To be frank, the poster that you cite doesn't know what he's talking about. He makes two elementary mistakes: (a) attempting to encode dimension/unit checking in an existing type system; (b) not appreciating the need for parametric polymorphism over dimensions/units. As others have pointed out, (a) doesn't work because the algebra of units of measure is not free - units form an Abelian group (if integer exponents are used) or a vector space over the rationals (if rational exponents are used) and so it's not possible to do unit-checking by equality-on-syntax or unit-inference by ordinary syntactic unification. Furthermore, parametric polymorphism is essential for code reuse - one can't even write a generic squaring function (say) without it. Best to ignore the poster and instead read the papers that contributors to this thread have cited :-) To turn to the original question, I did once give a moment's thought to the combination of type classes and types for units-of-measure. I don't think there's any particular problem: units (or dimensions) are a new "sort" or "kind", just as "row" is in various proposals for record polymorphism in Haskell. As long as this is tracked through the type system, everything should work out fine. Of course, I may have missed something, in which case I'd be very interested to know about it. - Andrew Kennedy.
-----Original Message----- From: andrew@andrewcooke.free-online.co.uk [mailto:andrew@andrewcooke.free-online.co.uk] Sent: Wednesday, February 14, 2001 5:02 PM To: haskell-cafe@haskell.org Subject: Re: Typing units correctly
Hi,
I don't know if this is useful, but in response to a link to that article that I posted on Lambda, someone posted a link arguing that such an approach (at least in Ada) was impractical. To be honest, I don't find it very convincing, but I haven't been following this discussion in detail. It might raise some problems you have not considered.
Anyway, if you are interested, it's all at http://lambda.weblogs.com/discuss/msgReader$818
Apologies if it's irrelevant or you've already seen it, Andrew
On Mon, Feb 12, 2001 at 01:51:54PM -0500, Dylan Thurston wrote: [...]
The papers I could find (e.g., http://citeseer.nj.nec.com/kennedy94dimension.html, "Dimension Types") mention extensions to ML. I wonder if it is possible to work within the Haskell type system, which is richer than ML's type system. [...]