How to use 'oneof' in quickCheck (Haskell)

functional-programming, ghci, haskell, quickcheck

Solution

What I was driving at is that you have a type mix-up. Namely, `aBlank` is not a `Pos`, but a `Gen Pos`, so `update su aBlank aCandidate` makes no sense! In fact, what you want is a way to generate a new sudoku given an initial sudoku; in other words a function

similarSudoku :: Sudoku -> Gen Sudoku

Now we can write it:

similarSudoku su = do aBlank <- elements (blanks su) 
                      -- simpler than oneOf [return x | x <- blanks su]
                      aCandidate <- elements (candidates su aBlank)
                      return (update su aBlank aCandidate)

or even simpler:

similarSudoku su = liftM2 (update su) (elements (blanks su)) (elements (candidates su aBlank))

And the property looks like

prop_similar :: Sudoku -> Gen Bool
prop_similar su = do newSu <- similarSudoku su
                     return (isSudoku newSu && isOkay newSu)

Since there are instances

Testable Bool
Testable prop => Testable (Gen prop)
(Arbitrary a, Show a, Testable prop) => Testable (a -> prop)

`Sudoku -> Gen Bool` is `Testable` as well (assuming `instance Arbitrary Sudoku`).

Problem

I am trying to write a prop that changes a Sudoku and then checks if it's still valid. However, I am not sure how to use the "oneof"-function properly. Can you give me some hints, please? ``` prop_candidates :: Sudoku -> Bool prop_candidates su = isSudoku newSu && isOkay newSu where newSu = update su aBlank aCandidate aCandidate = oneof [return x | x <- candidates su aBlank] aBlank = oneof [return x | x <- (blanks su)] ``` Here are some more info... ``` type Pos = (Int, Int) update :: Sudoku -> Pos -> Maybe Int -> Sudoku blanks :: Sudoku -> [Pos] candidates :: Sudoku -> Pos -> [Int] [return x | x <- (blanks example)] :: (Monad m) => [m Pos] ``` I have struggeled with this prop for 3 hours now, so any ideas are welcome!

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