iterators.nim

# These two are semantically equivalent:

proc makeIterator(x, y: int): Irerator[int] =
  result = iterator(): int {.closure.} =
    yield 1
    yield 2
    
iterator makeIterator(x, y): int =
  yield 1
  yield 2
  
# both of them can be passed to `sumValues`

proc sumValues(list: Iterator[int]): int =
  for e in list: result += e
  
# now this is different:

proc makeFancyIterator(x, y: int): iterator(s: string): int =
  result = iterator(s: string): int {.closure.} =
     yield x + s.len
     yeild y + s.len
     
# it has to be used like this:

var iter = makeFancyIterator(3, 5)
echo iter("test") # some(7), because x = 3 and s.len = 4
echo iter("a")    # some(6), because y = 5 and s.len = 1
echo iter("more") # none(), because the iteration has finished