Am Dienstag, 3. Februar 2009 15:54 schrieb Dave Bayer:
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?
GHC seems to have a special awareness of cyclic lists. For example, ghci computes
(zip (cycle [1..3]) (cycle [1..4])) !! (1000^1000)
No, it's that the type of (!!) is [a] -> Int -> a, and 1000^1000 :: Int is 0.
immediately, as if it knows enough to compute 1000^1000 mod 12, by repeated squaring.
On Feb 3, 2009, at 9:17 AM, Brent Yorgey wrote:
It doesn't?
cycle xs = xs' where xs' = xs ++ xs'
That sure looks like a cyclic data structure to me! xs' references a thunk which computes (xs ++ xs'); this thunk, in turn, references xs'. cycle is memory-efficient precisely because it *does* actually construct a cyclic data structure.