learn4haskell

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Mention where we need to be more careful when working with Ints:

• 0
• positive
• negative
• in some bounds

I'm pretty sure that's more of a general programming thing. But if Haskell has interesting ways of dealing with it, it may be useful to highlight it. Python for example famously allows the following shortcut for my_number < x && my_number > y

y < my_number < x 

which is much more "pythonic" and cool.

Yes, you are right! It just could be useful to mention just so people would setup on thinking more on the corner-cases while working on tasks 🙂

That is a very handy shortcut, indeed. Unfortunately, there is no such feature in Haskell 😞

I overlooked some of the negative numbers on my first run through of the tests (I didn't run the tests at all until I completed all 10 tasks of Chapter 1), but when I read through the output of the tests, it was readily apparent what cases I was missing and I added them easily enough. Although you could be more explicit in the task text what kinds of tests to expect, I think there is some benefit to be had from having students "solve" the task first and then read through the test output when there are (inevitable) failures, although perhaps it wouldn't hurt to have an explicit suggestion and example of test failure output in the ReadMe itself.

At least with the code I had been using:

firstDigit :: Int -> Int
firstDigit n = go n 0
  where
    go :: Int -> Int -> Int
    go 0 n = n
    go n _ = go (n `div` 10) n

The failure mode for the negative integers is to go into an infinite loop. The tests don't detect this, and it can look to a naive user as if the test for big integers is failing. I feel like detecting a loop would be a good part of any unit test.