Yep, this works. Of course, it assumes that fibonacci is memoized or
something to give good performance.
(: check-nat (-> Any Any))
(define (check-nat n)
(unless (exact-nonnegative-integer? n)
(raise-argument-error 'in-fibonacci "Natural" n)))
(raise-syntax-error #f "only allowed in a for clause" stx))
[[(name:id) (_ (~optional start-expr:expr
#:defaults ([start-expr #'0])))]
([(name) (fibonacci n)])
(for/list : (Listof Integer) ([fib (in-fibonacci 5)]
[i (in-range 10)])
On Mon, Apr 23, 2018 at 3:10 PM, Philip McGrath <phi...@philipmcgrath.com>
> It looks like you copied the code incorrectly: it should be (module
> adapter racket ...), but you have (module adapter racket/stream ...).
> To get the special performance you are referring to, you don't want to
> make a structure that's a sequence: you need to use
> `define-sequence-syntax` with `:do-in` (http://docs.racket-lang.org/
> rkt)._define-sequence-syntax))). I believe this should work with Typed
> Racket, but I haven't tried it.
> On Mon, Apr 23, 2018 at 3:00 PM, HiPhish <hiph...@openmailbox.org> wrote:
>> The adapter submodule does not work, I get the following error (in both
>> typed and untyped Racket):
>> fib.rkt:9:0: module: no #%module-begin binding in the module's
>> in: (module adapter racket/stream (provide stream-first stream-rest
>> (rename-out (stream-cons* stream-cons))) (define (stream-cons* make-first
>> make-rest/seq) (stream-cons (make-first) (sequence->stream
>> If I understand your code correctly, the idea is to define `fibonacci` as
>> a stream which returns a stream of Fibonacci numbers, correct? Won't that
>> incur a performance loss when used like that in a for-loop if the user does
>> not wrap it up in `in-stream`? The idea of `in-fibonacci` was to have a
>> form which can be used in a foor-loop the same way `in-naturals` can be.
>> Except `in-fibonacci` would have an optional argument at which Fibonacci
>> number to start counting.
>> On Sunday, April 22, 2018 at 10:48:12 AM UTC+2, Philip McGrath wrote:
>>> I hope there's a better way, but this works. The adapter submodule is
>>> needed because the normal `stream-cons` is a macro that expands into some
>>> private things that don't have types, and it requires that the rest
>>> expression produce a stream, not just any sequence. Note also, if you
>>> haven't worked with `racket/stream` before, that the arguments to the
>>> normal `stream-cons` are evaluated lazily.
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