According to the manual:
> An inline iterator is an iterator that's always inlined by the compiler
> leading to zero overhead for the abstraction, but may result in a heavy
> increase in code size. Inline iterators are second class citizens; They can
> be passed as parameters only to other inlining code facilities like
> templates, macros **and other inline iterators.**
Several topics have already be raised on similar questions:
* [pairs with iterators](https://forum.nim-lang.org/t/1273) in 2015
* [iterator as arg of iterator](https://forum.nim-lang.org/t/2027/1) in 2016
* [zip for iterators](https://forum.nim-lang.org/t/2769) in 2017
In the end, all solutions resorted to using closure iterators instead. So is it
possible to pass inline iterator to other inline iterator, if yes how? If no, I
think manual should be updated.
Test case with Python-like enumerate:
let s = @[1, 2, 3, 4, 5]
iterator values[T](s: seq[T]): T {.inline.}=
# copy of `items` to test without its special meaning
for val in s:
yield val
iterator enumerate[T](it: iterator {. inline .}): auto {.inline.}=
var i = 0
for val in it:
yield (i, val)
inc i
iterator enumerate2[T](s: seq[T], it: iterator(a: seq[T]): T {. inline .}):
T {.inline.}=
var i = 0
for val in s.it:
yield (i, val)
inc i
# for tup in s.values.enumerate:
# echo tup
# inline_it.nim(21, 20) Error: type mismatch: got (int)
# but expected one of:
# iterator enumerate[T](it: iterator): auto
for tup in enumerate2(s, values):
echo tup
# inline_it.nim(27, 22) template/generic instantiation from here
# inline_it.nim(16, 15) 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: set[T]): T
# iterator items[T](a: seq[T]): T
# iterator items(a: cstring): char
