Nested inline iterators - iterator

Question

Is it possible to have an inline iterator, which accepts another inline iterator as a parameter? My goal is to make a simple pipeline-like processing and get it unrolled into serial C code. An example which I can't get to work:
iterator test2(it: iterator(): int {.inline.}): int =
for i in it():
yield i*2
iterator test1(): int =
yield 10
yield 20
yield 30
for i in test2(test1()):
echo j
With the error:
test.nim(2, 14) Error: type mismatch: got (int) but expected one of:
iterator items[IX, T](a: array[IX, T]): T
iterator items[](E: typedesc[enum]): E:type
iterator items(a: string): char
iterator items[T](s: Slice[T]): T
iterator items[T](a: openArray[T]): T
iterator items[T](a: seq[T]): T
iterator items[T](a: set[T]): T
iterator items(a: cstring): char
> Process terminated with exit code 256

Answer 1

you have 2 options (at least)
define your iterator as {.closure.}
use templates to wrap the iterator and, which is a little ugly but works:
template runIterator(it, exec: untyped): typed =
block:
it(myIterator)
for i in myIterator() :
exec(i*2)
template defineTest1(name: untyped): typed =
iterator `name`(): int {.inline.} =
yield 10
yield 20
yield 30
template defineTest2(name: untyped): typed =
iterator `name`(): int {.inline.} =
yield 5
yield 10
yield 15
template exec(i: int): typed = echo i
runIterator(defineTest1, exec)
runIterator(defineTest2, exec)
edit:
the idea is to use a template instead of an inline iterator and to inject the code - so there is no second iterator but a template.
maybe this makes it clearer:
template test2(it, yielder, exec: untyped): typed =
for i in it() :
let `yielder` {.inject.} = 2 * i
exec
iterator test1(): int {.inline.} =
yield 10
yield 20
yield 30
iterator test1b(): int {.inline.} =
yield 5
yield 10
yield 15
test2(test1, i):
echo i
test2(test1b, i):
echo i