I recently implemented the bowling kata in Haskell. In the process, I found out how set up my environment to comfortably do Test Driven Development. Hopefully, others might find this post helpful to begin their journey with the language. I used the following components:
Haskell installation: Haskell Platform. This also gives you GHCi which you can use as an interactive environment and type inspector.
Test libraries: Hspec, which is based on RSpec. This can be installed using Haskell's package manager, cabal, from the command line with cabal install hspec.
Helper libraries: Printf for colourful command line output.
Using the example from Hspec's documentation, I began with this structure for my code:
module BowlingTests whereimport Bowling
import Text.Printf (printf)
testScoreGame :: String -> Int -> SpectestScoreGame game score =
it (printf “should return the score for game : %s → %d \n” game score) $
scoreGame game `shouldBe` score
main = hspec $ do
describe "scoreGame" $ do
So to test a function, you add a function in your test file, usually the same name with a ‘test’ prefix. This function takes the inputs to your function under test and the expected output as parameters. Then using Hspec you describe what you are testing. As you can see the ‘it’ part is written in the test function. You can of course omit this helper function and write all your tests under main = hspec $ do, which may be nicer if you want to describe in more detail what each individual test is testing.
module Bowling wherescoreGame :: String -> IntscoreGame game = 0
These files are in the same directory, now I can run my tests from the command line.
$ runhaskell BowlingTests.hs
should return the score for game : — — — — — — — — — — → 0
Finished in0.0000 seconds
1 example, 0 failures
There you have it. Now I can focus on writing a failing test and making it pass.