On 06/08/2004, at 6:56 PM, MR K P SCHUPKE wrote:
After all, Java basically does exactly what you're asking for with
Java's head/tail would be doing runtime checks if they are throwing exceptions, static guarantees mean the program would not be allowed to compile if it broke the static guarantees.
As Keith said, Java will check at compile time whether or not you handle the exception. My point is this: it is impossible to check whether the exception is "properly" handled. If you adjust Haskell's tail function to return (Maybe [a]) instead of just ([a]), you are doing the thing as Java from a pragmatic perspective: you are adding information to the type system that tells the programmer the function may fail. You also suffer the same consequence as Java: you have no idea whether the programmer properly handles the error situation. If I am writing a one-shot, never-use-again script that takes 3 minutes to write, and I _know_ that I'm not going to be feeding the tail function a non-empty list--e.g. because I'm writing a one-shot five-minute script to transform a file from one text format to another, as is the case for lots of Perl programs--then the extra Maybe type just gets in the way. I'll either ignore the Nothing case, or write `case tail foo of ... Nothing -> error "bleh"'. I will go so far to say that such a program can be considered "correct": it does exactly what I want it to do, in exactly the circumstances I desire (0 byte files being specifically excluded from the circumstances!).
Which is a bad thing! All programmers always have to consider error conditions, if they don't they write buggy code - that's the nature of the beast. I prefer making programmers expicitly face the decisions they are making, rather than have things implicitly handled in a way that hides what is going on from the programmer.
It's a question of whether the library designer should impose their will on the library user. As a library designer, do you feel that you are always making the right decision for the library user 100% of the time? I know I never feel like that when I write libraries ... -- % Andre Pang : trust.in.love.to.save