On Sun, 24 Jul 2011 07:30:56 -0700, Mark Spezzano
<[email protected]> wrote:
Hi all,
I would appreciate it if someone can point me in the right direction
with the following problem.
I'm deliberately implementing a naive Queues packages that uses finite
lists as the underlying representation. I've already read through
Hughes' paper and the article in The Fun of Programming, but I'm still
having some difficulties. Specifically:
1. I have a newtype Queue a = Queue [a] and I want to generate Queues of
random Integers that are also of random size. How do I do this in
QuickCheck? I guess that I need to write a generator and then make my
"Queue a" concrete type an instance of Arbitrary? How?
Mark,
One of the great things about QuickCheck is that it is automatically
compositional.
What I mean by this is that all you need in your instance is how to form a
"Queue [a]" given "[a]", because there are already QuickCheck instances
for forming lists, and as long as a is pretty standard (Integers is fine)
then there's likely an Arbitrary instance for that as well.
So (from my head, not actually tested in GHC):
import Control.Applicative
import Test.QuickCheck
instance Arbitrary Queue where
arbitrary = Queue <$> arbitrary
Then you can use this as:
testProperty "length is something" propQInts
propQInts t = length t == ....
where types = (t :: Queue Integers)
The where clause is a fancy way of specifying what the type of t should be
without having to express the overall type of propQInts. You could use a
more conventional type specification as well.
2. If I wanted to specify/constrain the ranges of random Integers
generated, how would I do this?
Probably something like this:
instance Arbitrary Queue where
arbitrary = do li <- listOf $ arbitrary
lr <- liftM $ map rangelimit li
return $ Queue lr
where rangelimit n = case (n < LOW, n > HIGH) of
(True,_) -> LOW
(_,True) -> HIGH
_ -> n
3. If I wanted to specify/constrain the Queue sizes how would I do this?
Similar to #2. Perhaps:
arbitrary = arbitrary >>= (return . Queue . take CNT . listOf)
--
-KQ
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