On Tue, 28 Aug 2001, Simon Marlow wrote: (snip)

Note that Haskell leaves the behaviour of these exceptional conditions undefined. Hugs for example will give a program error on divide by zero, and NHC has even more interesting behaviour:

Prelude> 1/0 :: Float 34.02823669209384634633746074317682114560000000

!! Damn - that's annoying. I've come to take the existence of reasonable IEEE 754 floating point support for granted - is it really generally that bad in Haskell? Are there any libraries and whatnot that will help fix this? I like at least for plus and minus infinities and NaNs to be part of things. (-: My first thought was to learn about the FFI and do actual arithmetic and the relevant reading and showing in another language that I link with, but I fear the speed penalty, and now I think about it I might be able to reasonably transparently deal with the cases I care about in pure Haskell. I also haven't yet worked out how to tell if a string is "read"able or not, yet - if (read "45")::Integer or whatever gives an error (e.g. if I'd put "af" instead of "45"), it seems to be pretty uncatchable, which is a pain. How do I tell if an instance of reading will work, or catch that it didn't? I know things must be better than this - I must be missing something? (-: Thanks. -- Mark

Mark Carroll wrote:

I also haven't yet worked out how to tell if a string is "read"able or not, yet - if (read "45")::Integer or whatever gives an error (e.g. if I'd put "af" instead of "45"), it seems to be pretty uncatchable, which is a pain. How do I tell if an instance of reading will work, or catch that it didn't?

You need to resort to reads :: Read a => ReadS a where ReadS is a typical parser type constructor (``replacing failure by a list of successes'', see BibTeX entry below), defined in the prelude: type ReadS a = String -> [(a,String)] You might want to wrap it in something like the following: read' :: Read a => String -> Maybe a read' s = case reads s of ((a,[]):_) -> Just a _ -> Nothing For understanding the machinery it is useful to look at the class and instance definitions for Read in the Prelude. Wolfram ========================================================= @InProceedings{Wadler-1985a, author = "Philip Wadler", title = "How to Replace Failure by a List of Successes --- {A} Method for Exception Handling, Backtracking, and Pattern Matching in Lazy Functional Languages", booktitle = {Proceedings of the Second Conference on Functional Programming Languages and Computer Architecture}, confaddress = "Nancy, France", publisher = Springer, series = LNCS, confdate = SEP # " 16--19,", year = "1985", pages = "113--128", }