On Thu, Apr 2, 2009 at 8:48 PM, Eli Barzilay <e...@barzilay.org> wrote: > On Apr 2, Sam TH wrote: >> >> Here's the implementation of `cycle' (which should probably be in the >> sequence library): >> [...] > > Below is a better implementation of `in-sequences' using a > `append-sequences' helper. It's also implementing `in-cycle', which > just uses the same `append-sequences' utility with a cyclic list of > the inputs. (And BTW, it doesn't cache the items, which looks like a > mistake to do.)
Thanks for the implementation. Caching the items is necessary if you want to work with a sequence of bytes from a port, for example (see `in-input-bytes-port'). > But there's a problem with this -- AFAICT, there is no way for a > sequence to just return a new sequence to iterate over, which would be > a tail-call kind of transfer to the new sequence. So the `in-lazy' > thing must create a sequence that forever wraps the input sequence. > The result is a growing chain of sequence proxies. Maybe there's a > nice way to extend the api so a sequence can stop and return a > "continuation sequence", but looking at the code that change doesn't > look easy. When I was writing my code, I also wanted that API addition. > (let loop ([m+g+r m+g+r]) > (if (and (pair? m+g+r) (not ((car m+g+r)))) > (seqs->m+g+r (cddr m+g+r)) > m+g+r))) The `loop' here is never used. -- sam th sa...@ccs.neu.edu _________________________________________________ For list-related administrative tasks: http://list.cs.brown.edu/mailman/listinfo/plt-dev
