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?
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