Quickcheck Preconditions - Beginners - Haskell.org

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Quickcheck Preconditions

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Guy

25 May 2011 25 May '11

6:40 a.m.

I have some functions with preconditions. For example, foo::[a]->IO Bool requires a list of at least three elements, and bar::[a]->Int->IO () requires that its second argument is some position within the list. How can I test these with quickcheck? If I use ==>, quickcheck gives up with very few test cases.

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Daniel Fischer

25 May 25 May

7:04 a.m.

On Wednesday 25 May 2011 12:40:20, Guy wrote:

I have some functions with preconditions. For example, foo::[a]->IO Bool requires a list of at least three elements, and bar::[a]->Int->IO () requires that its second argument is some position within the list.

How can I test these with quickcheck? If I use ==>, quickcheck gives up with very few test cases.

You could increase the maxDiscard value, ghci> quickCheckWith stdArgs{ maxDiscard = 5000 } (forAll arbitrary (\xs -> (length xs > 2) ==> whatever)) or create a generator that produces only output fulfilling the preconditions: atLeast3 :: Arbitrary a => Gen [a] atLeast3 = do x <- arbitrary y <- arbitrary z <- arbitrary ws <- arbitrary return (x:y:z:ws) ghci> quickCheck (forAll atLeast3 whatever)

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Jonas Almström Duregård

7:09 a.m.

You can write your own generators and use the forall function. Or for these simple examples you can just test on wrappers: testFoo :: (a,a,a,[a]) -> IO Bool testFoo (a1,a2,a3,as) = foo (a1:a2:a3:as) testBar :: (a,[a]) -> Int -> IO () testBar (x,xs) n = bar xs' (n `mod` length xs') where xs' = x:xs /J On 25 May 2011 12:40, Guy wrote:

I have some functions with preconditions. For example, foo::[a]->IO Bool requires a list of at least three elements, and bar::[a]->Int->IO () requires that its second argument is some position within the list.

How can I test these with quickcheck? If I use ==>, quickcheck gives up with very few test cases.

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

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Guy

8:13 a.m.

Thanks, mod is a neat trick for applying the size. Is there a simple way of specifying bounds for quickcheck, or would I have to write a custom generator to do that?

________________________________ From: Jonas Almström Duregård Sent: Wednesday, 25 May 2011, 14:09 Subject: Re: [Haskell-beginners] Quickcheck Preconditions

You can write your own generators and use the forall function. Or for these simple examples you can just test on wrappers:

testFoo :: (a,a,a,[a]) -> IO Bool testFoo (a1,a2,a3,as) = foo (a1:a2:a3:as)

testBar :: (a,[a]) -> Int -> IO () testBar (x,xs) n = bar xs' (n `mod` length xs') where xs' = x:xs

/J

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Jonas Almström Duregård

11:42 a.m.

Not sure what you mean by "specifying bounds for quickcheck". 2011/5/25 Guy :

Thanks, mod is a neat trick for applying the size. Is there a simple way of specifying bounds for quickcheck, or would I have to write a custom generator to do that?

...
________________________________ From: Jonas Almström Duregård Sent: Wednesday, 25 May 2011, 14:09 Subject: Re: [Haskell-beginners] Quickcheck Preconditions

You can write your own generators and use the forall function. Or for these simple examples you can just test on wrappers:

testFoo :: (a,a,a,[a]) -> IO Bool testFoo (a1,a2,a3,as) = foo (a1:a2:a3:as)

testBar :: (a,[a]) -> Int -> IO () testBar (x,xs) n = bar xs' (n `mod` length xs') where xs' = x:xs

/J

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Guy

12:13 p.m.

Simply stating that I need an integer between X and Y On 25/05/2011 18:42, Jonas Almström Duregård wrote:

Not sure what you mean by "specifying bounds for quickcheck".

2011/5/25 Guy:

...
Thanks, mod is a neat trick for applying the size. Is there a simple way of specifying bounds for quickcheck, or would I have to write a custom generator to do that?

...
________________________________ From: Jonas Almström Duregård Sent: Wednesday, 25 May 2011, 14:09 Subject: Re: [Haskell-beginners] Quickcheck Preconditions

You can write your own generators and use the forall function. Or for these simple examples you can just test on wrappers:

testFoo :: (a,a,a,[a]) -> IO Bool testFoo (a1,a2,a3,as) = foo (a1:a2:a3:as)

testBar :: (a,[a]) -> Int -> IO () testBar (x,xs) n = bar xs' (n `mod` length xs') where xs' = x:xs

/J

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Jonas Almström Duregård

3:27 p.m.

abs $ (n `mod` (y - x)) + x Provided y > x >= 0. /J On 25 May 2011 18:13, Guy wrote:

Simply stating that I need an integer between X and Y

On 25/05/2011 18:42, Jonas Almström Duregård wrote:

...
Not sure what you mean by "specifying bounds for quickcheck".

2011/5/25 Guy:

...
Thanks, mod is a neat trick for applying the size. Is there a simple way of specifying bounds for quickcheck, or would I have to write a custom generator to do that?

________________________________

...
From: Jonas Almström Duregård Sent: Wednesday, 25 May 2011, 14:09 Subject: Re: [Haskell-beginners] Quickcheck Preconditions

You can write your own generators and use the forall function. Or for these simple examples you can just test on wrappers:

testFoo :: (a,a,a,[a]) -> IO Bool testFoo (a1,a2,a3,as) = foo (a1:a2:a3:as)

testBar :: (a,[a]) -> Int -> IO () testBar (x,xs) n = bar xs' (n `mod` length xs') where xs' = x:xs

/J

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

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Daniel Fischer
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Jonas Almström Duregård