Hi, Attached are two programs that represent one-way and two-way traffic on a road. I can get the programs to produce reasonable results that match our intuitive ideas of roads. However, I have 2 question about the programs: 1)I cannot get the result suggested by the author that t1 should give true in prog1 and false in prog2. 2)My second question is more theoretical. It is stated by the author that type checking and excitability provide verification. In this case verification probably means checking that an instance is consistent with its type class. Does verification using these techniques in Haskell have any firmer logical foundation than say doing the verification in Java? I am aware that Haskell uses inference for type checking, but is the net result superior to compilers that do not use inference? Also, is Haskell execution based purely on logic? Regards, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie