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typelevel/mouse

A small companion to cats
Source code at GitHub null
typelevel/mouse.json
{
  "createdAt": "2016-05-05T13:14:54Z",
  "defaultBranch": "main",
  "description": "A small companion to cats",
  "fullName": "typelevel/mouse",
  "homepage": null,
  "language": "Scala",
  "name": "mouse",
  "pushedAt": "2025-11-26T16:07:48Z",
  "stargazersCount": 375,
  "topics": [
    "cats",
    "scala",
    "typelevel"
  ],
  "updatedAt": "2025-11-25T11:41:10Z",
  "url": "https://github.com/typelevel/mouse"
}

Mouse Continuous Integration Maven Central Cats friendly

Section titled “Mouse ”

Mouse is a small companion to the Cats functional programming library and the Scala standard library.

The library arose from this Cats issue and is a Typelevel member.

Mouse is published for Scala 2.12, 2.13 and 3.0. For Scala.jvm:

"org.typelevel" %% "mouse" % version

For scala.js 1.x:

"org.typelevel" %%% "mouse" % version

Learn more about Mouse at https://typelevel.org/mouse/.

Content

Section titled “Content”

Mouse includes enrichments for:

  • [Any]!(./shared/src/main/scala/mouse/any.scala)
  • [Boolean]!(./shared/src/main/scala/mouse/boolean.scala)
  • [Double]!(./shared/src/main/scala/mouse/double.scala)
  • [F[A] for any F, A]!(./shared/src/main/scala/mouse/anyf.scala)
  • [F[Either[A, B]]]!(./shared/src/main/scala/mouse/feither.scala)
  • [F[Option[A]]]!(./shared/src/main/scala/mouse/foption.scala)
  • [F[G[A]]]!(./shared/src/main/scala/mouse/fnested.scala)
  • [F[G[H[A]]]]!(./shared/src/main/scala/mouse/fnested.scala)
  • [F[TupleN]]!(./shared/src/main/scala/mouse/ftuple.scala)
  • [Int]!(./shared/src/main/scala/mouse/int.scala)
  • [List]!(./shared/src/main/scala/mouse/list.scala)
  • [Long]!(./shared/src/main/scala/mouse/long.scala)
  • [Map]!(./shared/src/main/scala/mouse/map.scala)
  • [Option]!(./shared/src/main/scala/mouse/option.scala)
  • [Set]!(./shared/src/main/scala/mouse/set.scala)
  • [String]!(./shared/src/main/scala/mouse/string.scala)
  • [Try]!(./shared/src/main/scala/mouse/try.scala)
  • [Tuple]!(./shared/src/main/scala-2/src/main/scala/mouse/tuple.scala)

Example:

Section titled “Example:”
scala> import mouse.all._
import mouse.all._


scala> true.option("Its true!")
res0: Option[String] = Some(Its true!)


scala> def makeEven(i: Int) = (i % 2 == 1).applyIf(i)(_ - 1)
def makeEven(i: Int): Int


scala> val e1: Either[String, Int] = Left("failed")
e1: Either[String,Int] = Left(failed)


scala> true.whenA(e1)
res0: Either[String,Unit] = Left(failed)


scala> false.whenA(e1)
res1: Either[String,Unit] = Right(())


scala> res0.cata(msg => s"Message received: ${msg}", "No message")
res1: String = Message received: Its true!


scala> "1.0".parseFloat
res0: Either[NumberFormatException, Float] = Right(1.0F)


scala> "foo".parseIntValidated
res1: cats.data.Validated[NumberFormatException,Int] = Invalid(java.lang.NumberFormatException: For input string: "foo")


scala> val t1 = scala.util.Try(new java.net.URL("https://www.github.com"))
t1: scala.util.Try[java.net.URL] = Success(https://www.github.com)


scala> t1.cata(msg => s"URL is valid!", error => s"URL is invalid: ${error.getMessage}")
res1: String = URL is valid!


scala> t1.toEither
res2: Either[Throwable,java.net.URL] = Right(https://www.github.com)


scala> val t2 = scala.util.Try(new java.net.URL("https//www.github.com"))
t2: scala.util.Try[java.net.URL] = Failure(java.net.MalformedURLException: no protocol: https//www.github.com)


scala> t2.cata(msg => s"URL is valid!", error => s"URL is invalid: ${error.getMessage}")
res3: String = URL is invalid: no protocol: https//www.github.com


scala> t2.toEither
res4: Either[Throwable,java.net.URL] = Left(java.net.MalformedURLException: no protocol: https//www.github.com)


scala> val intToBytes = 123456789.toByteArray
intToBytes: Array[Byte] = Array(7, 91, -51, 21)


scala> val longToBase64 = 123456789L.toBase64
longToBase64: String = AAAAAAdbzRU


scala> 5.squared
res0: Int = 25


scala> 1.5 |> (_.toInt) |> (_.toString)
res0: String = 1


//lift a partial function into a total function to an Either, when you want to treat unhandled input cases as an error
scala> liftEither[Option[Int]]!({case Some(n) => n}, a => s"Unexpected: $a")(Some(6))
res0: Either[String,Int] = Right(6)


scala> val mapped = Map(1 -> 2, 3 -> 4).mapKeys(_ * 2)
mapped: Map[Int,Int] = Map(2 -> 2, 6 -> 4)


scala> val foption = List(Option(1), Option(2), Option(4)).mapIn(_ * 2)
foption: List[Option[Int]] = List(Some(2), Some(4), Some(8))


scala> val feither = List(Either.cond(true, 1, "0")).mapIn(_ * 2)
feither: List[Either[String, Int]] = List(Right(2))


scala> val listOption = List(Option(1), Option(2)).mapNested2(_ * 2)
listOption: List[Option[Int]] = List(Some(2), Some(4))


scala> val listOptionList = List(Option(List(1)), Option(List(2))).mapNested3(_ * 2)
listOptionList: List[Option[List[Int]]] = List(Some(List(2)), Some(List(4)))


scala> val tupleHead = (1, 2, 4, 8).head
tupleHead: Int = 1


scala> val tupleLast = (1, 2, 4, 8).last
tupleLast: Int = 8


scala> Set(0).tailOrEmpty
val res3: Set[Int] = Set()


scala> Nil.tailOrEmpty
val res1: List[Nothing] = List()


scala> Set(0).tailOption
val res4: Option[cats.data.NonEmptySet[Int]] = None


scala> List(0,1,2).tailOption
val res5: Option[cats.data.NonEmptyList[Int]] = Some(NonEmptyList(1, 2))

Scope of Library

Section titled “Scope of Library”
  • Provide enrichments to classes from the Scala standard library that convert to Cats datatypes, or otherwise improve the functional programming experience.

Contributing

Section titled “Contributing”

Mouse is maintained by @benhutchison and @danicheg.

Issues and pull requests are welcome. Code contributions should be aligned with the above scope to be included, and include unit tests. See [contributing guide]!(./DEV.md) for more details.

This project supports the Scala code of conduct and aims that its channels (mailing list, Gitter, github, etc.) to be welcoming environments for everyone.