[Haskell-cafe] [Haskell] [ANN] quantities 0.3.0

[Auke Booij]

On 15 April 2014 20:29, John David Reaver <jdreaver at adlerhorst.com> wrote:
> Hello! I am happy to publicly announce the quantities package:

I think haskell-cafe is more appropriate for these kinds of libraries.
Anyway, welcome.

> From the description on Hackage:
>
>    A library for creating and manipulating physical quantities,
>    which are a numerical value associated with a unit of
>    measurement. Included is an expression parser and a huge list
>    of predefined quantities with which to parse strings into a
>    Quantity datatype. Once created, a quantity can be converted
>    to different units or queried for its dimensionality. A user
>    can also operate on quantities arithmetically, and doing so
>    uses automatic unit conversion and simplification.
>

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?

> Just to get a taste of how this package works, here are some
> examples:

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?

> >>> fromString "min => s"
> Right 60.0 second

can't we find some elegant way to express this in haskell, instead of
having to invent a scripting language of sorts?
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?

>From your documentation:

> >>> fromString "m => 3 ft"
> Left (ScalingFactorError 3.0 foot)

Why isn't the outcome of this 3.28084/3=1.09361 or something along
those lines? (this is just a suggestion, I'm not saying this is a bad
thing.)

> >>> fromString "2 ft + 6 in => ft"
> Right 2.5 foot

Oh, so you are trying to support arbitrary arithmetical input!
Then again, there is no way to add two "Quantity"s.


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?

In the former case, have you considered the discussion on the haskellwiki [1]?

In the latter case, why don't you write it on top of an existing units package?


Well, that was a big rant, I hope you can use some of it.

Auke.

[1]: http://www.haskell.org/haskellwiki/Physical_units