Check Expects in Haskell

This page explains how to write check-expect in Haskell in a basic way that can be used on repl.it (or anywhere). This method works for small numbers of tests. Hopefully it work well enough for now.

If you have your own computer, you can learn to use the HUnit testing system.

1. Write out tests in a list so each gives True when it passes.
2. Define a testing main function test_main.
3. Run the test_main yourself.

Example of Testing

This program is supposed to give True if every 5 is followed by a 6. It has a bug in it which we can uncover by testing.

-- Is every five followed immediately by a six?
fiveSix :: [Integer] -> Bool
fiveSix [] = True
fiveSix (5:6:xs) = fiveSix xs
fiveSix (5:ys) = False
fiveSix (a:b:xs) = fiveSix xs
fiveSix (a:xs) = fiveSix xs

Testing should make sure that the basic situation works, then check the “edge” cases, usually involving the start and end of the list.

1. Does it work in basic cases?
2. Does it work when there is a 5,6 at the start?
3. Does it work when there is a 5,6 at the end?
4. What if a 5 by itself is at the start?
5. What if a 5 by itself is at the end?

test56 = [ True  == fiveSix [5,6,5,6,5,6]
          ,True  == fiveSix [1,2,3,4,5,6,7,8]
          ,False == fiveSix [8,7,6,5,4,3,2,1]
          ,True  == fiveSix [4,6,7,8]  -- end basic
          ,True  == fiveSix [5,6,1,2,4]
          ,True  == fiveSix [1,2,4,5,6]
          ,False == fiveSix [5,1,2,4]
          ,False == fiveSix [1,2,4,5] 
          ]

The testing function:

testMain :: IO()
testMain = do
  putStr "All tests passed? "
  putStrLn $ case (and test56) of True -> "YES"; False -> "NO";
  putStrLn "Results:"
  print $ map show test56

You will see that this method has a lot of repetition. You could make functions that would handle the repetition for you. That is one of the things the HUnit package would do.