Re: [Haskell-cafe] quickCheck problem

On Tue, Oct 6, 2015 at 2:01 PM, Roelof Wobben wrote:

but why this part :

x (x + y) (x - (y + 1))

That is buggy. I was trying to quickly create three arguments that are guaranteed to be different by some random amount, but what I wrote doesn't always work (eg: if y = 0, then the first two arguments are the same, if y is -1, the first and 3rd are the same, if x is MAX_INT, then it may or may not be the same as other arguments, depending on Y, etc...) The predicate guard should work, though it may take longer to run. --Rogan

Roelof

Op 6-10-2015 om 22:31 schreef Rogan Creswick:

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On Tue, Oct 6, 2015 at 1:24 PM, Roelof Wobben wrote:

...

Hello,

I have written a function to test if three numbers are the same.

Now I want to test my functions by using quickCheck.

So I thought to test first if all three are the same.

but how can I tell quickcheck that all numbers are the same.

Just have quickcheck generate one number, and use it for all three arguments, eg:

prop_allSame :: Int -> Bool prop_allSame x = yourFn x x x

then test the other cases:

prop_different :: Int -> Int -> Int -> Property prop_different x y z = x /= y && y /= z && x /= z ==> not $ yourFn x y z

That's probably OK for ints, but generally the guard in that style isn't a great solution, since quickcheck will just keep generating inputs until the predicate is satisfied. That can take a while, so you could also manually offset the first input in various ways:

prop_different :: Int -> Int -> Bool prop_different x y = not $ yourFn x (x + y) (x - (y + 1)) -- I put a +1 in here to avoid returning true where y = 0,

There are probably other ways to mutate a randomly generated input for your problem that may work better, but that's the general idea.

--Rogan

...

and second question : hwo do the test likeif two numbers are the same.

I ask this because I try to solve a exercise of the programming Haskell book, IM have read chapter 3 now.

Roelof

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