[Haskell-cafe] quickCheck problem

[Rogan Creswick]

On Tue, Oct 6, 2015 at 2:01 PM, Roelof Wobben <r.wobben at home.nl> 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:
>>
>> On Tue, Oct 6, 2015 at 1:24 PM, Roelof Wobben <r.wobben at home.nl> 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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>>> Haskell-Cafe at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>>
>>
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>
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