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Simon Peyton Jones engineering, I think it s a qualitatively different experience. I think debuggers do require perhaps more engineering cycles to get to work well. But if you put them in, you do get something that is really quite remarkably more helpful. Maybe the people that you ve been mainly talking to are more in the academic software mold. And also perhaps have grown up with sophisticated debugging environments less. I wouldn t like to draw any general lessons. I certainly wouldn t wish to denigrate or downplay the importance of good debugging environments. Particularly in these rather complicated ecosystems where there are many, many, many layers of software. GHC is a very simple system compared to a full-on .NET environment with layers of DOMs and UMLs and I don t know what kind of goop. The real world gets so goopy that more mechanical support may well be really important. Seibel: Another approach to getting correct software is to use formal proofs. What do you think about the prospect of that being useful Peyton Jones: Suppose you declare that your goal is for everything to have a machine-checked proof of correctness. It s not even obvious what that would mean. Mechanically proved against what Against some specification. Well how do you write the specification This is now meant to be a specification of everything the program does. Otherwise you wouldn t have proved that it does everything that it should do. So you must have a formal specification for everything it should do. Well now how are you going to write that specification You ll probably write it in a functional language. In which case, maybe that s your program. I m being a bit fast and loose here because you can say some things in specification languages that you can t say in programs like, The result of the function is that y such that y squared equals x. That s a good specification for a square-root function but it s not very executable. Nevertheless, I think to try to specify all that a program should do, you get specifications that are themselves so complicated that you re no longer confident that they say what you intended. I think much more productive for real life is to write down some properties that you d like the program to have. You d like to say, This valve should
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Simon Peyton Jones never be shut at the same time as that valve. This tree should always be balanced. This function should always return a result that s bigger than zero. These are all little partial specifications. They re not complete specifications. They re just things that you would like to be true. How do you write those down Well, functional languages are rather good at that. In fact this is exactly what happens when you write a QuickCheck specification; you write down properties as Haskell functions. Say we want to check that reverse is its own inverse well, you might write checkreverse with type list of A to bool. So checkreverse of xs is reverse of reverse xs equals xs. So this is a function that should always return true. That s what a property function is. But it s just written in the same language so that s nice. Now you might hope to do some static checking on this. It might be hard or easy. But even having the property written down in a formal way is a real help. You can test it by generating test data, which is, indeed, just what QuickCheck does. So rather than trying to write down specifications for all that a program does I think it s much more productive to write down partial specifications. Perhaps multiple partial specifications. And then check them either by testing or by dynamic checks or by static checks. You never prove that your program is right. You just increase your confidence that it s right. And I think that s all that anybody ever does. Seibel: So you define however many properties, covering the things you care about. And then you can choose to confirm that those properties actually hold either statically or dynamically, depending on what s actually feasible. Because we may not know how to statically check them all Peyton Jones: Right. But in a functional setting, you have a better chance. But we ve still been dragging our feet a bit about demonstrating that. Nevertheless step one is to write down these properties in the first place. But I think the big thing is to get away from this monolithic, all-or-nothing story about specification and to say that you can do useful static and dynamic tests on partial specifications. These will increase your confidence in the correctness of your program and that is all you can possibly hope for.
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