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haskellquickcheck

Rewriting Arbitrary Double in QuickCheck2

Sorry for a newbie question, but how can I redefine Arbitrary Double to produce +/- infinity and NaNs as well as usual doubles? And how to use my version of this Arbitrary?

Solution

  • You don't need to make a new Arbitrary instance just to make a custom Generator for a type. You can just create it as a free-standing definition:

    evilDouble :: Gen Double
evilDouble = oneOf [ weirdDouble, arbitrary ]
  where
    weirdDouble = error "This is where you generate inf and NaN values"

    and then use it explicitly with QuickCheck's forAll:

    prop_foo = forAll evilDouble $ \x -> abs (foo x - 123) < 0.1