[Haskell-cafe] More QuickCheck Questions

[Ivan Lazar Miljenovic]

On 25 July 2011 20:01, Mark Spezzano <mark.spezzano at chariot.net.au> wrote:
> Hi all,
>
> I still don't understand exactly how the QuickCheck "sized" function is meant to work, and what it's useful for. Please explain!

The size parameter indicates how "large" a value can be generated
(e.g. how big an Int can be, how long a list should be, etc.).  As I
understand it, by default the size is passed around by QuickCheck in
how it uses the Gen Monad and used automatically, increasing as more
and more tests are done.

Sometimes you want to explicitly use this value to get an "upper
bound" when defining your own Arbitrary instances (e.g. see the Int
instance for Arbitrary).  Other times, you may wish to explicitly
_limit_ the size to a pre-defined value (to stop recursion or the
like; I have a couple of instances which use something like: \ s ->
liftM Foo $ resize (min 2 s) arbitrary).

Either way, you define a function of type "foo :: Int -> Gen a" and
then your arbitrary instance becomes "arbitrary = sized foo".

> Also, I think that class CoArbitrary can help to generate random *functions *(as opposed to values; but tell me if I'm wrong) but I don't see how this could be useful.

Can't really help you there as I've never used it.

> Also what does shrink do?

Oh no, QuickCheck has found an Arbitrary value that breaks your
property!  But it's absolutely _huge_: the output from show for it
takes up an entire page, and it will take _forever_ to track down the
specific little thing that caused the problem!  But if you implemented
the shrink method, QC will use it to find a minimal case: it shrinks
the broken value, and then tests the property on every possible
shrinked value, recursing until it finds one where every shrunken
value satisfies the property (if there are any); at which case it will
return that minimal broken case to you.

> Example code of these would be tremendously helpful understanding these concepts.

Real World Haskell has a chapter on QC.

-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
IvanMiljenovic.wordpress.com