Catchy combinators for HUnit

(inspired by ScalaTest's ShouldMatchers)

The three main primitives are shouldBe, shouldSatisfy and shouldThrow. They can be used with HUnit, or any framework that integrates with HUnit, like test-framework or Hspec.

An introductory example

Here is an example that uses Hspec. It's a partial specification of itself.

import Test.Hspec
import Control.Exception

main :: IO ()
main = hspec $ do
  describe "shouldBe" $ do
    it "asserts equality" $ do
      "foo" `shouldBe` "foo"

  describe "shouldSatisfy" $ do
    it "asserts that a predicate holds" $ do
      "bar" `shouldSatisfy` (not . null)

  describe "shouldThrow" $ do
    it "asserts that an exception is thrown" $ do
      evaluate (1 `div` 0 :: Int) `shouldThrow` (== DivideByZero)

shouldBe

shouldBe is just an alias for HUnit's @?=.

shouldSatisfy

shouldSatisfy asserts that some predicate holds for a given value.

"bar" `shouldSatisfy` (not . null)

It is similar to HUnit's assertBool, but gives a useful error message.

>>> 23 `shouldSatisfy` (> 42)
*** Exception: HUnitFailure "23 did not satisfy predicate!"

shouldReturn

shouldReturn asserts that an action returns a given value.

launchMissiles `shouldReturn` Left "permission error"

shouldThrow

shouldThrow asserts that an exception is thrown. The precise nature of that exception is described with a Selector.

error "foobar" `shouldThrow` anyException

A Selector is a predicate, it can simultaneously constrain the type and value of an exception.

throw DivideByZero `shouldThrow` (== DivideByZero)

To select all exceptions of a given type, const True can be used.

error "foobar" `shouldThrow` (const True :: Selector ErrorCall)

For convenience, predefined selectors for some standard exceptions are provided.

error "foobar" `shouldThrow` anyErrorCall

Some exceptions (like ErrorCall) have no Eq instance, so checking for a specific value requires pattern matching.

error "foobar" `shouldThrow` (\e -> case e of
    ErrorCall "foobar" -> True
    _ -> False
    )

For such exceptions, combinators that construct selectors are provided. Each combinator corresponds to a constructor; it takes the same arguments, and has the same name (but starting with a lower-case letter).

error "foobar" `shouldThrow` errorCall "foobar"