[Haskell-beginners] Circular Linked Lists

[Daniel Fischer]

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.
>