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forAll :: (Show a, Testable testable) => Gen a -> (a -> testable) -> Property
forAll argGen prop =
Property $ Gen $ \rand -> -- Create a new property that will
let (rand1, rand2) = split rand -- Split the generator in two
arg = runGen argGen rand1 -- Use the first generator to produce an arg
subProp = property (prop arg) -- Use the `a` to access the sub-property
result = runProp subProp rand2 -- Use the second generator to run it
in overFailure result $ \failure -> -- Enrich the counter-example with `arg`
failure { counterExample = show arg : counterExample failure }
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