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I've a function, rev , that returns some value for a type that is in three typeclasses: rev :: (Integral a, Show a, Read a) => a -> a rev = read . reverse . show I'd like to test some property about it with quickcheck. Though, I'm not interested in testing negative values of Integral types because I'm using Integer by lack of a Natural type in the base library. So I thought, let's take the opposite of the value generated when the value generated is negative and I'll be fine: prop_id :: (Integral a, Show a, Read a) => Positive a -> Bool prop_id n | n >= 0 = (rev.rev) n == n | otherwise = let n' = -n in (rev.rev) n' == n' (the property tested isn't important here - in particular it doesn't hold for very basic values and I'm aware of that, it's not the subject of this question) Then I ran into the Positive modifier and thought that although my test was now functionning, it'd be nice to implement it in a nicer way. So I tried: prop_id :: (Integral a, Show a, Read a) => Positive a -> Bool prop_id n = (rev.rev) n == n I must admit I was surprised when it compiled. But then an error popped when running the test: *** Failed! Exception: 'Prelude.read: no parse' (after 1 test): Positive {getPositive = 1} So I thought, "mmk, must declare this Positive thing an instance of Read ". So I did just that, but the instance is already declared in the quickCheck library it seems because ghci screamed at me. And at this point I'm lost, for I do not find good documentation (if any). Any pointer helping me to understand modifiers and other nice things in the quickcheck library will be appreciated.

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