Re: Unit validation in spreadsheets

["Paul A. Steckler" <steck stecksoft com>]

> We do handle circularity.  However, it's important to keep clear on
> the distinction between user visible expressions and calculation
> inside functions.  We'd need to handle those on a one off basis
> somewhat.  So if someone wanted to add a metre to a litre via '+' we
> could catch it, but SUM would take more work.

Right -- our design addresses exactly that issue.  For each 
function F, such as SUM, we generate a "unit transformer" F^ 
that does the right thing.  When there's circularity, this 
setup breaks down because the recursion doesn't terminate.
For that case, we use constraints that can be solved 
straightforwardly.  

I''ve been working out the details of how to generate 
unit transformers and constraints from a single specification.
You'd write a units spec symbolically, and generate the 
unit transformer and constraints automatically.  This isn't 
difficult, just grinding through details.

> Great.  The key will be in getting a coherent design.