[plt-dev] I love Haskell (not an April Fools Joke!), feature request for 'for'

From: Sam TH (samth at ccs.neu.edu)
Date: Thu Apr 2 21:12:40 EDT 2009

On Thu, Apr 2, 2009 at 8:48 PM, Eli Barzilay <eli at 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
samth at ccs.neu.edu

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