[Haskell-cafe] Why 'round' does not just round numbers ?

[Mitchell, Neil]

Hi,

That is a fairly standard implementation of rounding for financial
institutions. Consider

sum . map round

Over the list [3.5,2.5]

With rounding to the nearest even integer for 0.5's you get 6, otherwise
if you always round up you get 7. If you bias towards rounding up you
get a general upwards trend as numbers are rounded, which is bad, while
the even condition ensures that statistically it averages to the same
thing.

For more details see:
http://en.wikipedia.org/wiki/Rounding#Round-to-even_method

Thanks

Neil


> -----Original Message-----
> From: haskell-cafe-bounces at haskell.org 
> [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of L.Guo
> Sent: 27 October 2008 9:49 am
> To: MailList Haskell-Cafe
> Subject: [Haskell-cafe] Why 'round' does not just round numbers ?
> 
> Hi all:
> 
> 
> I just read about definitions of Prelude [1], and noticing that.
> 
> In 6.4.6 Coercions and Component Extraction, it discribes like this:
> 
> 
> "round x returns the nearest integer to x, the even integer 
> if x is equidistant between two integers."
> 
> 
> I think this is unresonable. then try it in GHC 6.8.3.
> 
> 
> Prelude> round 3.5
> 4
> Prelude> round 2.5
> 2
> 
> 
> Is there any explanation about that ?
> 
> 
> [1] The Haskell 98 Report: Predefined Types and Classes
>     http://haskell.org/onlinereport/basic.html
> 
>     Regards
> --------------
> L.Guo
> 2008-10-27
> 
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> 
> 

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