# [Haskell-cafe] Controlling how test data is generated in QuickCheck 2

Michael Litchard michael at schmong.org
Tue May 3 01:52:40 UTC 2016

```I have a problem similar to this question <
http://stackoverflow.com/questions/9977734/controlling-how-test-data-is-generated-in-quickcheck>.
Below I will articulate my specifics, the code I am using, and the
particular question I have.

I have written a fizz-buzz program that uses a Fibonacci sequence as input.
I would like to test two things. (1) Does my program emit the correct
string given an Int that meets a particular condition. (2) Is my Fibonacci
generator generating Fibonacci numbers?

The problem I am having is similar to the link above. The range of `Int`s
is too large. How do I constrain my tests to say, the first 1000 Fibonacci
numbers?

Here is code that I think is both adequate and minimal. Please let me know
if I need to elaborate.

import           Data.Numbers.Primes (isPrime)
import           Test.Hspec  (Spec,hspec,describe,it,shouldBe)
import           Test.Hspec.QuickCheck (prop)

qcheck :: Spec
qcheck = do
describe "QuickCheck test fiz" \$
prop "QuickCheck test" \$ modfiz

describe "QuickCheck test fib" \$
prop "QuickCheck test fib" \$ testfib

modfiz int
| int <= 0                                 = True  -- code smell,
should never generate number less than or equal to zero.
| int == 3                                 = test3
| int == 5                                 = test5
| int `mod` 15 == 0                        = testMod35
| int `mod` 3 == 0                         = testMod3
| int `mod` 5 == 0                         = testMod5
| isPrime int == True                      = testPrime
| otherwise                                = testRest
where
test3     =
Right "Buzz BuzzFizz"     == fizzbuzz 3
test5     =
Right "Fizz BuzzFizz"     == fizzbuzz 5
testMod3  =
Right "Buzz "             == fizzbuzz int
testMod5  =
Right "Fizz "             == fizzbuzz int
testMod35 =
Right "Buzz Fizz "        == fizzbuzz int
testPrime =
Right "BuzzFizz"          == fizzbuzz int
testRest  =
Right (show int)          == fizzbuzz int

testfib :: Integer -> Bool
testfib n =
case (fibb n) of
Left _ -> False
Right n' -> isFib n'

`fibb` takes an `Int` and finds that nth Fibonacci. So `fibb 6` would
return `Right 8`. The `Left` value is not relevant to this problem.

What I noticed was the answer suggested that one should write a `newtype`,
wrapping a `[Int]` and making a new `Arbitrary` instance. However, I also
noticed that the answer is from 2012, and `QuickCheck 2` has an `Args`
`datatype` that appears to be able to do what I need. So, can I create a
new `Args` to limit the range of tests, (just want the first 1000 Fibonacci
numbers), and also to limit the number of tests run? If not is the solution
from the above link the approach I would have to take?

the entire project here <https://github.com/mlitchard/swiftfizz>
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