Re: [Haskell-beginners] Spec with QuickCheck generationg list of list of the same length

15 Feb 2015


      Thanks, it sent me to the (higly probably) right direction:

newtype RectList  = RectList [LVector] deriving Show

instance Arbitrary RectList  where
     arbitrary = do
         m <- getPositive <$> arbitrary
         n <- getPositive <$> arbitrary

         RectList <$> replicateM n (vector m)

and the test can look like this:

it "average times number is sum" $
             property (\(RectList x) -> null $ nozeroMembers $ zipWith 
(-) ( map (* (fromIntegral $ length x)) (average x)) (sumVect x) )

Thanks again
    Ondrej
...
On 2015-02-15 at 10:52, Ondrej Nekola  wrote:
...
How can I generate list of lists, where all the included lists are of
the same length?
The general approach is to make a newtype, the Arbitrary instance of
which does what you want.  For example, QucickCheck includes newtypes
Positive[1] and NonNegative[2], that generate restricted numbers.
For your case, I'd write something like (untested):
```
newtype RectList a = RectList [[a]]
instance Arbitrary a => Arbitrary RectList a where
   arbitrary = do
     -- length of inner lists
     m <- getPositive <$> arbitrary
     -- length of outer list, can be zero
     n <- getNonNegative <$> arbitrary
     RectList <$> replicateM n (vector m)
```
vector here is another function provided by QuickCheck: [3].
Hope this helps.
bergey
...
Link for some tutorial *) can be, I believe, enough but I would be glad
for any help.
Thanks
     Ondra @satai Nekola
     ondra@nekola.cz
*) Maybe I have not trained my google good enough yet, but lack of
tutorials describing more than the most basic usecase is probably the
most problematic point in re-learning Haskell so far.
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Footnotes:
[1]  http://hackage.haskell.org/package/QuickCheck-2.7.6/docs/Test-QuickCheck.htm...
[2]  http://hackage.haskell.org/package/QuickCheck-2.7.6/docs/Test-QuickCheck.htm...
[3]  http://hackage.haskell.org/package/QuickCheck-2.7.6/docs/Test-QuickCheck-Arb...