On Sunday 03 April 2011 21:25:34, Patrick Browne wrote: > Daniel, > I think that the definition of other in Link makes bi-directional travel > possible in prog1. The function other takes an edge and the first node > and returns the other (second) node of that edge. So we can begin our > journey at the end and arrive at the start > > other (Edge (Node "end") (Node "start")) (Node "end") > =gives=> Node "start" > > Or we can begin at the start and end at the end. > > other (Edge (Node "start") (Node "end")) (Node "start") > =gives=> Node "end" > > So prog1 allows a car to go in both directions.
If you have edges in both directions, yes. But that's not the point for t1. In t1, you only have (Edge start end), and a car located at start. Then you (try to) move the car along that edge and check whether it is afterwards located at end. other (Edge start end) start =gives=> end, for both, one-way and two-way roads, so the move yields (Car end), its location is end and t1 == True. > > Pat > > On 03/04/2011 15:39, Daniel Fischer wrote: > > On Sunday 03 April 2011 16:04:22, Patrick Browne wrote: > >> 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. > > > > Only took a short look, but that'd probably be because it's wrong, t1 > > should give True for both. > > You have a road r from a to b and a car c at a. Then after you move > > the car along the road, it will be located at b, regardless of > > whether it's a one- way road or traffic may flow in both directions. > > 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 _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
