2010/5/19 Erik de Castro Lopo :

Dmitry Olshansky wrote:

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It seems that I saw something like this in Cafe recevtly. But I am not sure... In GHC 6.12.1 (Platform 2010 on Windows Vista) I have

<snip>

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Any comments?

The problem you point out is not a problem with Haskell, but a problem with the whole concept of floating point arithmetic as implemented on all modern CPUs. See:

   http://docs.sun.com/source/806-3568/ncg_goldberg.html

You would have got similar problems with just about any language running on the same hardware.

This is what used for Double list generation (Haskell Platform 2010): -------------------------------------------------- numericEnumFromThenTo :: (Ord a, Fractional a) => a -> a -> a -> [a] numericEnumFromThenTo e1 e2 e3 = takeWhile predicate (numericEnumFromThen e1 e2) where mid = (e2 - e1) / 2 predicate | e2 >= e1 = (<= e3 + mid) | otherwise = (>= e3 + mid) -------------------------------------------------- So normal C loop like for {double i = 1; i <= 10; i += 1+2/3) { insert_list(i); } won't generate the same list, as Haskell does in [1,1+2/3..10]. PS Rationals: Prelude> [1,1+2/3..10] :: [Rational] [1 % 1,5 % 3,7 % 3,3 % 1,11 % 3,13 % 3,5 % 1,17 % 3,19 % 3,7 % 1,23 % 3,25 % 3,9 % 1,29 % 3,31 % 3] Same result.

On Wed, May 19, 2010 at 10:57 AM, Serguey Zefirov wrote:

PS Rationals: Prelude> [1,1+2/3..10] :: [Rational] [1 % 1,5 % 3,7 % 3,3 % 1,11 % 3,13 % 3,5 % 1,17 % 3,19 % 3,7 % 1,23 % 3,25 % 3,9 % 1,29 % 3,31 % 3]

Same result.

This sounds like a bug to me. The section of the Haskell Report that deals with the Enum class mentions Float and Double, not Rational, and there's really no sensible reason why Rationals would exhibit this behaviour given that they don't have rounding error.

On 19/05/2010, at 23:44, Ben Millwood wrote:

On Wed, May 19, 2010 at 10:57 AM, Serguey Zefirov wrote:

...

PS Rationals: Prelude> [1,1+2/3..10] :: [Rational] [1 % 1,5 % 3,7 % 3,3 % 1,11 % 3,13 % 3,5 % 1,17 % 3,19 % 3,7 % 1,23 % 3,25 % 3,9 % 1,29 % 3,31 % 3]

Same result.

This sounds like a bug to me. The section of the Haskell Report that deals with the Enum class mentions Float and Double, not Rational, and there's really no sensible reason why Rationals would exhibit this behaviour given that they don't have rounding error.

From Section 12.1 of the Library Report: instance (Integral a) => Enum (Ratio a) where succ x = x+1 pred x = x-1 toEnum = fromIntegral fromEnum = fromInteger . truncate -- May overflow enumFrom = numericEnumFrom -- These numericEnumXXX functions enumFromThen = numericEnumFromThen -- are as defined in Prelude.hs enumFromTo = numericEnumFromTo -- but not exported from it! enumFromThenTo = numericEnumFromThenTo The numericEnum functions are defined in Section 8 of the Language Report and have semantics required for Float and Double. Roman

Ivan Lazar Miljenovic writes:

Dmitry Olshansky writes:

...

Hello all,

It seems that I saw something like this in Cafe recevtly. But I am not sure... In GHC 6.12.1 (Platform 2010 on Windows Vista) I have

Prelude> [1,1+2/3..10] [1.0,1.6666666666666665,2.333333333333333,2.9999999999999996,3.666666666666666,4.333333333333332,4.999999999999998,5.666666666666664,6.33333333333333,6.9999999999999964,7.6666666666666625,8.333333333333329,8.999999999999995,9.66666666666666,10.333333333333327]

-- It is a bug!

No it isn't, because of the dodgy Ord instance for Float and Double values.

And by "Ord", I of course mean "Enum"... -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com

Roman Leshchinskiy writes:

Personally, I consider the Enum class itself to be broken.

Oh? In what sense? It seems to work fine for data types representing bounded enumerable values with a proper mapping to/from Int (it's not bijective since there's no proper mapping from Int -> Bool for example); the problem is the fact that Double, etc. are made instances of Enum when this is not the case for those data types. -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com