November 2008 - Haskell-prime

Repair to floating point enumerations?
by Malcolm Wallace 10 Dec '08

10 Dec '08

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

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Suggestion: Syntactic sugar for Maps!
by circ ular 28 Nov '08

28 Nov '08

I suggest Haskell introduce some syntactic sugar for Maps. Python uses {"this": 2, "is": 1, "a": 1, "Map": 1} Clojure also use braces: {:k1 1 :k2 3} where whitespace is comma but commas are also allowed. I find the import Data.Map and then fromList [("hello",1), ("there", 2)] or the other form that I forgot(because it is to long!) to be to long... So why not {"hello": 1, "there": 2} ? Best regards Cirfan

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patch applied (haskell-prime-status): Add simonpj's ImportShadowing proposal
by Simon Marlow 28 Nov '08

28 Nov '08

Fri Nov 21 00:46:48 PST 2008 Simon Marlow <marlowsd(a)gmail.com> * Add simonpj's ImportShadowing proposal M ./status.hs +5 View patch online: http://darcs.haskell.org/haskell-prime-status/_darcs/patches/20081121084648…

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