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.
#lang typed/racket
(module adapter racket
(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 (make-rest/seq)))))
(require/typed
'adapter
[stream-first (All (A) (-> (Sequenceof A) A))]
[stream-rest (All (A) (-> (Sequenceof A) (Sequenceof A)))]
[stream-cons (All (A) (-> (-> A)
(-> (Sequenceof A))
(Sequenceof A)))])
(: fibonacci (Sequenceof Integer))
(define fibonacci
(stream-cons
(λ () 0)
(λ ()
(stream-cons (λ () 1)
(λ ()
(let calculate : (Sequenceof Integer) ([fibonacci
fibonacci])
(stream-cons (λ ()
(+ (stream-first fibonacci)
(stream-first (stream-rest
fibonacci))))
(λ () (calculate (stream-rest
fibonacci))))))))))
(for/list : (Listof Integer) ([fib fibonacci]
[i (in-range 10)])
fib)
-Philip
On Sun, Apr 22, 2018 at 3:01 AM, HiPhish <hiph...@openmailbox.org> wrote:
> Hello Racketeers,
>
> I have been playing around with the `math/number-theory` package and I
> wanted to use a `for`-loop of Fibonacci numbers. I had to write something
> like
>
> (for ([i (in-naturals)])
> (define fib (fibonacci i))
> ...)
>
> So I thought it would be nice to have an `in-fibonacci` sequence.
> According to the Racket reference [1] one can implement a custom sequence
> using structure type properties, but according to the Typed Racket guide
> [2] structure type properties are not supported in Typed Racket. Is there
> any other way to get `in-fibonacci` into the module or should I just live
> without it?
>
> [1] https://docs.racket-lang.org/reference/sequences.html?q=sequences
> [2] https://docs.racket-lang.org/ts-guide/caveats.html#%28part.
> _.Unsupported_features%29
>
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