[Haskell-cafe] ANNOUNCE: Facts

[Alexander Solla]

The Facts hierarchy is meant to contain commonly used, relatively  
static facts about the "real world". The facts are meant to be encoded  
using relatively simple Haskell constructs. However, we do make some  
promises: every data type our modules export will have instances of  
Data, Eq, Ord, Show, andTypeable. We will use explicit module export  
lists to control access to internal data structures.

As much of the data we are encoding is tabular, we use simple  
structures like lists and maps to encode the relations. This has two  
practical ramifications: the textual representation of the data can be  
very wide, but are also very easy to edit, with "block editing" tools  
like Vi's visual block mode. The other consequence is that the naive  
approach to writing queries can be tedious, and the resulting naive  
queries are slower than they could be. Template Haskell can eliminate  
much of this drudgery. Felipe Lessa has graciously donated some  
Template Haskell code which we have adapted.

The Facts.* hierarchy currently contains modules with geographical  
information, such as a data type of countries, cross references to  
various ISO-3166-1 names for each, a list of states in the United  
States, and the United States address format. Please see the module  
hierarchy for more specifics. Patches are welcomed, though prospective  
contributors are encouraged to encode data structures using lists of  
pairs to encode bijections, all exposed data types are instances of  
Data, Eq, Ord, Show, and Typeable, and use explicit exports to only  
export queries and their atoms. For now, we will add facts to the  
hierarchy lazily, as our projects need them.
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