Thanks for the feedback!
not sure if "quantities" is an appropriate name for your package: an integer, a float, complex numbers - those are quantities. doesn't this package parse units?
Wikipedia and this dictionary (http://dictionary.reference.com/browse/quantity) imply that quantities are associated with units of measurement. This package does indeed parse units. When a magnitude is associated with those units, you get a quantity. I think the name "quantities" is totally appropriate.
why can i only "unsafely" create quantities using strings? what if i know upfront what kind of a unit i want to deal with? from the looks of it, this library is only good for dealing with human-readable input - surely that's not the only use case you had in mind?
can't we find some elegant way to express this in haskell, instead of having to invent a scripting language of sorts?
Using the type system to handle units is incorporated into some other libraries. I couldn't find a library that parses units from strings, so I made one. I certainly plan to add this functionality in the future.
On a similar note, your "magnitude :: Quantity -> Double" sounds a bit like unsafeCoerce: why would you want to forget that your quantity has a unit?
You don't "forget" it has units. Sometimes you just want to get the magnitude, like after you perform a conversion operation, for instance.
Oh, so you are trying to support arbitrary arithmetical input! Then again, there is no way to add two "Quantity"s.
There is indeed: http://hackage.haskell.org/package/quantities-0.3.0/docs/Data-Quantities.htm...
I have the feeling the goal of your package is a bit unclear: do you want to implement a framework to type-safely compute with units? Or do you just want to write something that parses string input to typed data?
Currently, it parses string input. I am a contributor to a Python package called pint, which was a huge inspiration to this library. In my day job, I develop a commercial Python GUI application for petroleum engineering that needs to convert between many units. One feature of this application is a user can enter arbitrary units of their choice to for plots and data so they don't have to convert units by hand when using the application. I wanted to bring that functionality to Haskell.
Well, that was a big rant, I hope you can use some of it.
I did, thanks!