Running Haskell online (at Repl.it)

This is an alterantive to running GHCI

This avoids having to install GHCI, but it's also slower (for very large problems!) and requires you to revise how you write the code a bit.

1. Visit https://repl.it/languages/haskell
2. Enter the following code in main.hs:

   module Main where
   
   add a b = if a == b then 2 * a else a + b
   
   trials = ["Test 1: add 8 and 5: " ++ (show (add 8 5)),
             "Test 2: add -3 and 4: " ++ (show (add (-3) 4)),
             "Test 3: add 9 and 9: " ++ (show (add 9 9))]
   
   main :: IO ()
   main = putStrLn (show trials)
   

3. Click on the Run > button and confirm you get output like

           ["Test 1: add 8 and 5: 13","Test 2: add -3 and 4: 1","Test 3: add 9 and 9: 18"]
   

4. Replace the definition of add by your own functions.
5. Rewrite the trials variable to execute your code against various cases.
6. Run your code and debug as necessary.

You might also find it useful to use tuples for tests that return different types:

        f x = x * x
        main = putStrLn(show (f 3, f (-1), f 1.2))

Just in case you're having problems, here's what I see:

Notes

• The above code defines two functions, add and main, and then sets trails to a list of test runs. The show function converts values into strings (so the result can be added to a string). There are other ways to run multiple test cases, but this one uses no new constructs.
• To use repl.it, you must define a main with no parameters to run the code. The ::IO () code (above) just means that main is a function that does output. Repl.it will execute main to do the work.
• You can copy your code from the web page to a file in order to save it. But a better solution might be to set up an account (perhaps using a gmail or similar account) so the files can be saved in your web account. This will start a simple IDE for development.