Using QuickCheck’s Positive Type Level Modifier

One of my students this semester was struggling with the material. This student is a very diligent worker and has been researching the problems trying to get a better understanding of the material before digging in. Sometimes however, there is no replacement for just digging in and coding something. This does at least two things, it solidifies the material you’re read so far, and most importantly it shows you the limits of what you currently understand and urges you to learn more. Interestingly, I’ve found in my own intellectual endeavors some things just don’t make any sense until you’ve tried to use it in anger.


I gave this student the advice to

Put down the books. Pick up a pencil, some paper and a compiler Code it. Make your brain hurt.

As I was sharing this I realized how hypocritical I was. I’ve been learning Haskell, and yet I haven’t coded in Haskell in over a year. I’ve been reading books, watching videos.

I’ve been a Spectator. It’s time to be a Doer.

I started working through the codewars.com problems, specifically working on the test cases for this problem.

While coding the QuickCheck properties (tests) for this problem however, I was completely lost at trying to build a property with only positive generator values.

This particular test is trying to show that the height of the pyramid is the length of the out put array/list.

prop_heightIsLength :: Int ->  Bool
prop_heightIsLength height = length ( buildTower height ) == height

However, buildTower only defined for positive values. My first pass of the property is generates negative values (and 0 values) as input. After some searching I was able to find the Positive type modifier for quickcheck.

https://hackage.haskell.org/package/QuickCheck-2.4.1.1/docs/Test-QuickCheck.html#t:Positive

prop_heightIsLength :: Positive Int ->  Bool
prop_heightIsLength height = length ( buildTower height ) == height

However, this doesn’t compile.

Couldn't match expected type ‘Positive Int’
                  with actual type ‘Int’

Which brings me to my biggest challenge in learning anything new. Frequently in learning something new I don’t have enough knowledge to ask the right question, or even understand the answer. This is one of those cases.

Failed Questions

  1. haskell unwrap Positive newtype
  2. haskell use QuickCheck Positive
  3. haskell constraint quickcheck generator

This lead me to the closest answer

https://stackoverflow.com/questions/11910143/positive-integer-type#11910221

However, it is demonstrating how to build a custom generator. I just want to use the built in constraints!

Why is this so hard? Because I didn’t know how to read the type signature of the Postive type.

-- This property does not compile
prop_heightIsLength :: Positive Int ->  Bool
prop_heightIsLength height = length ( buildTower height ) == height


-- This property compiles
prop_heightIsLength' :: Positive Int ->  Bool
prop_heightIsLength' (Positive height) = length ( buildTower height ) == height

Naively, I thought the different is the parenthesis. Remember, parenthesis are only used for grouping in haskell. They have no other purpose. The different is what is bound to the name height. In the first version, I’m binding a Positive Int to the name height in the second version I’m binding merely an Int to height. The Positive type modifier assures that the values bound to it’s parameter are always positive. The bound value however will retain the Int type, even though it’s value is always positive.

I’m unstuck now. Moving on.