On Tue, Feb 03, 2009 at 09:54:46AM -0500, Dave Bayer wrote:
So the "repeat bars" are there until the first pass through the list completes, otherwise cycle would be bottom on infinite lists. Thereafter, you're saying that a core dump would reveal a completely homogeneous memory representation, just like C code, that one could pass through the foreign function interface to C code?
I'm not really sure what you mean by "repeat bars". There really is a cyclic data structure in memory at all times--it's just that until the first pass through the list, part of it is a thunk. After a complete pass to the list, however, a core dump would indeed reveal something like what you suggest.
GHC seems to have a special awareness of cyclic lists. For example, ghci computes
(zip (cycle [1..3]) (cycle [1..4])) !! (1000^1000)
immediately, as if it knows enough to compute 1000^1000 mod 12, by repeated squaring.
I came to the same conclusion as Daniel but it took me a few minutes of puzzlement. Besides, it should actually be equal to (2,1), not (1,1). =) -Brent