[Haskell-cafe] Bug with [Double]

[Dmitry Olshansky]

Thanks, it's clear now.

2010/5/19 Serguey Zefirov <sergueyz at gmail.com>:
> 2010/5/19 Erik de Castro Lopo <mle+hs at mega-nerd.com>:
>> Dmitry Olshansky wrote:
>>
>>> 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>
>>
>>
>>> 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.
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