Escape from Zurg: An Exercise in Logic Programming

Here are two Haskell modules that define a type class for programming search problems in Haskell. The module SearchStrat.hs is an extended version of the module Search.hs offering parameterization of search functions by a search strategy.

The implementation of an example application, a riddle, using either of the above modules can be found in the module Zurg.hs. (The module is ZurgStrat.hs is a variation that uses SearchStrat.hs.)

The type classes, the example application, the development of the programs, and a comparison with the Prolog approach are described in the paper Escape from Zurg: An Exercise in Logic Programming.

A Prolog solution to the riddle is contained in the file

If one is interested in solving only this particular riddle, the search need not be factored in a type class, and knowledge about the particular problem can be exploited to code a solution more directly. For example, since four toys have to cross and only two can go at the same time, it is clear that the pattern of crossings has to be 2 forward, 1 back, 2 forward, 1 back, 2 forward. Moreover, the names of the toys don't really matter, so that one can represent the toys directly by the times they need to cross the bridge. Here is a corresponding direct implementation in Haskell.

(See also the file README.)

last change: November 10, 2007 Martin Erwig