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Test.QuickCheck.Gen.Unsafe

Unsafe combinators for the Gen monad.

Gen is only morally a monad: two generators that are supposed to be equal will give the same probability distribution, but they might be different as functions from random number seeds to values. QuickCheck maintains the illusion that a Gen is a probability distribution and does not allow you to distinguish two generators that have the same distribution.

The functions in this module allow you to break this illusion by reusing the same random number seed twice. This is unsafe because by applying the same seed to two morally equal generators, you can see whether they are really equal or not.

promote :: Monad m => m (Gen a) -> Gen (m a)

Promotes a monadic generator to a generator of monadic values.

delay :: Gen (Gen a -> a)

Randomly generates a function of type Gen a -> a, which you can then use to evaluate generators. Mostly useful in implementing promote.

capture :: Gen Capture

A variant of delay that returns a polymorphic evaluation function. Can be used in a pinch to generate polymorphic (rank-2) values:

genSelector :: Gen (a -> a -> a)
genSelector = elements [\x y -> x, \x y -> y]

data Selector = Selector (forall a. a -> a -> a)
genPolySelector :: Gen Selector
genPolySelector = do
  Capture eval <- capture
  return (Selector (eval genSelector))
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