hspec- Behavior Driven Development for Haskell

Safe HaskellNone




Hspec is a framework for Behavior-Driven Development (BDD) in Haskell. BDD is an approach to software development that combines Test-Driven Development, Domain-Driven Design, and Acceptance Test-Driven Planning. Hspec helps you do the TDD part of that equation, focusing on the documentation and design aspects of TDD.

Hspec (and the preceding intro) are based on the Ruby library RSpec. Much of what applies to RSpec also applies to Hspec. Hspec ties together descriptions of behavior and examples of that behavior. The examples can also be run as tests and the output summarizes what needs to be implemented.



The three functions you'll use the most are hspec, describe, and it. Here is an example of functions that format and unformat phone numbers and the specs for them.

 import Test.Hspec
 import Test.QuickCheck
 import Test.HUnit

 main :: IO ()
 main = hspec spec

Since the specs are often used to tell you what to implement, it's best to start with undefined functions. Once we have some specs, then you can implement each behavior one at a time, ensuring that each behavior is met and there is no undocumented behavior.

 unformatPhoneNumber :: String -> String
 unformatPhoneNumber = undefined

 formatPhoneNumber :: String -> String
 formatPhoneNumber = undefined

The describe function takes a list of behaviors and examples bound together with the it function

 spec :: Spec
 spec = do
   describe "unformatPhoneNumber" $ do

A Bool can be used as an example.

     it "removes dashes, spaces, and parenthesies" $
       unformatPhoneNumber "(555) 555-1234" == "5555551234"

The pending function marks a behavior as pending an example. The example doesn't count as failing.

     it "handles non-US phone numbers" $
       pending "need to look up how other cultures format phone numbers"

An HUnit Assertion can be used as an example.

     it "converts letters to numbers" $ do
       let expected = "6862377"
           actual   = unformatPhoneNumber "NUMBERS"
       actual @?= expected

A QuickCheck Property can be used as an example.

     it "can add and remove formatting without changing the number" $ property $
       forAll phoneNumber $ \n -> unformatPhoneNumber (formatPhoneNumber n) == n

 phoneNumber :: Gen String
 phoneNumber = do
   n <- elements [7,10,11,12,13,14,15]
   vectorOf n (elements "0123456789")


data Pending Source

A pending example.

Setting expectations

Defining a spec

describe :: String -> Spec -> SpecSource

The describe function combines a list of specs into a larger spec.

context :: String -> Spec -> SpecSource

An alias for describe.

it :: Example v => String -> v -> SpecSource

Create a set of specifications for a specific type being described. Once you know what you want specs for, use this.

 describe "abs" $ do
   it "returns a positive number given a negative number" $
     abs (-1) == 1

pending :: String -> PendingSource

A pending example.

If you want to report on a behavior but don't have an example yet, use this.

 describe "fancyFormatter" $ do
   it "can format text in a way that everyone likes" $

You can give an optional reason for why it's pending.

 describe "fancyFormatter" $ do
   it "can format text in a way that everyone likes" $
     pending "waiting for clarification from the designers"

Running a spec

hspec :: Spec -> IO ()Source

Create a document of the given spec and write it to stdout.

Exit the program with exitSuccess if all examples passed, with exitFailure otherwise.