xpct provides an algebra that abstracts assertion of conditions and extraction of values from heterogeneous data types contained in a computation effect.

In more concrete terms, you can sequence IOs in a for-comprehension, adding an expectation to each step, while extracting values contained in Options or Eithers through typeclass based matchers:

import cats.implicits._

for {
  a <- IO.pure(Either.right("test")) must contain("test")
  b <- IO.pure(Option(5)) must beSome(be_>=(2))
  _ <- IO.pure(s"$a $b") must_== "test 5"
} yield ()

Module IDs

"io.tryp" %% "xpct-core" % "0.2.1"
"io.tryp" %% "xpct-klk" % "0.2.1"
"io.tryp" %% "xpct-specs2" % "0.2.1"
"io.tryp" %% "xpct-scalatest" % "0.2.1"
"io.tryp" %% "xpct-utest" % "0.2.1"


Matching and Extracting

Matches are based on the typeclass Match, where Predicate[_] is an arbitrary data type that represents a specific condition, like IsSome[Int](5) (here Target is Int):

trait Match[Predicate[_], Target, Subject, Output]
  def apply(a: Subject, fb: Predicate[Target]): AssertResult[Output]

Subject is the expectable value; matches can be performed on differing types, producing a third type Output that is extracted monadically. When a Predicate[Target] value is passed to the implicit must method on the expectable, an Xp value is produced, which uses the Output value returned from Match.apply for monadic composition, allowing you to use the expectation in a for-comprehension regardless of the type of Subject.

Nesting matchers is a mechanism that is implemented in an ad-hoc way in common spec frameworks. With xpct, a separate instance of Match can be defined that has another matcher type as its Target, allowing arbitrary nesting.

Included Matchers and Modifiers

All matchers and modifiers can be chained, i.e. applied to both IO and Xp[IO, *].

Match Types

Alternative syntaxes are available for matching:

Implicit Methods




IO and Retrying

When testing asynchronous programs, especially UIs, it is not unusual to wait for a condition to become fulfilled. In frameworks like specs2 and scalatest, this feature is implemented as a special case with severe limitations on composability. xpct treats retrying as a first class operation, allowing to retry a sequence of expectations with the same semantics as strict operations:

for {
  text <- {
    for {
      elem <- getUiElement(5) must beASome[Text]
      text <- IO.pure(elem) must containText("Cancel")
    } yield text
  }.retryEvery(10, 100.milliseconds)
  _ <- setUiElementText(6, text) must beRight
} yield ()

Aside from cats.effect.IO, arbitrary async effects can be used, as long as they implement the typeclass EvalXp:

trait EvalXp[F[_]]
  def apply[A](fa: F[A]): A

For the retry operation, an instance of cats.effect.Timer[F] is required.

Test Frameworks


kallikrein integration is the most seamless one, since it also focuses on IO programs.

The xpct-klk package contains instances of Compile[Xp] and TestResult[XpResult]. Either import the package xpct.klk._, mix in XpctKlk or subclass XpctKlkTest[F, SbtResources].


The xpct-specs2 package contains an instance of AsResult[Xpct], which is sufficient for automatic conversion of Xpct values to specs2 Fragments.

A convenience trait XpctSpec is provided, which includes the implicit conversion to the extension class with must methods.


The xpct-scalatest package contains the trait XpctSpec, providing a helper function xpct that converts an Xpct to a TestFailedException.


The xpct-utest package contains the trait XpctSpec, providing a helper function xpct that converts an Xpct to an exception.