[Haskell-cafe] money type ?

[Brandon S. Allbery KF8NH]

On Jul 12, 2007, at 4:40 , Bayley, Alistair wrote:

>> From: haskell-cafe-bounces at haskell.org
>> [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of Simon Michael
>>
>> Is there a type or library out there that's good for
>> representing money and
>> other quantities while avoiding rounding errors  ?
>
> I think Data.Fixed would be a good choice:
>
> http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data- 
> Fixed.ht
> ml
>
> I don't know /exactly/ how you'd go about adding a Money data type; I
> guess you'd have something like:
>
>> data E2 = E2
>> instance HasResolution E2 where resolution _ = 100
>> type Money = Fixed E2

I was playing with that when your message came in:

> mress:5003 Z$ cat foo.hs
> import Data.Fixed
>
> data E2 = E2
> instance HasResolution E2 where
>   resolution _ = 100
> type Money = Fixed E2
>
> mress:5004 Z$ ghci foo.hs
>    ___         ___ _
>   / _ \ /\  /\/ __(_)
> / /_\// /_/ / /  | |      GHC Interactive, version 6.7.20070322,  
> for Haskell 98.
> / /_\\/ __  / /___| |      http://www.haskell.org/ghc/
> \____/\/ /_/\____/|_|      Type :? for help.
>
> Loading package base ... linking ... done.
> [1 of 1] Compiling Main             ( foo.hs, interpreted )
> Ok, modules loaded: Main.
> *Main> :m +Data.List
> *Main Data.List> let l = [1.25 :: Money,2.00,4.46,12.80,1.15,6.00]  
> in sum l / genericLength l
> 4.61

That what you want?

-- 
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH