oleg:
Donald Bruce Stewart wrote:
So all this talk of locating head [] and fromJust failures got me thinking:
Couldn't we just use rewrite rules to rewrite *transparently* all uses of fromJust to safeFromJust, tagging the call site with a location?
I'm sorry for shifting the topic: I'm wondering if, rather than trying to make an error message more informative, we ought to make sure that no error will ever arise?
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. One solution would be to deprecate fromJust (we recently decided not to add fromLeft/Right for the same reasons). Having a compiler warning is a good way to encourage good behaviour :) But it seems hardly likely that head will be deprecated any time soon, and we have no support for checked non-empty lists in the base libraries. So how do we help out the beginners, other than warning about fromJust, and providing a useful error message as we can, for when they just go ahead and use head anyway? -- Don