Hello, I recently worked with QuickCheck for a while, But I still can't handle it well, And a few questions come to my mind. 1. How to find properties In QuickCheck examples on the codes or the papers, they find good properties easily. How did they find these properties? What property can make us believed its implementation is collect? 2. Property driven developments But finally, I could find some patterns of the properties. For example, occasionally, their shape is like unF . f == id or unF . f == Just I thought it could be applied to writing parser, and lead me to "Property Driven Developments." I tried writing S-expression parser in this way. First, I defined my property as parseSexpr . prettySexpr == Just where parseSexpr :: String -> Maybe Sexpr, prettySexpr :: Sexpr -> String. As you see, I should write printer before parser. Fortunately writing such printer for S-expr is easy, but I don't know if what I did is right. Do you think I wasted times? Have you ever tried PDD? And has it worked? If you have experience with TDD, how do you think about PDD? If you have any answers in any questions above, please tell me them. Thanks in advance. Cheers -nwn