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haskelltypesquickcheck

How do I constrain QuickCheck when using type synonyms?

I am using QuickCheck to run arbitrary test cases on my code. However, in one portion of my code I have the type synonym:

type Vector = [Double]

I also have a few functions that accept a number of Vectors as input. However, all of these functions require that the Vectors be of the same length.

Is there a way to constrain QuickCheck so that it only generates lists of length n?

Solution

  • A simple solution is to not have an arbitrary instance but instead to do something like

    import Test.QuickCheck
import Control.Monad

prop_vec :: Int -> Gen [Double]
prop_vec = flip replicateM arbitrary . abs


prop_addComm :: Int -> Gen Bool
prop_addComm i  = do
  v <- prop_vec i
  u <- prop_vec i
  return $ u + v = v + u --assuming you'd added a Num instance for your vectors

    There's never a typeclass so you get less helpful failures, but it's simpler to whip up.