Hi Malcolm, The current behaviour does sound like a bug, and the revised behaviour does sound like a fix - and one that has a sensible explanation if people trip over it. In general having floating point be a member of classes such as Eq has some obvious problems, but I realise is a necessity for practical programming. Given that we have Eq Double, then Enum Double seems no worse. If we don't alter H98 then a valid program under H98 vs H' will give different results without any warning - that seems really bad. In addition, having two instances around for one typeclass/type pair in a big library (base) which are switched with some flag seems like a nightmare for compiler writers. So I think a good solution would be to fix H98 as a typo, and include it in H'. Thanks Neil
-----Original Message----- From: libraries-bounces@haskell.org [mailto:libraries-bounces@haskell.org] On Behalf Of Malcolm Wallace Sent: 15 October 2008 10:41 am To: haskell-prime@haskell.org Cc: libraries@haskell.org Subject: Repair to floating point enumerations?
Dear Haskell-Primers (and libraries).
Recently, Phil Wadler has pointed out a weird anomaly in the Haskell'98 Prelude, regarding numeric enumerations for Floats/Doubles:
Prelude> [0, 0.3 .. 1.1] [0.0,0.3,0.6,0.899999,1.2]
What is odd is that the last value of the list is much larger than the specified termination value. But this is exactly as specified by the Haskell'98 Report.
http://haskell.org/onlinereport/basic.html#sect6.3.4
"For Float and Double, the semantics of the enumFrom family is given by the rules for Int above, except that the list terminates when the elements become greater than e3+i/2 for positive increment i, or when they become less than e3+i/2 for negative i.
We have discussed this question (and related ones, such as whether Float and Double belong in the Enum class at all) several times before, and I do not wish to rehash all of those points again e.g.:
http://www.cse.unsw.edu.au/~dons/haskell-1990-2000/msg07289.html http://www.haskell.org/pipermail/haskell/2001-October/008218.html http://www.haskell.org/pipermail/haskell/2002-October/010607.html
Phil proposes that, although retaining the instances of Enum for Float and Double, we simplify the definitions of the numericEnumFrom family:
numericEnumFromThenTo :: (Fractional a, Ord a) => a -> a -> a -> [a] numericEnumFrom = iterate (+1) numericEnumFromThen n m = iterate (+(m-n)) n numericEnumFromTo n m = takeWhile (<= m) (numericEnumFrom n) numericEnumFromThenTo n m p = takeWhile (<= p) (numericEnumFromThen n m)
The particular feature of note is that the odd fudge factor of (1/2 * the increment) is removed. The inexact nature of floating point numbers would therefore cause a specification like
[ 0.0, 0.1 .. 0.3 ]
to yield the sequence
[ 0.0, 0.1, 0.2 ]
that is, to omit the upper bound, because (3 * 0.1) is actually represented as 0.30000000000004, strictly greater than 0.3.
Phil argues that this behaviour is more desirable: "the simple fix is that the user must add a suitable epsilon to the upper bound. The key word here is *suitable*. The old definitions included completely bizarre and often highly unsuitable choices of epsilon."
This proposal seems to me to improve the consistency of the enumeration syntax across both the integral and floating types. Some users may still be surprised, but the surprise will be easier to explain.
I am bringing this question to the attention of all who are interested in Haskell Prime, because it seems like a sensible and well-reasoned change. Discussion on whether to adopt this proposal for H' is welcome.
But as maintainer and bug-fixer of the Haskell'98 Report, I have also been asked whether we should make this change retrospectively to the Haskell'98 language (as a "typo"). Since it involves not merely an ordinary library function, but a Prelude function, and moreover a function that is used in the desugaring of syntax, it is less clear to me whether to alter Haskell'98.
Thoughts?
Regards, Malcolm _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries
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