[Haskell-cafe] Request for Comments - hscurrency 0.0.1

[Max Cantor]

@Jason I'm not sure what you mean about exposing the type  
information.  Unless you mean that each currency would be a separate  
type somehow. While this is a similar problem to the dimensional  
issue, the problem is that the FX rates are changing all the time.   
While the conversion between a meter and a foot is always constant,  
with FX rates thats not the case.  So 2ft + 3m is always a well  
defined value but something like USD 1 + JPY 1 gives a function of the  
USDJPY exchange rate, not a constant value.

@ Antoine I'll add some comments.  you're right that doubles are not  
typically used nor would they be in a finished product.  for the time  
being, however, I dont know if its better to use Ratio's, Fixed's or  
what, so just settled on the most straightforward for now.


On Aug 16, 2009, at 1:26 AM, Jason Dagit wrote:

>
>
> On Sat, Aug 15, 2009 at 5:15 AM, Max Cantor <mxcantor at gmail.com>  
> wrote:
> Hi all,
>
> I'm putting together some simple tools to do safe calculations on  
> different currencies.  For instance, making sure that you dont add  
> something like 5 USD + 10 JPY without doing a proper conversion.
>
> I've put up some code on google code which probably explains what  
> I'm trying to do more succinctly than this email.  I'm curious what  
> poeple think about the library, its the first haskell code I've  
> written for the purpose of sharing and I intend to add it to hackage  
> once I finalize the interface a bit more.
>
> The code is at: http://bit.ly/1Cjjlj
>
> I'm very open to suggestions on improving the interface.  RIght now  
> its very simple and straightforward but potentially limited.
>
> Right now it looks like you have taken the approach of embedded  
> domain specific language.  You have not exposed the currency units  
> to the type system.  Therefore you rely on the rules of your  
> embedded language to do proper conversions.
>
> Have you considered exposing the type information to the type  
> checker?  A similar question came up recently on a Portland  
> functional programming mailing list and my reply can be found here:
> http://groups.google.com/group/pdxfunc/tree/browse_frm/month/2009-08/5c565768ecf30c57?rnum=1&_done=%2Fgroup%2Fpdxfunc%2Fbrowse_frm%2Fmonth%2F2009-08%3F#doc_5c565768ecf30c57
>
> The experimental code which resulted is here:
> http://gist.github.com/165691
>
> You may also want to look at the dimensional package:
> http://code.google.com/p/dimensional/
>
> Jason