Don't shoot me... The last exchange with Andrew Bromage made me recall a homework which was given to some students by a particularly nasty teacher I happen to know.
The question is to generate the whole infinite Rabbit Sequence in one shot (co-recursive, selbstverständlich). The Rabbit Sequence: 1,0,1,1,0,1,0,1,1,0,1,1,0,1,0,1,1,0,1,0,1,1,0,1,1,0,1,0,1,1,0,1,1,0,... may be obtained in two ways. A. 1. Begin with one *young* rabbit: 0. 2. In one unit of time a young rabbit grows, becomes *old*: 1. 3. In one unit of time an old rabbit has an offspring, transmutes into [1,0] (Yes, the rewriting order is meaningful). The evelution continues... So, after three units we have: 1 0 1. After four: 10110. Then 10110101. Etc. B. The n-th instance fulfils the recurrence rs 0 = [0] rs 1 = [1] rs n = rs (n-1) ++ rs (n-2) === That's it, you see now what is the relation between the Rabbit Sequence and Fibonacci. This nasty acquaintance of mine asked the students to write down a simple procedure which generates the sequence after the infinite number of units of time. Of course, any finite prefix of it. The pedagogical result was a disaster. Some students began to work, but then the teacher went crazy and demanded the solution (in Haskell) as a one-liner. Just one line, and standard Prelude functions, nothing more. So, the students thought that if it is a one liner, it must be stupid, and abandoned this exercise. Would somebody try to solve it, before I unveil the solution? It isn't difficult. Jerzy Karczmarczuk _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
