[Haskell-cafe] the purpose of QuickCheck's size parameter

[David Feuer]

data Foo a = Leaf a | Node [Foo a]

Without the size parameter, it's a bit tricky to control the distribution
to avoid generating extremely large trees. I certainly agree, however, that
the size parameter is an ugly and ill-specified hack.

On Thu, Jun 14, 2018, 4:20 PM Petr Pudlák <petr.mvd at gmail.com> wrote:

> Hi everyone,
>
> I'd like to better understand the principles behind the 'size' parameter.
> Looking at quickCheckWithResult [1], its computation seems to be somewhat
> non-trivial, or even arbitrary. As far as I understand it, the size is
> varied throughout tests, increasing from small to larger values. I see two
> main purposes:
>
> - Test on smaller as well as larger values. But with generators having
> proper distribution of values, this should happen anyway, just as if we had
> a constant, larger 'size' parameter.
> - Starting with smaller sizes allows to find smaller count-examples first.
> But with shrinking, it doesn't matter that much, big counter-examples are
> shrunk to smaller ones anyway in most cases.
>
> So is this parameter actually necessary? Would anything change
> considerably if it was dropped?
>
> Thanks,
> Petr
>
> [1]
> http://hackage.haskell.org/package/QuickCheck-2.11.3/docs/src/Test-QuickCheck-Test.html#quickCheckWithResult
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