I can marshall that as well:

val result = Map(
  "id" -> "deadbeef-dead-beef-beef-deaddeadbeef",
  "createdAt" -> "2016-03-25T07:17:27.209-0700",
  "amount" -> "237.99",
  "currency" -> "USD"
//result: Valid(Sale(...))

In that case, my ADT can also include Option, and will pass validation even if optional fields are missing from the map:

case class SaleOptional(id: UUID, createdAt: LocalDate, amount: Option[BigDecimal], currency: Currency)

val result = Map(
  "id" -> "deadbeef-dead-beef-beef-deaddeadbeef",
  "createdAt" -> "2016-03-25T07:17:27.209-0700",
  "currency" -> "USD"
//result: Valid(SaleOptional(deadbeef-dead-beef-beef-deaddeadbeef, 2016-03-25T07:17:27.209-0700, None, USD))

Reeds also knows about default values, so if I have an ADT with defaults:

case class SaleDefault(id: UUID, createdAt: LocalDate, amount: BigDecimal, currency: Currency = Currency.getInstance("USD"))

Then it will be provided if the input doesn't specify it:

val result = Map(
  "id" -> "deadbeef-dead-beef-beef-deaddeadbeef",
  "createdAt" -> "2016-03-25T07:17:27.209-0700",
  "amount" -> "237.99"

//result: Valid(Sale(..., USD))

Finally, if I have a Map[String, F[String]] for some Functor[F], I can have that F embedded in my ADT:

import cats.std.list._

case class SaleList(id: UUID, createdAt: LocalDate, amounts: List[BigDecimal], currency: Currency)

val result = Map(
  "id" -> List("deadbeef-dead-beef-beef-deaddeadbeef"),
  "createdAt" -> List("2016-03-25T07:17:27.209-0700"),
  "amounts" -> List("220.99", "42.22", "47.47"),
  "currency" -> List("USD")).readMap[SaleList]
//result: Valid(SaleList(deadbeef-dead-beef-beef-deaddeadbeef, 2016-03-25T07:17:27.209-0700, List(220.99, 42.22, 47.47), USD))


reeds-circe provides a (low-priority) circe Decoder for anything that has a Reads instance. This gives you a free Decoder for all the java.time types, UUID, URI, and URL, assuming that they are typically stored as strings in JSON. Also, if you happen to define a custom Reads, you'll get a free Decoder for it as well.

import reeds.circe._
import java.time.OffsetDateTime

case class Foo(id: Int, createdAt: OffsetDateTime)

val result = circe.parser.parse("""{"id": 10, "createdAt": "2016-03-25T07:17:27.209-0700"}""").as[Foo]

//result: Right(Foo(10, 2016-03-25T07:17:27.209-0700))


Because reeds intends to be a go-to implementation of Reads, we welcome contributions of new instances. Please see the contribution guide for more information about how to contribute to reeds.

Code of conduct

The reeds project supports the Typelevel Code of Conduct and wants all its channels to be welcoming environments for everyone.


Reeds is released under the Apache License Version 2.0

Copyright 2016 Acorns Grow, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
See the License for the specific language governing permissions and
limitations under the License.