Hello, To let the type checker do some work for you, without getting all the way into the territory of the dimensional package, you can use newtypes and a Units class with methods for wrapping and unwrapping Doubles; we use this approach at work and find it strikes a nice balance between useful (static typechecking) and usable (unobtrusive to introduce to existing codebase). Just as in your code, conversions are written explicitly by us and can change when needed (with a recompile), or can require an IO action to make use of changing data. -Jeff On Sun, Aug 16, 2009 at 9:55 AM, Max Cantor wrote:

@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 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

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