[racket] Can't `yield' multiple values when `in-generator' is returned from a function?
At Fri, 14 Sep 2012 08:20:22 -0400, Greg Hendershott wrote:
> tl'dr: I can `yield' multiple values when an `in-generator' sequence
> is used directly in a comprehension, but not when it is returned from
> a function. I'm stumped.
>
>
> Let's try using the `in-generator' sequence directly in a comprehension:
>
> ;; Yield 1 value, directly using `in-generator`
> (for/list ([x (in-generator (for ([i (in-range 4)])
> (yield i)))])
> x)
> ;; => '(0 1 2 3)
>
> ;; Yield 2 values, directly using `in-generator`
> (for/list ([(x y) (in-generator (for ([i (in-range 4)])
> (yield i 0)))])
> (cons x y))
> ;; => '((0 . 0) (1 . 0) (2 . 0) (3 . 0))
>
> Great!
>
> Next, let's try using the `in-generator' sequence as returned from a
> function. As best I can understand from the docs, this should work. It
> does for the case of yield-ing 1 value -- but not 2 values:
>
> ;; Yield 1 value, using `in-generator' returned from a function:
> (define (in-gen-1)
> (in-generator (for ([i (in-range 4)])
> (yield i))))
> (for/list ([x (in-gen-1)])
> x)
> ;; => '(0 1 2 3)
>
> ;; Yield 2 values, using `in-generator' returned from a function:
> (define (in-gen-2)
> (in-generator (for ([i (in-range 4)])
> (yield i 0))))
> (for/list ([(x y) (in-gen-2)])
> (cons x y))
> ;; ==> EXCEPTION:
> ; .../private/for.rkt:1046:17: arity mismatch;
> ; the expected number of arguments does not match the given number
> ; expected: 1
> ; given: 2
> ; arguments...:
> ; 0
> ; 0
>
> Why?
It's a problem in `in-generator' --- specifically in the way that
`in-generator' expands to `in-producer' when the `in-generator'
expression is not in a `for' right-hand side. In that case, the
`in-generator' expansion doesn't know how many "stop" values it's
supposed to produce.
To work around the problem, you can use `in-producer' and `generator':
;; Yield 2 values, using `in-generator' returned from a function:
(define (in-gen-2)
(in-producer (generator ()
(for ([i (in-range 4)])
(yield i 0))
(values 'done 'done))
(lambda (a b) (eq? a 'done))))
(for/list ([(x y) (in-gen-2)])
(cons x y))
I'll work on a repair for `in-generator'.