[Haskell-cafe] the purpose of QuickCheck's size parameter
lysxia at gmail.com
Thu Jun 14 22:22:21 UTC 2018
The purpose of the size parameter is not well-defined formally, but it
is a very convenient knob to easily tune the test suite in various
situations, that is definitely worth the negligible cost of having it
Without a size parameter, a fixed distribution means that if we want the
generator to cover all possible cases, we have to keep a small
probability of generating humongous examples and thus go OOM. We can
avoid that by making the size of generated values bounded by the size
parameter (or a function thereof), which we can increase easily when
more resources become available.
Furthermore, if we really want to generate very large examples, the only
way with a fixed distribution is to wait longer. Instead, using a size
parameter, we can make smaller values less likely to target further
regions in the search space more accurately.
Some properties can take a while to test on larger examples (either
because of the generators or the actual verification process) so we
might like to keep the size small during development, and raise it again
once we're done.
The Arbitrary class assigns a "default" generator for every type. While
it is not always a good choice, having a parameter to tweak makes
Arbitrary more generally useful.
As for your last point, small examples are faster to generate and check,
so it seems like a decent strategy to start small by default.
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