| > The fromJust and `head of empty list' errors are totally equivalent to | > the dereferencing of zero pointer in C++ or NullPointerException in | > Java. It pains me to see that exactly the same problem arises in | > Haskell -- keeping in mind that already in C++ and Java one may | > exterminate these errors given right encapsulations. Languages like | > Cyclone or Cw use the type system to eliminate such errors. Surely | > Haskell can do something about this? | | Yes, these techniques are fairly well known now, and hopefully some of | the more experienced Haskellers are using them (I certainly use the | non-empty list tricks). Any anyone with more than 6 months Haskell knows | to avoid fromJust. | | The problem I see is that head/fromJust errors are usually caused by | *beginner* Haskellers, who don't know the techniques for statically | avoiding them. I don't agree. My programs have invariants that I can't always express in a way that the type system can understand. E.g. I know that a variable is in scope, so searching for it in an environment can't fail: head [ v | (n,v) <- env, n==target ] (Maybe if I had an Oleg implant I could express all this in the type system -- but I don't.) But yes, we should have more sophisticated techniques to express and check these invariants. With Dana Xu I'm working on this very thing (see her Haskell Workshop paper http://www.cl.cam.ac.uk/~nx200/research/escH-hw.ps); and Neil Mitchell is doing complementary work at York. So I think there is reason to be hopeful. Simon